Excessive trabeculation, often referred to as “noncompacted” myocardium, has been described at all ages, from the fetus to the adult. Current evidence for myocardial development, however, does not support the formation of compact myocardium from noncompacted myocardium, nor the arrest of this process to result in so-called noncompaction. Excessive trabeculation is frequently observed by imaging studies in healthy individuals, as well as in association with pregnancy, athletic activity, and with cardiac diseases of inherited, acquired, developmental, or congenital origins. Adults with incidentally noted excessive trabeculation frequently require no further follow-up based on trabecular pattern alone. Patients with cardiomyopathy and excessive trabeculation are managed by cardiovascular symptoms rather than the trabecular pattern. To date, the prognostic role of excessive trabeculation in adults has not been shown to be independent of other myocardial disease. In neonates and children with excessive trabeculation and normal or abnormal function, clinical caution seems warranted because of the reported association with genetic and neuromuscular disorders. This report summarizes the evidence concerning the etiology, pathophysiology, and clinical relevance of excessive trabeculation. Gaps in current knowledge of the clinical relevance of excessive trabeculation are indicated, with priorities suggested for future research and improved diagnosis in adults and children.  •Current evidence shows that the extent of trabeculated vs compact myocardium in the adult is determined by differential  growth of each myocardial layer. Because trabeculated myocardium does not coalesce to form the compact myocardial wall, the term “left ventricular noncompaction” is inaccurate and use should be discouraged.•Excessive trabeculation, as diagnosed using existing criteria, may be present as a normal variant or as a response to preload conditions.•In adults incidentally found to have excessive trabeculation with normal myocardial function and morphology, clinical management is determined by other cardiovascular symptoms or abnormalities, without regard to trabecular pattern.•In adults diagnosed with hypertrophic or dilated cardiomyopathy where excessive trabeculation is also present, the extent of ventricular trabeculation has not been demonstrated to alter management nor prognosis. Following guidelines developed specifically for those conditions is recommended.•In infants and children, caution is warranted because fewer data are available regarding the etiology and prognosis of excessive trabeculation. Clinical evaluation for occult neuromuscular disease or another genetic/metabolic etiology may be appropriate. Loci associated with lower fractal dimension conferred higher risk of dilated cardiomyopathy and heart failure in both observational and Mendelian randomization analyses. This suggests a potential role of normal trabecular structure in maintaining cardiac output, or alternatively, that observed associations with certain genetic forms of cardiomyopathy and excessive trabeculation may be mediated by the roles that these genes have in development.In the community-based UK Biobank CMR study, left ventricular trabeculation measured in fractal dimension was observed to have ∼20% heritability.For patients in whom so-called noncompaction is diagnosed, causative genetic sequence variations are reported in approximately one-third of individuals, although these estimates are confounded by inclusion bias. Such biases include patient age, family screening, heterogeneity in case ascertainment, variability in content and size of genetic screening panels, and inconsistent interpretation of the pathogenicity of genetic variants.,,,Gene sequence variation associated with so-called noncompaction were recently evaluated in several systematic reviews.,MYH7, MYBPC3, ACTC1, and TTN represented just over two-fifths of the identified sequence variations.MYH7, ACTN2, and PRDM16 were detected in some cases, but such truncating variants have not been identified as causes of more specific cardiomyopathies, suggesting that they may associate more specifically with excessive trabeculation.PRDM16 has been shown to be crucial for normal mural development in mouse models.In 2 of these recent systematic reviews,Due to the aforementioned considerations, contemporary guidelines advocate genetic testing according to the presence of the features of a conventional cardiomyopathy,,,,Excessive trabeculation has been observed in several neuromuscular disorders, including specific genetically determined conditions such as Barth syndrome,,,A retrospective analysis of CMR scans of another large cohort with Duchenne muscular dystrophy showed almost 30% of patients had excessive trabeculation in at least 1 cardiac segment when using Petersen criteria.Reversible excessive trabeculation is known to develop secondarily to increased preload in a sizeable proportion of individuals who are pregnant with otherwise normal hearts. The feature had usually resolved by 12 weeks subsequent to delivery, although with some variability in the regression of the trabecular layer, this being similar to the variation in the reduction of the overall left ventricular mass.,A reversible phenotype of excessive trabeculation has been reported in athletes. This is recognized as a morphologic epiphenomenon related to high cardiac preload demand associated with intensive physical exercise.In a younger (mean age 48 years), community-based cohort of physically active participants involved in the PESA  study, which objectively measured vigorous recreational physical activity, was associated with higher prevalence of isolated excessive trabeculation. This was double in those making up the highest quintile achieving vigorous physical activity compared to those with no vigorous physical activity.At the levels of physical activity reported by individuals who were nonathletic making up the community-based UK Biobank study, there was no evidence to suggest a dose-response relationship between physical activity intensity and the extent of left ventricular trabeculation.Excessive trabeculation has been described in up to one-sixth of patients with β-thalassemia, raising the issue of a differential diagnosis between cardiomyopathy with excessive trabeculation and thalassemic cardiomyopathy.,Chemotherapy-related cardiac dysfunction is now also emerging as being potentially associated with excessive trabeculation.Ultimately, in patients with hemoglobinopathies and other chronic hematologic disorders, the finding of excessive trabeculation should be interpreted as an adaptive response to increased cardiac preload.A number of reports have described a possible association between polycystic kidney disease and a cardiomyopathy said to be caused by excessive trabeculation.Arrhythmias are common in heart failure, in part because of coexistent myocardial fibrosis.,,There are numerous reports on presence of thrombus in the trabecular layer of patients with cardiomyopathy.,,,,Trabeculations have also been suggested to reduce the compliance of the ventricular wall,Zemrak et al,,,Multiple studies have identified the presence of left ventricular dysfunction as the principal mediator for adverse outcomes in the presence of excessive trabeculation.,Few studies have compared the prognosis for patients with cardiomyopathy and excessive trabeculation to that of other nonischemic cardiomyopathies. Among patients with dilated cardiomyopathy, the extent of trabeculation did not influence event-free survival in either unadjusted or adjusted models.The bulk of current evidence suggests that the phenotypic feature of excessive trabeculation has no independent prognostic relevance in otherwise healthy individuals with no clinical suspicion of inherited cardiac conditions or symptoms. In patients with excessive trabeculation and a known cardiomyopathy, in contrast, the risk for major adverse clinical events appears to be associated with the latter and is apparently independent of the coexisting trabeculation  and 2. ILike adults, children with normal ventricular size and function may have excessive trabeculations, frequently representing a normal variant. Congenital heart defects, such as Ebstein malformation and isomerism of the atrial appendages, may coexist with excessive trabeculation and complicate the picture.Using echocardiography, the prevalence of excessive trabeculation fulfilling the criterion for recognition as a potential cardiomyopathy among competitive athletes ranges from around 1%Among athletes, excessive trabeculation is of concern in individuals who have either left ventricular systolic dysfunction or dilation, cardiac symptoms, or abnormal electrocardiographic findings unrelated to training . Alterna,,,Athletes with symptoms, abnormal electrocardiographic findings, or a family history of cardiomyopathy should undergo cardiopulmonary exercise testing, Holter monitoring, and CMR.Controversies remain regarding the clinical significance of excessive trabeculation in both adults and children. For adults, patients who are asymptomatic with excessive trabeculation but normal ventricular function and normal chamber size appear to require little or no long-term follow-up unless other clinical concerns exist, such as family history or electrocardiogram abnormalities. No prospective cohort or registry data are currently available to validate this approach over a 5- or 10-year period. Ongoing collection of outcomes and covariates  in large studies such as UK Biobank may prove beneficial to understanding the independent physiologic impact of excessive trabeculation in these otherwise healthy individuals.For adults with cardiomyopathies combined with excessive trabeculation, questions remain regarding the implications of the trabecular phenotype. As diagnostic and genetic testing has become more sophisticated, the underlying etiology of these and other cases of cardiomyopathy will increasingly be understood. This may result in an ability to make more specific etiologic diagnoses as opposed to a general statement of cardiomyopathy with excessive trabeculation. Large, multi-institutional databases of patients with cardiomyopathy and excessive trabeculation would allow tracking of long-term outcomes, final diagnoses, and potential treatments. Further research to determine whether cardiomyopathy with excessive trabeculation have a different natural history compared to dilated cardiomyopathy without excessive trabeculation may be desirable. Such studies require careful statistical adjustment for confounders, thus increasing complexity.For neonates, children, and adolescents, the research focus is somewhat different. Longitudinal studies of the early development of the myocardium may shed light on the origin of varying phenotypes. Severe malformations of the trabecular layer, poorly developed left ventricular papillary muscles, along with frequent congenital heart disease and sudden death have been suggested as a distinct cardiomyopathy in neonates and young children.This review summarizes the evidence and uncertainties regarding the phenotypic feature of excessive trabeculation and its potential associations with cardiomyopathies . BecauseDr Petersen has received support from the National Institute for Health and Care Research Barts Biomedical Research Centre; and has consulted with Circle Cardiovascular Imaging Inc. Dr Friedrich has received support from the McGill Health Centre Foundation; has served as an advisor to and is a shareholder of Circle Cardiovascular Imaging Inc; and is a founder and shareholder of Area19 Medical Inc. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.


