title: StartReact effects support different pathophysiological mechanisms underlying freezing of gait and postural instability in Parkinson's disease.
abstract: BACKGROUND
  The pathophysiology underlying postural instability in Parkinson's disease 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 patients, 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 patients with postural instability compared to patients without postural instability, but did not differ between freezers and non-freezers. In contrast, the StartReact effect was intact in patients with postural instability but was attenuated in freezers.
  
  
  CONCLUSIONS
  We suggest that the mechanisms underlying freezing of gait and postural instability in Parkinson's disease 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.
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Rehabilitation, Nijmegen, The Netherlands.
authors: Jorik
authors: J
authors: Nonnekes
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Rehabilitation, Nijmegen, The Netherlands.
authors: Digna
authors: D
authors: de Kam
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Neurology, Nijmegen, The Netherlands.
authors: Lars B
authors: LB
authors: Oude Nijhuis
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Rehabilitation, Nijmegen, The Netherlands.
authors: Karin
authors: K
authors: van Geel
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Neurology, Nijmegen, The Netherlands.
authors: Bastiaan R
authors: BR
authors: Bloem
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Rehabilitation, Nijmegen, The Netherlands; Sint Maartenskliniek Research, Development & Education, Nijmegen, The Netherlands.
authors: Alexander
authors: A
authors: Geurts
authors: Radboud University Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Rehabilitation, Nijmegen, The Netherlands; Sint Maartenskliniek Research, Development & Education, Nijmegen, The Netherlands.
authors: Vivian
authors: V
authors: Weerdesteyn
mesh_terms: D000161:Acoustic Stimulation; D000704:Analysis of Variance; D001933:Brain Stem / Q000502:physiology*; D004576:Electromyography; D020233:Gait Disorders, Neurologic / Q000209:etiology / Q000503:physiopathology*; D006801:Humans; D010300:Parkinson Disease / Q000150:complications / Q000503:physiopathology*; D004856:Postural Balance / Q000502:physiology*; D011930:Reaction Time; D013216:Reflex, Startle / Q000502:physiology*