Objective: The evaluation of the neurophysiological criteria for the diagnosis of cortical myoclonus in patients with a clinical evidence of cortical myoclonus. 

Background: Myoclonus is defined as a brief and jerky shock-like involuntary movement which can be present in a variety of neurological disorders and is classified according to different clinical characteristics such as distribution and eliciting factors (1). Based on presumed sources myoclonus can be divided in cortical, subcortical, spinal or peripheral. Cortical myoclonus is the most common and typically affects distal limbs, it is evident during action and elicited by somatosensory stimuli, and can have a positive and negative component. Neurophysiological criteria for cortical myoclonus include the presence of EEG discharges time-locked to individual myoclonic jerks, giant somatosensory evoked potentials (SEP) and increased amplitude of long-latency reflexes (C-reflex) (2). However, all these neurophysiological abnormalities may not always be present.

Methods: We reviewed 44 patients who underwent neurophysiology testing between January 2012 to September 2016 at the National Hospital of Neurology and Neurosurgery to investigate a clinical suspicion of cortical myoclonus. 

Results: All the 44 patients reviewed presented with the clinical features suggestive of cortical myoclonus. Forty-three patients were recorded with EMG-EEG polymyography, and 81% of them showed a cranial-caudal progression of the jerks with a mean duration of myoclonic EMG bursts of <50 ms. EEG-EMG jerk-locked back averaging showed a positive correlation in only 22% of the patients, but in many cases the use of this technique was limited by technical difficulties (58%). Giant SEPs were detected in nearly 43% and the C-reflex was seen in the 55% of the patients studied. 

Conclusions: According to our results, the neurophysiological findings more often in agreement with clinical characteristics were burst duration and cranial-caudal progression of myoclonic jerks, while others, such as EEG-EMG jerk-locked back averaging and giant SEPs were not always present. These data suggest that clinical-neurophysiological agreement is generally quite low and that further refinement of neurophysiological investigations is needed.

References:

1. Marsden CD, Hallett M, Fahn S. The nosology and pathophysiology of myoclonus. In: Marsden CD, Fahn S, eds. Movement disorders. London: Butterworths, 1982: 196–248.
2. Shibasaki H, Hallett M. AAEE monograph #30: electrophysiological studies of myoclonus. Muscle Nerve 2005;31:157–174.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/reappraisal-of-cortical-myoclonus-a-retrospective-study-of-clinical-neurophysiology/