StartReact Effects Support Different Pathophysiological Mechanisms Underlying Freezing of Gait and Postural Instability in XXXX .

@entity1 @entity191
INTRODUCTION: The pathophysiology underlying postural instability in @entity191 is poorly understood. The frequent co-existence with freezing of gait raises the possibility of shared pathophysiology. There is evidence that dysfunction of brainstem structures contribute to freezing of gait. Here, we evaluated whether dysfunction of these structures contributes to postural instability as well. Brainstem function was assessed by studying the StartReact effect (acceleration of latencies by a startling acoustic stimulus (SAS)). METHODS: We included 25 @entity1 , divided in two different ways: 1) those with postural instability (HY = 3, n = 11) versus those without (HY<3, n = 14); and 2) those with freezing (n = 11) versus those without freezing (n = 14). We also tested 15 matched healthy controls. We tested postural responses by translating a balance platform in the forward direction, resulting in backward balance perturbations. In 25% of trials, the start of the balance perturbation was accompanied by a SAS. RESULTS: The amplitude of automatic postural responses and length of the first balance correcting step were smaller in @entity1 with postural instability compared to @entity1 without postural instability, but did not differ between freezers and non-freezers. In contrast, the StartReact effect was intact in @entity1 with postural instability but was attenuated in freezers. DISCUSSION: We suggest that the mechanisms underlying freezing of gait and postural instability in @entity191 are at least partly different. Underscaling of automatic postural responses and balance-correcting steps both contribute to postural instability. The attenuated StartReact effect was seen only in freezers and likely reflects inadequate representation of motor programs at upper brainstem level.
