Abnormal Blink Reflex and Intermuscular Coherence in Writer's Cramp.

Supriyo Choudhury,Shantanu Shubham,Stuart N Baker,Payel Chatterjee,Mark R Baker,Ravi Singh,Hrishikesh Kumar,Santosh Trivedi

doi:10.3389/fneur.2018.00517

Abstract

Background: Writer's cramp (WC) is a task-specific focal hand dystonia presenting with pain, stiffness and/or tremor while writing. We explored the involvement of cortical and brainstem circuits by measuring intermuscular coherence (IMC) and pre-pulse inhibition (PPI) of the blink reflex.Methods: IMC was measured in 10 healthy controls and 20 WC patients (10 with associated tremor) while they performed a precision grip task at different force levels. Blink responses were evaluated in 9 healthy controls and 10 WC patients by stimulating the right supraorbital nerve and recording surface EMG from the orbicularis oculi muscles bilaterally. PPI involved conditioning this stimulation with a prior shock to the right median nerve (100 ms interval), and measuring the reduction in the R2 component of the blink reflex.Results: Significant IMC at 3–7 Hz was present in WC patients, but not in healthy controls. Compared to healthy controls, in WC patients the R2 component of the blink reflex showed significantly less PPI. IMC at 3–7 Hz could reliably discriminate WC patients from healthy controls.Conclusion: Cortical or sub-cortical circuits generating theta (3–7 Hz) oscillations might play an important role in the pathogenesis of WC. Moreover, the lack of PPI implicates abnormalities in brainstem inhibition in the emergence of WC. IMC may merit further development as an electrodiagnostic test for focal dystonia.

Highlights

Writing is a highly developed motor task requiring fractionated contraction of hand and forearm muscles
writer’s cramp (WC) is more formally classified as a task specific focal hand dystonia [2, 3], of which there are other examples reported in typists, musicians and golfers [4]
Whilst inhibition of the R2 response may be reduced in some focal dystonias [e.g., blepharospasm and cervical dystonia [11,12,13]], the only previous study in WC concluded that pre-pulse inhibition (PPI) was normal [11]

Summary

Introduction

Writing is a highly developed motor task requiring fractionated contraction of hand and forearm muscles In some, this fine balance of activity is lost, writing becomes increasingly difficult and a disorder popularly known as writer’s cramp (WC) emerges. This fine balance of activity is lost, writing becomes increasingly difficult and a disorder popularly known as writer’s cramp (WC) emerges These patients usually present with difficulty in writing, abnormal posturing, painful spasms and muscle cramps or tremor affecting the forearm or hand whilst writing but not during other activities involving the same muscle groups. These signs are associated with co-contraction of antagonistic muscles [1]. We explored the involvement of cortical and brainstem circuits by measuring intermuscular coherence (IMC) and pre-pulse inhibition (PPI) of the blink reflex

Methods

Results

Discussion

Conclusion