Many older adults require post-acute care from a nursing home or home health agency following hospitalization. Recent trends show providers are increasingly relying on home health agencies rather than institutionalized settings, with home health volume surpassing skilled nursing facility (SNF) volume since 2017. Using MedPAR patient encounter data from 2016–2019 and provider data from CMS, we analyze changes in the profile of patients receiving home health over time, showing that individuals discharged to home health are increasing in complexity based on hospital length of stay, comorbidities, and use of critical care services. Mixed effects models additionally suggest that grouping patients at the hospital and market level helps to account for unexplained variation in whether a patient is likely to receive home health versus SNF services. Examining the characteristics of hospitals and provider markets with increasing rates of home health referrals over time, we found that hospitals with increasing rates of discharge to home health were more likely to be for-profit facilities in urban areas with higher operating margins. However, this increase was not consistently tied to a corresponding decrease in rates of discharge to SNF, suggesting that hospitals are experiencing a combination of both patient-shifting across post-acute settings as well as an overall increase in baseline complexity of hospitalized patients over this time period. These results have implications for understanding the how policies currently being considered to improve value in the post-acute sector filter through heterogeneous market structures and complex organizational environments to impact patient care decisions.


Phytoene synthase (PSY) catalyzes the first committed step in the carotenoid biosynthesis pathway and is a major rate-limiting enzyme of carotenogenesis. PSY is highly regulated by various regulators and factors to modulate carotenoid biosynthesis in response to diverse developmental and environmental cues. Because of its critical role in controlling the total amount of synthesized carotenoids, PSY has been extensively investigated and engineered in plant species. However, much remains to be learned on its multifaceted regulatory control and its catalytic efficiency for carotenoid enrichment in crops. Here, we present current knowledge on the basic biology, the functional evolution, the dynamic regulation, and the metabolic engineering of PSY. We also discuss the open questions and gaps to stimulate additional research on this most studied gene/enzyme in the carotenogenic pathway. Carotenoids are a group of lipophilic isoprenoid metabolites. They play diverse roles in plants as essential photoprotective and light-harvesting pigments in photosynthesis, color agents, and precursors of phytohormones, aroma/flavor compounds, and signaling molecules. Carotenoids are also important to human nutrition and health as dietary precursors of vitamin A and antioxidants in preventing vitamin A deficiency and reducing the risk of various chronic diseases . The cride novo synthesized in nearly all kinds of plastids and are abundant in chloroplasts and chromoplasts in plant cells  was characterized as a bifunctional enzyme, which catalyzes two tightly coupled reactions of dimerization of GGPP into prephytoene diphosphate and its subsequent conversion into phytoene . Enzymatplastids . PSY alsplastids . In geneplastids . This isplastids .2+ binding site   . Most reth Mg2+) . The thrth Mg2+) .PSY genes in plant genomes.Gene duplication plays a prominent role in generating evolutionary novelty, facilitating acclimation and adaptation to adverse environments and contributing to the emergence of new agronomic traits . Gene duPSY paralogs belonging to three subgroups  were found to directly bind to the promoters of by light . Two keyian cues , 2014. Ato shade . In addi network .PSYs are transcriptionally upregulated during fruit ripening  carotenoids are present in plastids, an attractive hypothesis includes shuttling of regulatory, hydrophilic carotenoid degradation products via plastid membranes , but include amphiphilic intermediates like GGPP, free substrate diffusion can be excluded and metabolite channeling between different enzymes of the pathway plays an essential role. Importantly, the PSY substrate GGPP is shared with other pathways , such asPSY is the main target for manipulation as it is the major rate-limiting and key enzyme driving the flux into carotenogenesis. Because PSY genes are not expressed in rice endosperm in all rice germplasms, the use of genetic engineering to obtain the capacity for carotenoid biosynthesis is required for Golden Rice  generated β-carotene enriched Golden Rice  to increase antioxidant vitamin E provides a strategy to improve β-carotene content and stability as shown in sorghum grain (PSY with HisOR, a nature variant of OR that induces chromoplast biogenesis, offers an additional strategy to significantly boost carotenoid accumulation and stability during seed maturation and storage as documented in Arabidopsis seed (OR mutant variant has also shown to enhance carotenoid levels and stability in potato and sweetpotato (Manipulation of nd maize . Co-exprum grain . In addisis seed . Expressetpotato . No othevia heterologous expression in bacterial test system and in callus system, which seems transferable to various plant systems (The intrinsic properties of PSY determine its efficiency. Numerous examples exemplify a simple change in PSY amino acid sequences that profoundly alters its activity , which p systems . ApplicaPSY is known to be the key rate-limiting enzyme of carotenogenesis. However, the availability of precursors and products following altering pathway enzyme expression and activities can create new metabolic bottlenecks in the pathway , 2017. IPSY gene family members and to reveal their tissue-specific expression patterns in plant species. Extensive studies of PSY in carotenogenesis have also established various regulatory tiers acting at transcriptional, post-transcriptional, post-translational, and epigenetic levels. At the same time, great success has been made in engineering PSY expression to enhance crop nutritional quality. However, considerably more efforts are still required to delineate the mechanistic details of various regulatory controls and to develop/select highly efficient PSY enzyme (Because of its critical role in carotenogenesis, PSY is the most well characterized and studied gene/enzyme in the carotenoid biosynthesis pathway. The accelerated sequencing of numerous plant genomes and transcriptomes makes it straightforward to identify Y enzyme . Open qucis-motifs in the promoters of PSY, additional regulators that may have dominant or additive roles in transcriptionally regulating PSY remain uncovered. In addition to a few post-translational regulators identified, many more are expected to be identified for rapid regulation of PSY activity and fine-tune of carotenogenesis.What are the additional regulators that govern PSY gene expression, protein stability, and activity? The crucial role of PSY in carotenogenesis and multiple signals/factors affecting its expression suggest a complex regulatory control machinery and network. Although a number of TFs have been shown to bind directly to the What are the key amino acid residues that give high PSY activity? The intrinsic enzymatic efficiency of PSY enzymes from various plant species varies greatly to give different biosynthetic capacity as exemplified with daffodil and maize PSY used for Golden Rice 1 and 2, respectively. Single amino acid sequence changes can profoundly alter PSY activity. Therefore, identification and optimization of the key amino acid residues will not only unravel the intrinsic features of PSY activity, but also facilitate the design of highly efficient PSY for development of carotenoid enriched crops.How does PSY work with other enzymes in the pathway to drive the metabolic flux and what are the mechanisms controlling metabolon assembly? It has long been hypothesized that carotenogenic enzymes form enzyme complexes although solid evidence is still lacking. GGPS-PSY metabolon can efficiently drive GGPP substrate into the carotenogenic pathway, supporting an important role of the complexes for carotenogenesis.PSY is often associated with increased β-carotene accumulation in many non-photosynthetic tissues or organs. What are the regulatory mechanisms for this favorable phenomenon? Identification of the mechanisms involved might further help to develop additional approaches for crop nutritional quality improvement.One interesting phenomenon observed in many studies is that overexpression of Phytoene synthase activity needs to be precisely coordinated with carotenoid end products. Of special interest for future research also includes how the rate-limiting function of PSY is dynamically regulated with the total carotenoid content and the relative levels of individual carotenoids.The fast advance in techniques and multi-omics information from many plant species will be promising in elucidating the unknown regulators of PSY and unraveling the intrinsic features underlying its activity and function. The information obtained would greatly enrich our current knowledge of the regulatory mechanisms and lay down novel strategies for predictable carotenoid improvement in crops. Integrating both key regulators and targeted carotenogenic genes into a synthetic biology framework will allow us to achieve the targeted biofortification goals.XZ wrote the Sections “Functional Evolution of Phytoene Synthase” and “Genetic Engineering of Phytoene Synthase” as well as formatted the references. SR wrote the Sections “Basic Biology of Phytoene Synthase” and “Multifaceted Regulation of Phytoene Synthase.” EW wrote the Section “Introduction.” TS designed RW was employed by ScreenSYS GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.


