Afferent inhibition and cortical silent periods in shoulder primary motor cortex and effect of a suprascapular nerve block in people experiencing chronic shoulder pain.
OBJECTIVE
To characterise short afferent inhibition (SAI) and the cortical silent period (CSP) in the primary motor cortex representations of the infraspinatus muscle in healthy adults and people experiencing chronic shoulder pain, to determine the impact of a suprascapular nerve block (SSNB).


METHODS
Neurophysiological measures were obtained in 18 controls and 8 patients with chronic shoulder pain, pre and post SSNB and 1 week later. Pain intensity was assessed by a visual analogue scale.


RESULTS
SAI was apparent in controls (all P<0.03) and a CSP was observed which reduced in the presence of SAI (all P<0.0001). Compared to controls, shoulder pain patients demonstrated higher active motor threshold (P=0.046), less SAI (P=0.044), a longer CSP (P=0.048) and less modulation of the CSP by SAI (P=0.045). Higher motor thresholds were related to higher pain scores (P=0.009). The SSNB immediately restored SAI (P=0.013), with a positive relationship between increased SAI and reduced pain (P=0.031). The SSNB further reduced modulation of CSP by SAI at 1 week post injection (P=0.006).


CONCLUSIONS
SAI and the CSP were present and demonstrated robust interaction in controls, which was aberrant in patients. The SSNB transiently restored SAI but had no effect on the CSP; however CSP modulation by SAI was further attenuated 1 week post injection.


SIGNIFICANCE
The current findings improve understanding of the neurophysiology of the shoulder motor cortex and its modulation by chronic pain. The effect of SSNB in shoulder pain patients should be interpreted with caution until proven in a larger population. Interventions that target intracortical inhibition might increase efficacy in people with chronic shoulder pain.
['Bradnam|Lynley|L|', 'Shanahan|E Michael|EM|', 'Hendy|Kirsty|K|', 'Reed|Amalia|A|', 'Skipworth|Tegan|T|', 'Visser|Anri|A|', 'Lennon|Sheila|S|']
D000328:Adult D000344:Afferent Pathways D000368:Aged D059350:Chronic Pain D015331:Cohort Studies D004576:Electromyography D005260:Female D006801:Humans D008297:Male D008875:Middle Aged D009044:Motor Cortex D009407:Nerve Block D009433:Neural Inhibition D010147:Pain Measurement D012540:Scapula D020069:Shoulder Pain D050781:Transcranial Magnetic Stimulation D016896:Treatment Outcome D055815:Young Adult
Cortical silent period Suprascapular nerve block Chronic shoulder pain Short afferent inhibition Infraspinatus Transcranial magnetic stimulation