Background: Acoustic startle reflex (ASR) can be used as a tool to examine reticulospinal excitability. The potential role of reticulospinal mechanisms in the development of spasticity has been suggested but not tested.
Objective: To examine reticulospinal excitability at different stages of motor recovery in patients with chronic stroke using the ASR.
Methods: Sixteen subjects with hemiplegic stroke participated in the study. We examined ASR responses at rest and contralateral motor overflow during voluntary elbow flexion.
Results: ASR responses in impaired biceps muscles showed different patterns at different stages. In subjects without spasticity, ASR responses were less frequent (10% on impaired side) and had normal duration (<200 ms). In subjects with spasticity, the responses were more frequent (58.3% on impaired side) and longer lasting (up to 1 minute). However, no correlation between exaggerated reflex responses and Modified Ashworth Scale (MAS) scores was observed. During voluntary elbow flexion on the impaired side, similar positive linear force-electromyogram (EMG) relationships were found in subjects with and without spasticity. Electromyographic activity of the resting nonimpaired limb increased proportionally in subjects with spasticity (r = 0.6313, P = .0004), but no such correlation was found in subjects without spasticity (r = 0.0191, P = .9612).
Conclusions: Preliminary findings of exaggerated ASR responses and associated contralateral overflow only in spastic biceps muscles in patients with chronic stroke suggest the important role of reticulospinal mechanisms in the development of spasticity.
Keywords: acoustic startle reflex; motor recovery; spasticity; stroke.