Dairy is a high-potential food for addressing child malnutrition in low and middle income countries (LMICs), but consumption varies greatly across LMICs.I explore economic drivers of cross-country disparities in dairy consumption among young children.Wealth differences across LMICs are the strongest predictor of variation in dairy consumption, followed by dairy price differences.Refrigeration also predicts dairy consumption while water quality influences powdered milk consumption.LMICs should better leverage dairy policies to improve nutrition, focusing on dairy value chain constraints, industrial policy, trade policy and nutrition education and awareness campaigns.Agricultural policies are increasingly being asked to do more to address the extensive global burden of undernutrition . UndernuOne area within agriculture with tremendous potential to influence early childhood nutrition is the dairy sector. Dairy products have a range of nutritional and physical characteristics that make them an almost ideal complementary food. Undernourished children in poor countries are often deficient in foods rich in high-quality proteins comprised of essential amino acids that constitute the building blocks for linear growth and cognitive development . Dairy hConsistent with the biological importance of milk for nutrition, a diverse and growing body of evidence links dairy consumption to faster growth in early childhood. A public health-nutrition literature has engaged in efficacy and programmatic trials of dairy products on growth in different stages of childhood, and across diverse populations. It finds significant impacts of dairy on child growth . An exteYet despite their nutritional potential, dairy consumption in developing regions is highly variable and often very low, as I demonstrate below. Many parts of the developing world have little or no history of dairy consumption, especially in South-East Asia and much of sub-Saharan Africa where lactose intolerance in the adult population is widespread . NeverthIn this paper, I explore why dairy consumption is still low in so much of the developing world where malnutrition is most prevalent, and in spite of abundant evidence of dairy being highly effective in redressing child stunting in particular. Previous reviews on dairy and nutrition in developing countries focus principally on the nutritional properties of dairy , the effSince malnutrition is especially harmful in early childhood, this section explores the potential determinants of variation in child dairy consumption, as measured by a simple yes/no indicator of whether children 12 to 23 mo consumed milk, yoghurt, or cheese in the past 24 hours as measured by Demographic Health Surveys (DHS) in 59 LMICs  in the 2The remaining indicators in r = 0.62, significant below the 1% level), so separate models with GDP and wealth as interchangeable substitutes are reported in Economic studies on the demand for dairy products suggest that consumption rises quite sharply with income, particularly at low levels of income, even in Africa where lactose intolerance is high . MoreoveIs wealth also a strong predictor of dairy consumption within countries for the pooled multicountry sample of 114,560 children? Poverty is always a constraint to purchasing healthy foods because almost all healthy foods are relatively expensive sources of calories , and pooP-value = 0.30). Hence, consumption seems more sensitive to the price of fresh milk than to prices of powdered milk, perhaps reflecting parental preferences for fresh milk.In A limitation of the consumer price data described above is that the milk prices refer only to marketed retail products, whereas rural households may be much more dependent on milk sold through informal market channels or obtained from a farm household’s own cattle. In the rural Ethiopian highlands, for example, In brief, the cross-country regression evidence in Previous research has found strong associations between nutritional knowledge—or its proxies such as formal schooling—and child nutrition outcomes . AldermaThere is no evidence that cross-country differences in dairy consumption are explained by these two knowledge proxies. However, child-level regressions reveal that maternal education typically increases the probability of dairy consumption by 4 to 10 percentage points , Panel BIn countries that do not produce milk at scale, imported milk powder is often the main alternative. However, since young children are highly sensitive to infection, milk powder requires reconstitution with clean water. Hence, poor access to clean water could add significantly to the implicit cost of milk powder, especially if households—and female caregivers in particular—bear high implicit costs in water collection . I thereStrikingly, the descriptive statistics in Given the apparent interaction between dependence on dairy powder imports and water quality, I estimated cross-country regressions that include both fresh milk and long-life milk prices in the same model, before introducing interactions between milk prices and access to piped water . ConsistGiven that many consumers may have a strong preference for fresh or long-life liquid milk over powdered milk, refrigeration could be an important magnifier of demand for dairy products . I therefore use household ownership of a refrigerator (from the DHS survey) as an explanatory variable in both the cross-country and within-country regressions. Fridge ownership is strongly correlated with ownership of other household assets, particularly electricity access, but the goal here is to again test for independent predictive power controlling for household asset ownership.r = 0.86) with the DHS wealth index, which is statistically significant at the 1% level. However, the child level regressions results reported in Panel E of In the cross-country regressions, we observe mixed evidence on the importance of refrigeration. In the model with GDP per capita as a control , the relative price of fresh milk (13 points) and differences in piped water access (around 10 points). Clearly, long-run economic development  will eventually help redress the imbalance of dairy consumption observed across countries, but the importance of relative prices arguably offers more scope for increasing consumption quickly through subsidies, trade reform or agricultural investments, as I discuss further below.This study first identified significant differences in child dairy consumption across regions and showed that these differences are strongly explained by income/wealth differences across countries, as well as wealth differences across households within countries. Although poverty is a lamentable barrier to increasing dairy consumption, the strong wealth gradient for dairy at least suggests that parental demand for their children to consume milk is almost universally strong. However, even for a given per capita income, there are still large variations in dairy consumption across regions, with countries in West and Central Africa and South East Asia (where dairy traditions are weak and cattle ownership not widespread) having particularly low consumption levels compared to similarly poor countries in Eastern Africa or South Asia (with strong dairy traditions).Given that milk demand rises sharply with income, consumption differences that persist net of income effects likely reflect a combination of formal and informal price differences, with “informal” prices including prices in informal milk markets or the implicit cost of consuming milk that is produced by the household itself. In most of Africa and Asia fresh milk is exceptionally expensive in caloric terms, even in countries where cattle ownership is widespread. This is partially explained by the extreme perishability of fresh milk, and the high transport and trade costs of imported powdered milk, especially in rural areas. However, the low consumption of powdered milk in still somewhat puzzling given that it could surely be consumed on a greater scale in many LMICs, and could also be subsidized for nutritional purposes. Our research hints at water quality as one additional constraint to powdered milk consumption, but one area for future research is better understanding consumer demand for powdered versus fresh milk, as well as demand for nutrient-fortified dairy products targeted at young children.One plausible conjecture is that in regions where milk is traditionally consumed—such as in East Africa—the preference for fresh milk is stronger than it is for powdered milk. At the same time, many low-income countries at low levels of development have largely failed to modernize their domestic dairy production, collection, processing, storage, transport, and marketing systems , resultiIn countries where milk production is minimal and dairy is not a traditionally consumed good, the challenge of raising dairy consumption is very different to countries with strong dairy traditions like India, Kenya, or Ethiopia. Direct sales of relatively cheap milk powder may not be the most viable way to increase consumption, especially if there is strong and growing demand for liquid milk. In such contexts, one can conjecture that industrial-scale reconstitution of milk, including blending powdered milk with local fresh milk, is a more viable option for improving accessibility and affordability of dairy products. Exemplars in this regard are Thailand and Vietnam. In both countries dairy is not a traditional food, but rising incomes, consumer perceptions of the nutritional importance of milk, and favorable policy environments have led to improved supply and rapid growth in consumption. The broader industrial policy strategies for dairy promotion in these countries is still not well-documented and the existing literature largely focuses on the benefits of these strategies for smallholders, rather than consumers . In ThaiThe rapid transformation of dairy systems in both traditional and nontraditional countries shows that dairy consumption patterns are not immutable, that there are policy levers for overcoming longstanding constraints to consumption, and that dairy can play a central role in nutrition-sensitive agricultural policies. There is, however, still much to learn about how best to exploit the full potential of dairy for addressing the very extensive and very costly global burden of malnutrition.vfac083_suppl_Supplementary_AppendixClick here for additional data file.


