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  • 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)


Last reviewed 01/2018