Excerpt from Myokymia

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Background

Myokymia, a form of involuntary muscular movement, usually can be visualized on the skin as vermicular or continuous rippling movements.

The word myokymia was used first more than 100 years ago, when Schultze described continuous, slow, undulating muscular contractions in small muscles of hands and feet. Kny used the term myoclonus fibrillaris multiplex to describe similar clinical manifestations. For the past century, different authors applied the term myokymia to different involuntary muscular movements. Most of them showed electromyographic (EMG) evidence of spontaneous group discharges. This led to tremendous confusion in conceptually defining this particular clinical entity and its electrophysiologic features.

Pathophysiology

The clinical phenomenon is characterized by its classic quivering movement of the involved muscle without movement of the joint. Myokymia can be seen in muscles innervated by cranial or spinal nerves. The distribution can be either regional or generalized, depending on the etiology. Also, it can be seen transiently in healthy subjects after strenuous exercise.

The exact mechanism(s) of myokymia is not well understood. Myokymia of the facial muscles is believed to originate from the facial nucleus or from some contribution by a supranucleus process; however, the presence of myokymia in polyradiculopathy indicates the possibility of a more distal generator. Most authors agree that myokymia in other parts of the body is generated by distal motor axons, either by a primarily axonal process or by segmental demyelination with secondary axonal dysfunction. Some have postulated that transaxonal ephaptic excitation occurs peripherally after focal nerve damage leads to formation of an artificial synapse.

Myokymia is believed to be associated with generation of spontaneous activity, including myokymialike discharge in the dystrophic mouse whose nerve root axons have no Schwann-cell enwrapment. By this mechanism, spontaneous discharge could initiate volleys of activity or afferent fibers could directly stimulate efferent fibers in the vicinity of the lesion and produce a self-perpetuating reverberating circuit.

The central nervous system's electrotonic spread of discharge from rhythmic generators toward anterior horn cells also might play a role in generation of the spontaneous discharge. Each patient may have a different operating mechanism, depending on the particular areas involved and the different etiologies. The fact that patients with Isaacs syndrome respond dramatically to treatment of myokymia with phenytoin and/or carbamazepine suggests a possible abnormality of the potassium channel in this particular entity.

Frequency

United States

Although myokymia can be seen in patients with different neurological and medical conditions and occasionally even in healthy subjects, it is a relatively rare clinical manifestation.

Mortality/Morbidity

• Most of the diseases associated with myokymia are not life threatening.
• The prognosis is solely dependent upon the underlying etiologies.
• Myokymia is considered benign when detected in patients after strenuous exercise.

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