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. [1] Kny used the term myoclonus fibrillaris multiplex to describe similar clinical manifestations. [2] 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.
Etiology
Facial myokymia
Eyelid myokymia is the most common type of facial myokymia. [3]
Facial myokymia has been reported to be associated with inflammatory demyelinating diseases, [4] brainstem neoplasms, Guillain-Barré syndrome, or other intramedullary pontine lesions. Facial myokymia also has been reported in patients with history of radiotherapy, with findings similar to those of more common brachial or lumbar radiation plexopathies.
Focal or segmental myokymia
The majority of patients with a history of radiation therapy have myokymic discharges detected within the field of radiation. Metastatic lesions generally are believed to be less likely to generate myokymia. The amount of radiation ranges widely, though myokymia rarely is reported with radiation doses less than 10 gray (Gy).
Electrodiagnostic findings are usually consistent with plexopathy. Other less common causes include acute or chronic inflammatory polyradiculoneuropathy [5] (with or without coexistent systemic vasculitis), ischemic or traumatic focal neuropathy, entrapment neuropathy, polyradiculopathy secondary to torticollis, syringomyelia, and chronic idiopathic plexopathy.
Transient myokymia, described in the calf or hand muscles, was reported after brief strenuous exercise, hypokalemia, hypomagnesemia, and increased caffeine intake. It usually resolves spontaneously over weeks to months.
Isaacs syndrome
Generalized myokymia is one of the cardinal features of Isaacs syndrome, which is a rare clinical entity with no known common etiologies. [6] Congenital and acquired forms are described.
The acquired form has been associated with neoplasms, thymoma, myasthenia gravis, lymphomas, and a variety of autoimmune nervous system disorders.
Spinal anesthesia and peripheral nerve block fail to abolish the myokymic discharges. Blocking the motor end plate transiently terminates the spontaneous activities. Evidence from muscle and nerve biopsies also favors a neurogenic origin.
Generalized myokymia also can be seen in patients with systemic illnesses (eg, thyrotoxicosis, uremia) and following binge consumption of alcohol, exposure to toxins, timber rattlesnake bite, gold therapy, and penicillamine therapy.
One case of myokymia as an initial and predominant manifestation of dermatomyositis has been reported.
Pathophysiology
The clinical phenomenon of myokymia 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 [7] suggests a possible abnormality of the potassium channel in this particular entity.
Epidemiology
Prevalence
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.
Prognosis
Prognosis is related directly to the underlying etiology. Myokymia is reversible with successful treatment of the cause. [8]