Advances in intracranial electroencephalography (iEEG) and neurophysiology have enabled the study of previously inaccessible brain regions with high fidelity temporal and spatial resolution. Studies of iEEG have revealed a rich neural code subserving healthy brain function and which fails in disease states. Machine learning (ML), a form of artificial intelligence, is a modern tool that may be able to better decode complex neural signals and enhance interpretation of these data. To date, a number of publications have applied ML to iEEG, but clinician awareness of these techniques and their relevance to neurosurgery, has been limited. The present work presents a review of existing applications of ML techniques in iEEG data, discusses the relative merits and limitations of the various approaches, and examines potential avenues for clinical translation in neurosurgery. One-hundred-seven articles examining artificial intelligence applications to iEEG were identified from 3 databases. Clinical applications of ML from these articles were categorized into 4 domains: i) seizure analysis, ii) motor tasks, iii) cognitive assessment, and iv) sleep staging. The review revealed that supervised algorithms were most commonly used across studies and often leveraged publicly available timeseries datasets. We conclude with recommendations for future work and potential clinical applications. Intracranial electroencephalography (iEEG) provides exquisite detail with which to study neural activity. Stereotactic EEG (sEEG), strips, grids and depth electrodes are among the most common iEEG modalities. Modern diagnostic and treatment strategies for neurological conditions such as epilepsy rely heavily on interpreting iEEG signals to extract meaningful information that can impact clinical decision making  is a rapidly growing field that has shown significant potential in complex biomedical applications  between nodes are assigned weights during training analogous to the strength of synaptic connections between neurons. A primary advantage of deep learning compared to standard ML is that these algorithms can learn from the raw data and automatically extract meaningful information thereby bypassing human-intensive feature selection steps. However, it is important to consider that deep networks can take significantly longer to train and often require larger sample sizes than commonly available in neurosurgical populations.ML algorithms can be dichotomized into “supervised” and “unsupervised” learning approaches. An overview of these methodologies is illustrated in Despite the potential for application of ML to iEEG data, evidence to support the utilization of different ML techniques is limited, and considerable heterogeneity has been reported in the literature. As such, the present systematic review aims to: 1) assess the current landscape of ML utilization in iEEG and 2) examine the relative benefits and limitations of ML approaches as they relate to specific neurosurgical problems.th, 2020. Our inclusion criteria comprised of two categories of keywords which must appear in either the article title, abstract or article keywords. The first category comprised of machine learning keywords , while the second category comprised of keywords relevant to intracranial EEG . Relevant articles were required to contain at least one keyword from the first category and at least one from the second category. A detailed summary of our search strategy and yield is provided in the In order to identify published, peer-reviewed articles that employ ML in the context of iEEG data, we performed a systematic search of articles using 3 commonly accessed medicine and engineering databases: OVID MEDLINE, IEEE, and Web of Science. We selected these databases as we aimed to capture the breadth of applications of this technology in the fields of science, medicine, and engineering. The search was conducted on August 18The PRISMA guidelines . The country of origin was defined by the affiliation of the first author.Data acquisition comprised of intracranial recording method. This includes sEEG, and electrocorticographic (ECoG) strips or grids, or depth electrodes. A fourth option was available if the studies did not specify the type of recording method used. The source of EEG data was also compiled as free-text entries to track whether studies used their own data, or data obtained from online databases.The application of machine learning algorithms was classified into four broad categories. Due to the heterogeneity of studies in these fields, studies in the seizure and motor categories were further subdivided as follows:Seizure: seizure prediction/detection, surgical outcome prediction, pathological tissue detection, high-frequency oscillation (HFO) detection/classification, seizure onset zone (SOZ)/epileptic focus localization, bad channel detection, spike detection, tumor tissue detection.Motor: movement classification, motor imagery, speech production.Cognitive tasksSleep Staging.The type of ML algorithm was categorized into four groups: Standard supervised ML includes popular algorithms such as support vector machines (SVM), K-nearest neighbors (KNN), single-layered artificial neural networks (ANN), linear discriminant analysis (LDA) and any others that did not fit the definition of deep learning. Deep learning algorithms include recurrent neural networks (RNN), convolutional neural networks (CNN) and multi-layered neural networks  higher order feature selection algorithms; and 2) physically interpretable features (raw or hand-crafted). Studies that used higher-order feature-selection algorithms were defined as those in which the feature selection/pre-processing workflows contained one or more of: i) non-standard time-frequency transforms; ii) dimensionality reduction of feature space; iii) abstract feature selection algorithms; or iv) abstract features .Performance metrics of the best final algorithms from each paper were recorded to gain insight on relative performance based on algorithmic and feature selection decisions. As algorithm performance can only be fairly compared between studies where both studies use the same dataset, this step was only done in studies employing the same publicly available datasets further discussed in Section Intracranial EEG Datasets. Although accuracy may be considered a sufficient measure of performance, it holds many caveats whereby it may give a false impression of performance in highly unbalanced data. As such, we opted to record all performance metrics presented within each study to provide a more objective picture. These include accuracy, area-under-ROC-curve, sensitivity, specificity, precision, positive predictive value, and/or negative predictive value. As model specificity was most commonly reported in all included studies, this metric is used to compare performance of algorithms between studies. If this metric was not provided, accuracy is used for comparison. Where studies solely reported cross-validation performance, a single mean performance metric was tabulated. In others that split their data into validation and testing sets, both the validation and testing performance were compiled and the testing performance was used for comparison.One-hundred-seven articles were included in the systematic review . The 50 remaining reported the use of at least one pre-existing dataset, either from their own research group or a freely available dataset available online. Some publications use more than one source of data.Pre-existing iEEG datasets are utilized in a number of these papers. The most popular of these include the Freiburg  and SOZ/epileptic focus localization  were the most popular. The motor category is composed of 3 subcategories where movement classification makes up half of the applications .The majority of articles reviewed in this study employed standard supervised ML methods only , followed by DL only . Fourteen articles employed both standard ML and DL in order to compare algorithm performance. Unsupervised learning made up a minority of articles found . An illustration of this distribution is presented in Performance metrics for each study employing a publicly available dataset are summarized in p = 0.019 by Student's t-test). Few of these studies, however, reported specificity thereby making a performance comparison based on this metric more difficult. In those using the Freiburg dataset and exclusively investigating standard machine learning  , and interictal epileptiform discharges (IED). In particular, HFOs have been proposed as epileptogenic biomarkers with emerging prognostic utility in cortical resection outcomes  and ripple (135-200Hz) bands were most useful in seizure prediction  or its variants as a data pre-processing step . This suggests that while higher-order input features have few biological and physical correlates, this penalty to interpretability does not necessarily translate into better model performance.Measures such as fractal dimension and higher-order moments have unintuitive physical correlates and may not correlate with underlying biological signals. Thus, we compared “higher-order” feature selection algorithms with approaches with physically interpretable features to determine if differences in model performance arise. Studies that used higher-order feature-selection algorithms were defined as those in which the feature selection/pre-processing workflows contained one or more of: i) non-standard time-frequency transforms; ii) dimensionality reduction of feature