Neuromyotonia or Isaac’s syndrome is a rare condition of peripheral nerve hyperexcitability characterized by continuous muscle fibre activity. It can be inherited or acquired, in which case it may be paraneoplastic or autoimmune in origin, due to antibodies against voltage-gated potassium channels. Patients complain of twitching muscles, cramps and stiffness. Myokymia may be observed. This is a continuous rippling of muscles that can resemble a bag of worms under the skin. Patients may also have increased sweating. In some, the antibodies also affect the brain, causing an autoimmune encephalitis (Morvan’s syndrome). Electromyography can be useful in diagnosis and antibodies are detected in about 40% of those with the acquired syndrome.

Patients may respond to anticonvulsants such as phenytoin, carbamazepine, valproate, lamotrigine, or the carbonic anhydrase inhibitor, acetazolamide. Immunosuppression or plasmapheresis may be useful in those with an autoimmune cause. Andrew Kirk, MD

Reference

• Maddison P. Clin Neurophysiol 2006;Oct;117(10):2118-27.

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