Neuromyotonia and myokymia

Robert Layzer MD (Dr. Layzer of the University of California, San Francisco, has no relevant financial relationships to disclose.)
AHM M Huq MD PhD, editor. (Dr. Huq of Wayne State University has no relevant financial relationships to disclose.)
Originally released January 7, 2003; last updated January 25, 2015; expires January 25, 2018

Overview

Neuromyotonia and myokymia are related disorders of peripheral nerve hyperexcitability manifest respectively 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. In this update, the author offers a clinical and electromyographic guide to the diagnosis and treatment of these intriguing syndromes.

Key points

 

• 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.

 

• 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).

 

• These phenomena also occur in episodic ataxia type 1, an autosomal dominant hereditary disease.

 

• Autoimmune neuromyotonia responds to treatment with sodium channel-blocking drugs (carbamazepine, phenytoin, mexiletine), corticosteroids, plasmapheresis, and high-dose intravenous human immunoglobulin.

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" (Isaacs 1961) and "myokymia with impaired muscle relaxation" (Gardner-Medwin and Walton 1969), but it is now mostly called "neuromyotonia" (Mertens and Zschocke 1965). 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.

Video: Continuous muscle fiber activity (Isaacs syndrome)

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