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Isaacs’ syndrome is an extremely rare neuromuscular disorder that causes significant muscle stiffness, slower reflexes, and constantly contracting or twitching muscles. Other names for this disorder are neuromyotonia, Isaac’s-Metern’s syndrome, and continuous muscle fiber activity syndrome. Though the exact prevalence of Isaacs’ syndrome is unknown, there are between 100 and 200 reported cases in the current medical record. Because it is so rare, research is still ongoing to discover more information about this condition.

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1. Symptoms

Isaacs’ syndrome is a diverse disorder. Its characteristic symptom is myokymia, continuous muscle twitching that many describe as “bag-of-worms” movements. This descriptor comes from the undulating or rippling motions visible under the skin. Even during sleep or under general anesthesia, the muscles continue twitching. Myotonia-like symptoms also occur, which means that voluntary muscles take additional time to relax after contracting. For example, a person with Isaacs’ syndrome may not be able to open their hands or eyes after closing them for a short period.

muscle twitch leg PeopleImages / Getty Images
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