Objective: • To compare Stop Signal Reaction Time (SSRT) among patients with Hyperkinetic Movement Disorders and Healthy participants. • To explore the effect of Botulinum toxin treatment on SSRT in Cervical Dystonia patients.

Background: The neurobiology of movement is believed to be a balance between, the process of initiation (go signal) and efficiency of stopping (stop signal) in response to external stimuli. [1] Technically, quantification of the latency of ‘go signal’ is possible through electromyography but the estimation of stopping process is challenging. ‘Stop signal reaction time (SSRT)’ is a validated statistical measure, which quantifies hypothetical idea of efficiency of stopping. The stopping ability depends on complex interaction of cortico-subcortical structures. [2][3]

Methods: The patients and healthy participants were seated on a fixed chair in a quiet room. The SSRT task is operated by simple digital battery operated box with a push button at the centre. They were asked to release a button as fast as possible, in response to a green LED cue. They were also instructed to inhibit the ‘release response’ in case the green LED was followed by a red LED cue. The red LED used to illuminate in random interval followed by the green LED cue. 180 responses were recorded in all participants.

Results: We compared Median Response Time and Optimal Combination SSRT among cohorts of 20 healthy participants, 20 cervical dystonia (CD) and 10 Writer’s cramp (WC) patients. Median Response Time and Optimal Combination SSRT were also compared in Pre and post Botulinum toxin injection of 20 CD patients. We observed significantly prolonged Median Response Time and Optimal Combination SSRT (p< 0.0001) in CD (563ms,373ms respectively) and WC patients (481ms,306 ms respectively) compared to healthy participants (350ms,238 ms respectively) (p< 0.0001). One month after Botulinum toxin injection CD patients demonstrated significantly reduced Optimal Combination SSRT (p<0.006) But Median Response Time remained unaltered after one month of Botulinum toxin injection (p=0.254).

Conclusions: Response inhibition of already planned movement in response to external cueing is deficient in focal dystonia. Partial correction of SSRT without concomitant correction of the Median Response Time suggests an independent central action of Botulinum Toxin over and above its classical peripheral action.

References: 1. Jahanshahi M, Obeso I, Rothwell JC, Obeso JA. A fronto-striato-subthalamic-pallidal network for goal-directed and habitual inhibition. Nature Reviews Neuroscience. 2015 Dec 1;16(12):719-32. 2. Tolleson C, Turchan M, van Wouwe N, Isaacs D, Phibbs F, Wylie S. Parkinson’s Disease Subtypes Show Distinct Tradeoffs Between Response Initiation and Inhibition Latencies. Journal of the International Neuropsychological Society. 2017 Jun:1-0. 3. Rae CL, Nombela C, Rodríguez PV, Ye Z, Hughes LE, Jones PS, Ham T, Rittman T, Coyle-Gilchrist I,Regenthal R,Sahakian BJ. Atomoxetine restores the response inhibition network in Parkinson’s disease. Brain. 2016 Jun 24;139(8):2235-48.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/stop-signal-reaction-time-ssrt-in-dystonia-and-its-correction-after-botulinum-toxin-treatment/