space; iii) abstract feature selection algorithms; or iv) abstract features . When comparing seizure prediction/detection algorithms that were tested on all 21 patients from the Freiburg iEEG dataset, “standard” and “higher-order” feature selection approaches did not differ in overall sensitivity , accelerometry (ACM), electrocardiography (ECG), near-infrared spectroscopy (NIRS), video, and electrodermal activity (EDA) inputs beyond what can be achieved by a human interpreter (Ulate-Campos et al., Enhanced SOZ localization is an emerging area for increased ML utilization. Indeed, in medically-refractory epilepsy, post-operative seizure freedom rates remain in the range of 50–70% despite multimodal analytic tools to guide localization and resection of the putative epileptogenic zone (Englot et al., Beyond epilepsy, the breadth of ML use in neurosurgical outcome prediction spans multiple fields from neoplasms to spinal disease to arteriovenous malformations, with promising success. (Senders et al., Despite the significant potential of deep learning, there are several challenges to its clinical deployment. First, deep learning generally requires a significantly larger pool of data compared with standard ML algorithms, and datasets of this size may be clinically unavailable. A second limitation of deep learning relates to its status as a “blackbox” algorithm whereby the underlying decision-making process of the algorithm cannot be readily understood. A great deal of contemporary literature has examined this subject and is producing algorithmic approaches to explain such “blackbox” models. Examples include game theoretic “Shapley” values and permutation importance which provide insight through backpropagation of altered to retrospectively determined feature importance (Wong et al., Interestingly, the realm of neuro-oncology may also be impacted by these advanced AI technologies. A study reviewed herein presented a novel proof of concept using ML to differentiate healthy tissue from brain tumor tissue during awake craniotomy (Boussen et al., A significant number of machine learning applications to iEEG relate to motor tasks. Our analysis of the model performances in this context provided a reminder on the importance of model generalizability. Although Demirer et al.'s SVM (Demirer et al., The majority of movement classification studies employ algorithms to differentiate motor tasks such as upper limb movement (Yanagisawa et al., The relevance of combining iEEG signals with ML in these domains can relate to better tailoring of neuroprostheses to individual patient dynamics. Whereas, current myoelectric prostheses follow a “one-size-fits-all” approach, the adaptability of ML-based neuroprotheses could learn from the patient's individual movements and adapt to become more responsive and precise over time. Indeed, a recent survey of upper limb prothesis users reveals a desire for adaptable grip strength and individual finger and thumb movements (Cordella et al., The last sub-category of motor tasks relates to speech production (Ramsey et al., The study of cognitive performance is unique in that the neural circuitry and activity patterns remain poorly understood. This has limited the development of neuromodulatory interventions, such as deep brain or closed-loop stimulation that could potentially treat deficits in subdomains of cognition. A key role for ML therefore lies in its potential to identify activity distributed across neural assemblies related to specific tasks.Within the studies we report, memory was the most common cognitive task studied (Kragel et al., The complexity of neural activity during cognitive tasks is reflected in the relatively modest improvements in algorithm classification performance when compared to chance. However, despite these challenges, these models have shown practical utility in closed-loop stimulation for memory (Ezzyat et al., Other studies in the cognitive realm employed machine learning for unique purposes. Saboo et al.  employedIn another, SVMs were used to differentiate whether participants were performing a mathematical or a memory task and achieved best accuracies of 89.51, 87.18 and 76.80% for 3 participants (Schrouff et al., Studies identified in this review employed supervised (Kremen et al., The primary advantage of ML in this context is the speed of classification and accuracy of sleep-stage detection which would be of importance to a variety of neuromodulatory therapeutics that function during specific stages of sleep (Kremen et al., The literature search conducted herein revealed that multiple different types of ML algorithms can be employed in the context of iEEG analysis and classification for variety of applications. However, each algorithm faces its own set of limitations. Generally, standard algorithms including SVM, KNN, and decisions trees may not be suitable for very large datasets. Training an SVM also involves numerous parameters which can be a lengthy process to optimize, and they can be also be biased if the number of features provided is greater than the number of samples in the training set. KNNs also perform poorly when using a large number of features and they are particularly sensitive to outliers or missing values. Decision trees can become hypersensitive to its input data whereby small changes in the input can have drastic effects on the output classification. As an ensemble of decision trees, RFs are can be limited by their interpretability and are often considered a black box. Similarly, deep learning models are generally criticized for their lack of interpretability, although novel methods attempting to elucidate how these complex multilayered models make decisions are discussed in the literature (Montavon et al., Exploring the current ML literature, we identified three recommendations which may be useful for future work in ML and iEEG. First, we believe that future studies should aim to not only report performance metrics but provide a specific reference point for comparison–whether the best previous ML performance or contemporary expert performance. Such comparisons can help ground clinical interpretation of the data and identify the potential practical utilizations of ML in routine clinical practice. Second, large datasets representative of the target population may be advantageous over small datasets as these are more likely to generalize well when applied to new patients. This reinforces the importance of internal and external validation to ensure that algorithms can perform equally well with previously unseen data. Third, it may be advantageous to employ explainable ML algorithms rather than “black box” models in order to shed light on algorithms' decision-making processes. More transparent models may not only offer new knowledge on important features of disease but may also improve the trust from the medical community to employ these models clinically. A study centered around classification performance may employ a more convoluted algorithm while sacrificing interpretability while one aiming to provide new knowledge on the classification process may sacrifice some performance for interpretability.Artificial intelligence is expected to have a significant impact on the future practice of neurosurgery. By identifying complex and hidden patterns that may be inconspicuous to human counterparts, ML algorithms may not only offer more accurate and more rapid tools to assist decision making but may also elucidate the underlying mechanisms involved in a number of neurological disorders.This study achieved both of our aims: to assess the current landscape of ML in iEEG, and to examine the relative benefits and limitations of ML approached as they related to neurosurgical problems. We conducted a systematic search across three databases encompassing the fields of medicine, science and engineering and identified 107 articles. We nonetheless recognize that some articles may have been missed within our systematic search considering the rapid rate at which papers within the realm of AI are currently being published.The studies reviewed herein reveal a promising future of AI in iEEG by shedding light on the variety of ML algorithms that can be employed to classify data in the context of seizures, motor tasks, cognitive tasks and sleep. Of these, supervised algorithms were most commonly employed with most articles published in the realm of seizures and epilepsy. The merits of using ML to tackle neurosurgical problems are broad including the development of more personalized neuroprostheses, standardizing seizure detection compared to human analyses, predicting surgical outcomes, and improving the speed and classification of sleep stages.As AI becomes increasingly utilized in medicine and surgery, we intend for this work to serve as a resource to aid in the interoperation and development of relevant technologies employing ML in the context of iEEG analysis.The original contributions presented in the study are included in the article/NM and NW led the study and conducted the literature search, review and analyses. FZ conducted the statistical analyses and contributed to extracting content from the literature. SW and LE provided extensive input on machine learning. SW, HS, KM, LE, and GI assisted and provided input throughout the review process. NM, NW, and FZ wrote the manuscript with input and feedback from all authors. GI was in charge of the overall direction and planning. All authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.


