Neuromyotonia and myokymia are related disorders of peripheral nerve hyperexcitability. Clinically they manifest as muscle stiffness and twitching. Both conditions are typically related to disorders of the voltage-gated potassium channel and caused by autoimmune, toxic, or genetic processes. Whereas neuromyotonia is specific to these disorders, myokymia can be seen in many neurologic conditions. In this update, the author offers a clinical and electromyographic guide to the diagnosis and treatment of these intriguing syndromes.
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• Generalized neuromyotonia is usually an autoimmune disease characterized by widespread muscle stiffness and delayed muscle relaxation after voluntary movement. It is accompanied by continuous muscle twitching known as myokymia.
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• Electromyography of the affected muscles shows either electrical neuromyotonia (high-frequency trains of decrementing motor unit discharges that start and stop abruptly) or electrical myokymia (grouped discharges recurring semi-rhythmically at a rate of 2 to 10 Hz).
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• Neuromyotonia is specific to peripheral nerve hyperexcitability syndromes, whereas myokymia can be seen in a diverse group of neurologic disorders.
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• Neuromyotonia may respond to symptomatic treatment with sodium channel-blocking drugs (carbamazepine, phenytoin, mexiletine). If antibodies are identified, it may respond to immune-based therapies (corticosteroids, plasmapheresis, high-dose intravenous human immunoglobulin, rituximab, and other immunosuppressive medications).
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• These phenomena also occur in episodic ataxia type 1and a form of hereditary neuropathy.
Historical note and terminology
The terms “neuromyotonia” and “myokymia” have both been used to describe clinical phenomena as well as distinct patterns of abnormal electrical discharge recorded during needle electromyography. This dual nomenclature has created confusion over the years, but no other set of clearer definitions has yet been universally accepted. In this review, we will address and distinguish the clinical syndromes of neuromyotonia and myokymia, the electromyographic discharges defined by these terms, and their relationships.
Clinical neuromyotonia is a syndrome of persistent muscle stiffness, delayed muscle relaxation, and continuous muscle twitching due to abnormal electrical discharges of motor nerves. Originally described by Gamstorp and Wohlfart in 1959, it has also been called "Isaacs syndrome" (28) and "myokymia with impaired muscle relaxation" (15), but it is now mostly called "neuromyotonia" (39). Needle EMG recordings from affected muscle show abnormal electrical activity of either the type known as electrical neuromyotonia or the type known as electrical myokymia, or both. These EMG findings are discussed in this article.
Clinical myokymia refers to the presence of focal or generalized continuous muscle twitching, often exhibiting a rippling, “bag of worms” appearance under the skin. Needle EMG recordings from the twitching muscle can show either very frequent fasciculations, electrical neuromyotonia, or electrical myokymia.