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Focal myoclonus 

Focal myoclonus
Chapter:
Focal myoclonus
Author(s):

Ivan Donaldson

, C. David Marsden

, Susanne A. Schneider

, and Kailash P. Bhatia

DOI:
10.1093/med/9780192619112.003.0770
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date: 22 October 2020

Myoclonic jerking restricted to one part of the body may be due to a lesion anywhere from the peripheral nerve to the cerebral cortex. Clinical characteristics, in particular the range of muscles involved, may give some clue as to the site of origin, but extensive electrophysiological investigation usually is required. Polymyography will identify whether the affected muscles are in the territory supplied by a single nerve, a segment of a plexus, or nerve roots. Alternatively, the distribution of the myoclonus may not conform to any such peripheral origin. Myoclonus innervated by one or more spinal segments may suggest a spinal origin. Myoclonus affecting the palate and other central bulbar structures suggests the palatal myoclonus/tremor syndrome. Myoclonus affecting one part of the limb, particularly those parts with large cortical innervation such as the hand, suggests a cortical origin. Stimulus sensitive myoclonus, especially if it affects the distal limbs, is thus likely to arise in the cerebral cortex. Somatosensory evoked potentials may be abnormal due to peripheral nerve or spinal cord lesions, and characteristically are enlarged in cases of cortical myoclonus. Back averaging of ongoing electroencephalographic (EEG) activity in relation to the myoclonic jerks may reveal a cortical correlate. The results of physiological investigation will direct appropriate attention for imaging studies.

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