Its efferent arm influences GI motility and secretion, cardiac ionotropy, chonotropy and heart rate variability, blood pressure responses, bronchoconstriction and modulates gag and cough responses via palatine and pharyngeal innervation. Vagus nerve stimulation has been utilized as a successful treatment for intractable epilepsy and treatment-resistant depression, and new non-invasive transcutaneous (t-VNS) devices offer equivalent therapeutic potential as invasive devices without the surgical risks. t-VNS offers exciting potential as a therapeutic intervention in cognitive decline and aging populations, classically affected by reduced cerebral perfusion by modulating both limbic and frontal cortical structures, regulating cerebral perfusion and improving parasympathetic modulation of the cardiovascular system. In this narrative review we summarize the research to date investigating the cognitive effects of VNS therapy, and its effects on neurocardiovascular stability.The vagus nerve is the longest nerve in the human body, providing afferent information about visceral sensation, integrity and somatic sensations to the CNS  Vagus nerve stimulation (VNS) as a neurostimulation technique and has received renewed attention in recent years. Traditionally invasive VNS (iVNS) devices were sutured under the skin of the chest with a lateral left neck dissection undertaken to expose the left cervical vagus nerve and wrap a stimulating electrode around it. Each iVNS device is costly and up to 30% of patients have side effects post implantation . Since tvia the central autonomic network.Declining cognition associated with aging is a burgeoning global health crisis, with at least 152.8 million persons projected to have dementia worldwide by the year 2050 . There aStudies of patients with intractable epilepsy and treatment -resistant depression treated with iVNS devices showed signals indicating increased alertness and potentially cognitive improvements  and a smThe purpose of this narrative review will be to outline the research to date investigating both cognitive outcomes of VNS in healthy and clinical populations, and the effect VNS has on HRV as a measure of autonomic tone. The mechanisms of action of VNS including neurotransmitter release, local increased cerebral blood flow and modulation of peripheral hemodynamics are discussed and future research recommendations outlined.The longest nerve in the body, the vagus nerve derives its name from the Latin for ‘straying’ or ‘wandering.’ Aptly named, the nerve has an extensive course, traveling from the medulla to the gut. The vagus nerve’s function is to transmit information to and from the central nervous system (CNS) regarding control of the gastrointestinal, cardiovascular, and respiratory systems. It is comprised of approximately 80% afferent and 20% efferent fibers  includinAfferent vagus fibers enter the medulla at the level of the olive, and terminate primarily in the NTS . Each vavia descending pathways projecting onto sympathetic pre-ganglionic neurons in the spinal cord and onto the DMN at the origin of vagal efferents , a motor nucleus located in the reticular formation of the medulla which gives rise to preganglionic neurons innervating the heart and lungs  which is thought to be the origin of autonomic, behavioral, cognitive, and endocrine responses, capable of modulating the functioning of the autonomic nervous system (ANS) fferents . The cenfferents . Whilst nd lungs  which ex ganglia . The rig ganglia . The dorSee th century  or the neck . For the purposes of this narrative review non-invasive VNS will refer to auricular t-VNS.Invasive VNS (iVNS) received United States regulatory approval for the adjunctive treatment of refractory seizures in 1997 and for use in treatment resistant depression in 2005 . Howevervia this non-invasive method  at the ear  see Figue method .Transcutaneous auricular vagus nerve stimulation waveforms can be delivered at a variety of different parameter settings which vary frequency (Hz), amplitude (mA), pulse width (μs-msec) and duration of stimulation. It is currently being investigated as a therapeutic intervention for a variety of medical disorders including epilepsy, migraine and cluster headaches, tinnitus, atrial fibrillation, Parkinson’s disease, schizophrenia, impaired glucose tolerance, obesity, and pain . There iSee via response and attention-inhibition , and Simon tasks, i.e., forced-choice reaction time tasks that require participants to selectively attend and respond to target stimuli whilst ignoring goal-irrelevant distracting stimuli .Enhanced response times, as reflected by participants’ ability to stop a process and change to another response simultaneously and sequentially, and increased post error slowing were demonstrated during t-VNS . Post erIn a similar experimental set up, increased attention, globally enhanced accuracy and reduced performance costs were demonstrated in a Stop-Change paradigm with t-VNS .Results in this area have not been uniformly positive. In a testing paradigm in healthy volunteers using higher than average amplitude settings see  there weThe most recent studies in this area have involved a spatial stimulation and response inhibition multitask, with notable improved results in accuracy with 25 min pre-assessment t-VNS stimulation  and imprvia synaptic plasticity. A recent investigation of word retention, stimulating the left tragus with t-VNS at again similar parameters but wider amplitude found improved accuracy in word retention but only in items that rhymed, i.e., were phonologically similar  was also significantly improved . However similar .Transcutaneous auricular vagus nerve stimulation has been tested in a group of healthy older adults to determine the technique’s impact on performance in a face-name association task . VNS wasvia the Reading the Mind in the Eyes test . A further potential confounder is the impact some of these underlying pathologies have on cognition, the altered medial temporal anatomy especially in cases of epilepsy and the medications used to manage these conditions can also have deleterious effects on cognition.Vagus nerve stimulation has been shown experimentally to have mixed results when examining the subdomain of decision making, specifically on the Iowa Gambling Task (IGT). In one paradigm eleven patients with refractory epilepsy and iVNS devices completed a gambling task involving control and experimental trials with active VNS synchronized to stimulate in the latter. Whilst improved performance was demonstrated in the earlier part of the task, this trend was reversed later in the experimental trial with active stimulation trending toward being detrimental to performance . TechnicWorking memory refers to a cognitive process that provides temporary storage and manipulation of the information necessary for complex cognitive tasks . LiteratThe effect of active iVNS on response inhibition was also assessed by employing a classic stop-signal task in participants with refractory epilepsy . Quickervia an experimental protocol whereby participants with iVNS devices inserted for epilepsy control, were required to read a series of paragraphs, and subsequently identify words that were highlighted in the text. The study population comprised two groups of patients who were administered active (0.5–1.5 mA) or sham VNS, delivered 2-min after learning in the memory consolidation phase. An inverted U-shaped relationship was demonstrated regarding stimulus intensity and modulation of cognitive performance, with memory enhancing effects demonstrated only at moderate intensities, namely 0.5 mA  was assessed in a group of adolescents diagnosed with major depressive disorder (MDD). In non-depressed controls t-VNS delivered at 1Hz, 0.5 mA 30 s block with 15 min lead in time, demonstrated enhanced recognition of emotions but notably led to a significant decrease in the ability of those with MDD to recognize sad emotions .See via modulation of peripheral hemodynamics. For the purposes of this narrative review, we will analyze the link between cerebral blood flow, cerebral autoregulation and cardiac modulation. Beyond the scope of this review is how t-VNS may therapeutically affect the inflammatory cascade via activating the cholinergic anti-inflammatory pathway and the beneficial effects this may have in aging populations.There are many potential mechanisms through which VNS may exert its cognitive enhancing effects, including direct neurotransmitter release, increased cerebral perfusion to discreet neuroanatomical structures, reduced neuro-inflammation and via the afferent projections of the vagus nerve are thought to be GABA and Norepinephrine (NE). For a comprehensive review of the preclinical and clinical studies detailing the evidence supporting the modulation of these neurotransmitters during iVNS and t-VNS see . There are two distinct modes of LC firing that are associated with equally distinct modes of attentional strategy. Connections with the orbitofrontal cortex and anterior cingulate cortex are thought to drive the LC-NE system into one of these two stable states of activity, a high tonic (sustained) mode or a phasic (bursting) mode accompanied by moderate tonic activity  had overall better performance on a reversal memory tasks  and had via the LC-NE system increasing frontal NE release and thus executive functioning  despite variations in perfusion pressure. The aim of autoregulation is to protect the brain against hypoxia and edema as a result of decreased or critically high arterial blood pressures respectively. Multiple factors physiologically modify autoregulation including blood CO2 levels, hypoxia etc. While still controversial, the ANS may play a prominent role in cerebral autoregulation in response to such stimuli, inducing vasodilation or constriction, and parasympathetic and sympathetic nerves are anatomically located in the same perineural sheath innervating cerebral arteries . The meaMultiple modalities have been utilized to assess for CBF changes due to vagus nerve stimulation, including position emission tomography (PET), functional magnetic resonance imaging (fMRI) and single photon emission computed tomography (SPECT) studies and trials of