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Isaacs syndrome

Issacs syndrome (also known as Isaacs' syndrome, Isaacs-Mertens syndrome, neuromyotonia, continuous muscle fiber activity syndrome, and quantal squander syndrome) is a rare neuromuscular disorder in which the peripheral nerve cells become overly excited (hyperexcitability) and continuously fire causing muscle fibers to activate.

Symptoms include:

  • Progressive muscle stiffness
  • Continuously contracting or twitching muscles (myokymia)
  • Cramping
  • Increased sweating
  • Delayed muscle relaxation

Symptoms occur even during sleep or when individuals are under general anesthesia. Many people also develop weakened reflexes and muscle pain, but numbness is relatively uncommon. In most people with Issacs syndrome, stiffness is most prominent in limb and chest muscles, although symptoms can be limited to cranial muscles.

Speech and breathing may be affected if pharyngeal or laryngeal muscles are involved. Onset is between ages 15 and 60, with most individuals experiencing symptoms before age 40.

There are hereditary and acquired (occurring from unknown causes) forms of the disorder. The acquired form occasionally develops in association with peripheral neuropathies or after radiation treatment, but more often is caused by an autoimmune condition.

There is no cure for Isaacs syndrome. Anticonvulsants usually provide significant relief from the stiffness, muscle spasms, and pain. Plasma exchange may provide short-term relief for individuals with some forms of the acquired disorder.

How can I or my loved one help improve care for people with Isaacs syndrome?

Consider participating in a clinical trial so clinicians and scientists can learn more about Isaacs syndrome and related disorders. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.

For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with Isaacs syndrome at

Where can I find more information about Isaacs syndrome?

The following organizations and resources can help people with Isaacs syndrome and their families, friends, and caregivers:

Genetic and Rare Diseases (GARD) Information Center


National Organization for Rare Disorders (NORD)
Phone: 203-744-0100 or 800-999-6673; 844-259-7178 Spanish

Content source: Accessed June 23, 2023.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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