History
Patients with myokymia may present with symptoms of pain, cramps, spasms, weakness, stiffness, or twitching.
Sensory symptoms are reported rarely, unless the underlying etiology(ies) includes sensory nerve involvement.
Typical myokymic discharges also can be seen in the EMGs of patients referred for totally unrelated complaints.
Physical
Findings of facial myokymia, segmental or focal myokymia in other areas of the body, and generalized myokymia are somewhat different in physical examination and in their potential etiologies; therefore, they are discussed separately.
Facial myokymia
Affected muscles show slow, undulating, fine movements on the surface of the skin due to activation of the most superficial muscle layers. Facial weakness can be present in the involved muscles.
EMG study shows typical myokymic discharges of spontaneous, rhythmic/semirhythmic bursts of normal-appearing potentials of 30-60 Hz. The bursts of each group of potentials are followed by a period of silence, with subsequent repetition of grouped discharges of identical potentials. The spontaneous activities are not altered by voluntary activation of the muscles.
Focal or segmental myokymia
These types of myokymia commonly are seen in the limbs or particular segmental level(s). Physical findings mostly are related to the underlying etiology(ies), which is usually asymptomatic and not a major concern of the patient.
EMG study shows myokymic discharges similar to those recorded in facial myokymia.
Generalized myokymia
A triad of myokymia, muscular stiffness, and decreased deep tendon reflexes was first described by Isaacs in 1961; it also is called Isaacs syndrome. [4, 5, 6, 7]
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Muscle weakness and atrophy and excessive sweating are frequently associated features. Smooth muscles and cardiac muscles typically are spared.
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Sensory symptoms are rarely present.
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The muscular stiffness is different than that seen in myotonia, both clinically and electrodiagnostically. Although both can be exacerbated by cold, myokymia can be detected when the muscle is at rest and during sleep. Patients with myotonia are normal at rest; the stiffness is induced by mechanical stimulation.
The myotonic discharge recorded by EMG ceases upon relaxation of the muscle, while spontaneous grouped discharges of myokymia persist for some time, well above the abnormal pattern that was present before the voluntary contraction.
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The EMG features encountered in patients with generalized myokymia include the previously described typical myokymic discharge or neuromyotonia, which has a much higher frequency of discharges (up to 300 Hz).
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Generalized myokymia in other clinical entities shares less consistent clinical manifestations.
Causes
Facial myokymia
This type of myokymia is seen more commonly than other types.
Facial myokymia has been reported to be associated with inflammatory demyelinating diseases, 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 [8] (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. 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.