patients with iVNS treatment for epilepsy and depression have demonstrated a variety of CBF modulatory effects at specific cortical and subcortical areas. Increased CBF at the orbitofrontal cortex , temporaNotably analysis undertaken during acute iVNS has noted bilateral decreased hippocampal CBF . This haIt is worth considering that intermittently stimulating neurons at different frequencies produces drastically different changes in neuronal behavior with low frequency stimulation inducing long term depression (LTD) and less connectivity while intermittent high frequency stimulation produces long term potentiation (LTP) and increased signaling . TherefoBoth dementia and even its prodromal stage, MCI, are characterized by a reduction in cerebral blood flow . A meta-via this route, however further dedicated studies are required to precisely delineate this relationship.As well as modulating central neurotransmitter release and cerebral blood flow, VNS has been shown to have positive peripheral modulatory effects in pathological states characterized by impaired autonomic regulation including postural orthostatic tachycardia syndrome (POTS)  specificvia a variety of approaches and is based on the extrapolation of time intervals between each R wave peak  HRV measures . In a caTranscutaneous auricular vagus nerve stimulation devices and their stimulation effect on HRV have been examined in several experimental paradigms involving multiple auricular positions, left vs. right ear stimulation, and different stimulation settings. There is a trend toward positive findings, i.e., improved HRV indices, with t-VNS in healthy volunteer populations when the right auricular branch of the vagus is stimulated . It is nWhen specific parameters of stimulation at the left tragus were sequentially analyzed, the settings that had the most significant impact on heart rate analysis in young volunteers were 500 μs at 10 Hz . StudiesResearch in this area has not been consistent. Some initial findings indicated improved HRV measures with t-VNS to the left cymba conchae but ultimately no difference compared to sham and at multiple intensities . In an evia its left cervical branch, thereby appropriately reducing adverse cardiac effects but also not demonstrably influencing HRV measures in these populations.Initial studies in clinical populations involved patients with iVNS devices inserted for control of refractory epilepsy. The earliest study demonstrated a reduction in LF:HF ratio and significantly higher HF power was noted in the higher stimulation group than lower stimulation see . These rPlease see r = 0.09) between vagally mediated HRV indices and emotional regulation processes  in mostly healthy participants across a number of age groups  indicating that higher HRV was related to better cognitive and behavioral scores, which was not influenced by mean age or cognitive status , vascular dementia, Parkinson’s disease and multiple sclerosis found a significant, moderate effect  and the amygdala  scores of 21 (range 16–24) had iVNS devices implanted and the median change in MMSE and Alzheimer’s Disease Assessment Scale-cognitive subscale (ADAS-cog) scores among a battery of tests was assessed at 3 and 6 months, with improvements in both assessments noted in the majority (6 out of 10) of cases  as defined by the criteria of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer’s Disease and Related Disorders Association (NINCDS-ADRDA), with a view to assessing its impact on cognition of cases . The folof cases . There aof cases ) howevervia its ability to regulate both cardiac autonomic function and increase cerebral perfusion. Dementia is a multifactorial process and together with reduced cerebral perfusion is associated with neuroinflammation and altered synaptic plasticity, both of which may also be favorably modulated by VNS. It has been noted that perfusion to cortical and subcortical areas increases with VNS, specifically to areas that modulate executive function and attention, i.e., insular, orbitofrontal and prefrontal cortex. These areas are hypothesized by the neurovisceral integration model to be crucial areas in modulating the ANS (via norepinephrine release and improved executive performance, it is notable that the earliest stages of pathological tau accumulation in Alzheimer’s disease are seen in the LC. Whether this small midbrain nucleus will prove to be pivotal in our understanding of how to modulate the vagus nerve and harness its benefits cognitively remains to be elucidated. VNS can now be delivered safely and non-invasively via t-VNS devices with equivalent neuromodulatory effects on brain imaging as invasive devices, which broadens its therapeutic applicability considerably, especially to an older population with cognitive complaints for whom device implantation may not be feasible. Globally, the need for effective therapies to both treat the cause and symptoms of cognitive decline are needed urgently as rates of dementia increase due to population expansion. Dedicated studies into the potential therapeutic effects of t-VNS in early cognitive decline and dementia are needed. Research to date has been limited by myriad issues, including studies on cognition in clinical populations with altered neuroanatomy, lack of standardization in device usage, parameter settings, frequency of use, duration of stimulation. Minimum reporting standards have recently been published to help ameliorate some of these issues. Further rigorous studies of the therapeutic benefit of VNS are required, especially in populations with autonomic instability and cognitive decline.There is mounting evidence of the potential benefits of VNS in myriad disease states, with notable promise in the area of cognition. VNS shows promise as a neuromodulatory technique in cognitive decline and this may be  the ANS . Given tHD did most of the research, writing, and editing of the article. Significant contributions were made by each author, specifically TD with manuscript reading, editing, and direction, SC with direction RE psychological assessments and plasticity, CF with neurocardiovascular assessments, ANS testing. PM and SK assisted significantly with overall editorial support and guidance. All authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.


The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon’s perspective and give an outlook toward future challenges in the field. Peripheral nerve injury (PNI) occurs predominantly in the upper limb, including digital nerves and nerves of the brachial plexus and its terminal branches and results in pain, loss of motor functions and sensation of the limb, significantly impacting the patient’s quality of life . PNI occPNI regeneration is often categorized by the Sunderland classification, where grade I is defined as a defect of the myelin layer without any damage to the axons and grade II describes injury to the axons without disruption of supporting connective tissue sheaths such as the epi-, peri-, and endoneurium. Grades III-V describe additional injury to the endoneurium, perineurium, or complete transection of the nerve and are associated with significantly worse healing outcomes . This obAxonal injury triggers a series of events especially at the distal nerve segment called Wallerian degeneration, which is driven by Schwann cells. Activated Schwann cells increase their mitotic rate with upregulation of several genes to orchestrate the degeneration and repair process. Following migration of macrophages, Schwann cells and macrophages work together to clear all myelin lipid and other axonal debris and prepare the nerve for regeneration .Proliferating and migrating Schwann cell later develop glial bands of Büngner which encase basal lamina. These bands provide neurotrophic and structural support and guide the regrowing axon back to innervate its former target . AlthougIndeed, although peripheral nerves possess the theoretical ability to regenerate at a rate of about 1 mm/day, successful regrowth is dependent on surgical reconstruction of these anatomical guiding sheaths . HoweverA “critical nerve gap” is defined as a nerve gap over which no recovery will occur without nerve grafting or bridging. It is generally accepted that all vertebrate species posess the same velocity of nerve regeneration. However, intrinsic differences of each species result in different nerve gap size being considered critical. In rats, the critical nerve gap is considered ∼1.5 cm, in rabbits ∼3 cm, and in pigs and humans ∼4 cm .While in terms of healing outcome the autograft approach is still the gold standard to bridge a critical gap defect, it creates another nerve defect at the donor site and in most cases consists of a small-diameter cutaneous nerve unsuitable to repair large-diameter nerves. Therefore, there is a need for bioartificial, off-the-shelf nerve guidance conduits (NGCs) that promote nerve regeneration. In this review we provide an overview of classic and emerging conduit design strategies, highlight how principles derived from the regenerative medicine field are being used to augment bioactivity of the implants and discuss their potential value for clinical practice.Years of preclinical research in nerve regeneration have resulted in only few products on the market. Perhaps the most popular and widely accepted one is the decellularized allograft from deceased human donors, known as Avance . Although the use of AxoGen eliminates donor site morbidity, it has the limitation of usage of biological material that can be characterized only to a certain extent . AnotherCurrently, more biocompatible synthetic polymer product made from polyglycolic acid (PGA) or poly-lactidecaprolactone (PLCL) are gaining popularity in nerve gap repair. Neurotube is produced from woven polyglycolic acid (PGA); by Synovis Micro Companies Alliance  while Neurolac is a PLCL conduit . The newin vivo evaluation of NGCs, several general requirements and desirable properties have been identified  range . Ideallyin vivo.