Objective: In this study, we explored the involvement of brainstem in Writer’s cramp (WC) patients through pre-pulse inhibition of blink reflex and Start-React paradigm. 

Background: WC might result due to reduced inhibition of motor output and sensory motor disintegration at various anatomical levels. Pre-pulse inhibition of blink reflex is a physiological phenomenon where the blink reflex (R2 component) is inhibited by preceding sensory stimulation. Start-React is an experimental technique which evaluates brainstem function through quantification of reaction latency to startling acoustic stimulation. 

Methods: Eight patients of WC and eight healthy participants were evaluated for pre-pulse inhibition of blink reflex. Extent of blinks was measured through surface EMG placed on Orbicularis oculi muscles of both eyes. Half of the Supraorbital nerve stimulation was randomly conditioned with preceding low intensity median nerve stimulation. The extent of inhibition was estimated through an Index (R2 area at conditioned stimuli/ R2 area at test stimulus). 17 patients with WC were compared with 12 healthy participants through Start-React paradigm. The participants were asked to perform a task of pinching on a force transducer immediately on illumination of a LED light. On some trials, the visual stimulus was co-presented with either a quiet acoustic stimulus or a startling acoustic stimulus. The reaction latency was quantified through EMG placed on first dorsal interosseous muscle.   

Results: Median nerve stimulus inhibited the blink reflex in healthy as well as WC patients. However, the extent of inhibition was significantly less in WC patients (average pre-pulse index in WC 0.44, healthy participants -0.10, p 0.01). The onset of ipsilateral R2 component of blink reflex was 46.3 s for WC patients and 38.2 s for healthy participants (p 0.02). The attenuation of reaction latency to acoustic stimulus was consistently present in both the groups. Nevertheless, in WC patients the attenuation of latency was higher than healthy participants (41.6 s, 28.6, p 0.03).

Conclusions: The sensory inhibition of motor volley was inadequate at brain stem level in WC patients, as reflected through deranged pre-pulse inhibition of blink reflex. Whereas, increased startling reaction to acoustic stimulus was possibly related to the reduced inhibitory influence at the level of brainstem in WC patients.    

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MDS Abstracts - https://www.mdsabstracts.org/abstract/brainstem-connection-of-writers-cramp-electrophysiological-correlates/