Abnormal Blink Reflex and Intermuscular Coherence in Writer's Cramp

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.

Force variability is mostly not motor noise: Theoretical implications for motor control

“…Finally, it is important to mention that, although synchronized activity above 5 Hz has limited effects on the overall amplitude of force variability in healthy neuromuscular systems, many neurological disorders have been associated with amplified synchronization/force variability above 5 Hz (e.g. [ 226 , 228 – 230 ].…”

Minimum Electromyographic Burst Duration in Healthy Controls: Implications for Electrodiagnosis in Movement Disorders

“…EMG burst duration is a simple, widely used guide to diagnosis (and localization) of myoclonus 4 and has the advantage of requiring little expertise to measure. However, our data suggest that EMG burst duration <50 milliseconds is not a reliable criterion for deciding whether a movement is pathological, particularly when applied to certain muscle groups (as is evident from a previous study 22 ).…”

“…Finally, as noted by others, 1,4 in the context of FMD it is important that EMG burst duration is not interpreted in isolation and where possible the presence or absence of co‐contraction of agonist–antagonist muscles should also be assessed.…”

“…Although there is increasing interest in the application of complex signal processing techniques to such data (eg, refs. 3,4), analysis limited to the simple parameter of electromyogram (EMG) burst duration is widely used to guide diagnosis (and localization) of myoclonus 5 and has the additional advantage of being simple to measure. Typically, EMG is recorded simultaneously with video‐electroencephalography and only a limited number of channels are available for EMG (sometimes a single channel).…”

Spasms after spinal cord injury show low-frequency intermuscular coherence

“…Intermuscular coherence at low frequencies in these patients correlates with the presence of dystonia ( Foncke et al 2007 ). Low frequency intermuscular coherence is also of greater amplitude in subjects with writer’s cramp (a focal task-specific dystonia) than in healthy controls ( Choudhury et al 2018 ). Dystonia is believed to occur because of basal ganglia dysfunction, which results in decreased cortical inhibition ( Morgante and Klein 2013 ).…”