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Isaac syndrome causes neuromuscular manifestations, including myokymia.
Isaacs syndrome (neuromyotonia) is generally thought to be a channelopathy and sometimes occurs as a paraneoplastic syndrome. It may also occur in other disorders (eg, myasthenia gravis, thymoma, cancer, amyloidosis) or can be inherited. Cause is unknown. Abnormalities are thought to originate in a peripheral nerve because they are abolished by curare but usually persist after general anesthesia.
The limbs are most affected. The sine qua non is myokymia—continuous muscle twitching described as bag-of-worms movements. Other symptoms include carpopedal spasms, intermittent cramps, increased sweating, and pseudomyotonia (impaired relaxation after a strong muscle contraction but without the typical waxing-and-waning EMG abnormality of true myotonia).
Carbamazepine or phenytoin may relieve these symptoms. Plasma exchange and IVIG are usually beneficial.
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