neuromyotonia (Isaacs syndrome) is a disorder of peripheral nerve hyperexcitability characterized by myokymia, muscle cramps and stiffness, delayed muscle relaxation after contraction (pseudomyotonia), and hyperhidrosis, associated with well described spontaneous electromyographic features
usually neuromyotonia is an acquired disorder associated with autoantibodies against neuronal voltage-gated potassium channels
familial neuromyotonia occurs very rarely
mutations of KCNA1, encoding the K+ channel subunit hKv1.1, have been reported in rare families with neuromyotonia
clinical features are diverse and include cramps, generalized muscle twitching, stiffness, and pseudomyotonia
some patients experience paresthesias or hyperhidrosis that may be secondary to muscle overactivity
underlying electrophysiologic basis of Isaacs’ syndrome is the spontaneous repetitive burst of single motor-unit potentials, with high intraburst frequency
most common electromyographic abnormalities are myokymic and neuromyotonic discharges
both terms are used to describe this spontaneous electrical activity, and both forms of discharge are sometimes seen in the same patient
main difference between them arises from the higher frequencies (150-300 Hz) and waning features of neuromyotonic discharges
distinction between neuromyotonia and other entities, such as cramp-fasciculation syndrome and stiff-man syndrome, lies in the electromyographic and clinical findings
rippling-muscle disease is a rare autosomal-dominant disorder (characterized by mechanically induced, involuntary muscle contractions) which should always be included in the differential diagnosis of Isaacs’ syndrome
in addition to conventional pharmacotherapy with anticonvulsants such as phenytoin or carbamazepine, immunomodulatory treatments using high doses of intravenous immunoglobulin and plasmapheresis have also been employed (3)
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