- Biodegradability: If the implanted material is not degradable it needs to be removed surgically in time before it starts to compress the newly sprouted axon bundles. Silicone tubes are an early historic example of non-degradable synthetic conduits. Since then, biodegradable polymers have been investigated for PNI repair . Becausein vivo, no matter if natural or synthetic, low toxicity of degradation products should be a top priority , polylactic acid (PLA), polycaprolactone (PCL) and copolymers such as polylactide-co-glycolide (PLGA) or polylactide-co-caprolactone (PLCL) are all hydrolytically degraded within weeks to months and absorbed in a controlled process that is accompanied by only a moderate inflammatory reaction  or proteins . Some of these natural polymers like collagen and laminin are physiologically present within the nerve and are natural candidates for re-engineering the damaged microanatomy . Other nBecause natural polymers need to be harvested form animal sources, their purity and their physicochemical and immunological properties are more difficult to control, making translation potentially challenging. An alternative to harvested materials are recombinant sources, which offer potentially well-defined biopolymers with minimized impurity, but are significantly more costly and require advanced production methods. For example, production of recombinant collagen from bacterial, yeast or mammalian cells allows to recreate a variety of its native structures or even introduce new derivatives. Despite these advancements and great demand for collagen in both the biomedical and the food industry, both industries still rely on animal-derived sources, because there is no consensus on the preferred structure or production method and production of recombinant protein remains more expensive than the well-established harvesting from animal herds, as discussed in detail by As an alternative to the autograft, allografts from cadaveric donors could avoid donor site morbidity but would be rejected by the host immune system due to human leukocyte antigen (HLA) mismatch. To avoid graft rejection, the tissue needs to be decellularized, removing the HLA epitopes presented on the cell surface but processed nerve allografts maintain the extracellular matrix and its structure, which is advantageous as discussed in Decellularized allograft resulted in regeneration superior to bioartificial collagen-based conduits, but still inferior to autograft in rat models . More reThe most abundant extracellular matrix (ECM) protein Collagen type I is widely implemented as a biomaterial for tissue repair. Collagen consists of a triple helix of three polypeptide chains. Fibrils are formed by cross-links between telopeptides that contribute to the relatively high moduli of telo-collagen, as discussed in depth elsewhere .Collagen provides attachment sites for neurons, that have been shown to contribute to neurite outgrowth . AdditioA collagen nerve guidance conduit was developed for the regeneration of a 4 mm nerve gap and demonstrated comparable nerve growth to the autograft method in rats and monkeys . This prAs gelatin is denatured collagen that lacks the triple helix structure, it seems that it could be used for NGC construction as well. However, its weak mechanical properties and its tendency to move quickly away from the implant site make additional modification and cross-linking necessary to increase mechanical stability of the NGC . A clearLaminin, a glycoprotein of the basal lamina secreted by Schwann cells, has been described as a particularly favorable substrate for adhesion, migration, and regeneration of axons . LamininHA is a highly hydrophilic glycosaminoglycan of the ECM, that contributes to the cushioning function of cartilage due to its high-water content. Already applied clinically in several indications reviewed by Chitosan is a linear polysaccharide derived from chitin, the major component of the exoskeleton of crustaceans and insects, through chemical or enzymatic processes. Its reactive groups make chitosan accessible to a myriad of chemical modifications and biofabrication techniques, resulting in fibers, beads, films, gels, scaffolds or nanoparticles . AdditiBombyx mori or the spider Nephila clavipes  respond to the electrical stimulation , acts mostly on sensory and sympathetic neurons by promoting neurite sprouting and elongation. in vitro . Howeverin vitro . Brain Din vitro . Lopes ein vivo and determine the optimal time and mode of delivery.Neurotrophin-3 (NT-3) can enhance Schwann cell migration and ensure their survival. in vitro , a neurotrophic factor produced by Schwann cells (SCs) has been shown to promote the survival of sensory neurons and axon outgrowth from DRG explants in vitro  but when treated with recombinant growth factors cannot always be directly translated into conclusions about the factor’s beneficial role in a therapeutic application. The optimal dose, timing, release kinetic and localization may be entirely different for each therapeutic protein and the current challenge lies in understanding and modulating the application-specific pharmacokinetics and pharmacodynamics. While there is a strong consensus on the pro-regenerative potential of soluble factors such as neurotrophic growth factors or the matrix remodeling enzyme chondroitinase in PNI, these potent factors are not a one one-size-fits-all solution and their biological function is dependent on the timing, microanatomical localization and often gradient.This last example reminds us, that observations from in vitro and promote peripheral nerve regeneration, as well as Schwann cell proliferation and migration  and its receptor “Deleted in colorectal carcinoma” have been shown to attract the tip of a sprouting axon igration . Other cigration . Lykissaigration . Thus, cBecause every growing tissue is dependent on nutrient supply, sufficient vascularization has an important supportive role in the process of nerve regeneration. As chondroitin sulfate proteoglycans (CSPG) and glycosaminoglycans can interfere with axon regeneration even after successful readaptation of transected nerve ends. Chondroitinase ABC, an enzyme derived from the bacterium Proteus vulgaris, catalyzes the degradation of the polysaccharides. Enzymatic removal of the CSPG scar by chondroitinase treatment is an established strategy in the field of spinal cord repair, but also in PNI  and ensure the formation of myelin sheaths around the axons and the accelerated conduction of nerve impulses as reviewed elsewhere . A recengned SCs . An earlneration . The impiPSCs are a personalized self-renewing source of stem cells which enables autologous treatment. iPSCs can be reprogrammed from patient-derived somatic cells and then differentiated into a broad spectrum of cell types, enabling researchers to generate large quantities of cells that are difficult to harvest, such as SCs . On the Because generating patient-derived iPSCs and differentiating them into a desired cell type takes time, their usefulness for PNI management is limited. Strategies for off-the-shelf clinical use and for complete depletion of undifferentiated iPSCs needs to be developed to fully unleash their potential.MSCs are adult multipotent stromal cells isolated from bone marrow, adipose or perinatal tissues. MSCs have attracted attention due to their well-documented clinical safety, rich pro-regenerative secretome, and immunomodulatory properties . Additioin vivo, protective carrier materials can shield MSCs from the harsh microenvironment, hold them in place and stimulate them to augment their growth factor secretion  anatomy and prevent axon dispersion and can be introduced into the scaffold either by precise 3D bioprinting or through more conventional fabrication such as directional freeze drying or molding  Hu et a. Chang eThese studies show that different fabrication techniques can be used to achieve an internal microchannel structure that mimics the natural anatomy of guiding sheaths. Emerging study design combining different modalities of bioactive cues stress the importance of synergistic biological and structural cues for NGC constructs that could promote nerve regeneration across larger gaps.® 3D Nerve Guide Matrix), which is composed of a bioactive collagen-glycosaminoglycan blend and has substantially improved the conduit’s performance (Engineering a clinically usable alternative to the autograft for repair of large peripheral nerve gap injuries still poses an unsolved challenge. The first FDA-approved artificial NGCs were implants fabricated from biodegradable synthetic polymers that could bridge a short defect but failed at promoting axon regrowth across larger gaps. In an effort to increase the conduits’ bioactivity, the design of synthetic NGCs has progressed over the past decades from hollow tubes to the implementation of bioactive substrates and intraluminal structures for refined topographical guidance. In 2014, the FDA approved the first NGC with a porous lumen filler (NeuraGenformance . Since tformance . While sin vivo model for translation to human application. Most preclinical studies show efficacy of their NGC in a rat sciatic nerve gap injury, usually of 10 mm gap size (A major translational challenge lies in choosing the correct preclinical gap size . (Kaplangap size .Additionally, while many studies show efficacy of their design and use meaningful internal controls, they are hardly comparable with other studies in the field as there is little consensus on which readouts, time points and effect sizes are considered most relevant for clinical translation. The evaluation timepoint following nerve repair is crucial for thoroughly understanding the effect of the experimental technique on nerve regeneration. in vitro comparisons and mode-of-action studies with a functional in vivo investigation.As discussed in the “structural design” section of this review, synergistic biological effects of multimodal NGC functionalization are still poorly understood and would require extensive comparisons and scrutinous experimental design to optimize each component . Furtherin vivo live monitoring of peripheral nerve regeneration. This could be achieved through theranostic implants that track the regenerative process in vivo (Finally, in order to identify both biological mechanisms as well as translationally relevant readout, some attention should be brought to  in vivo . For exa in vivo , but oth in vivo .