neuromyotonia

• 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)

Reference:

• (1) Falace A et al. Inherited neuromyotonia: A clinical and genetic study of a family Neuromuscular Disorders 2007; 17: 23-27
• (2) Am J Med Sci. 2000 Apr;319(4):209-16.
• (3) Gonzalez G et al. Acquired neuromyotonia in childhood: case report and review.Pediatr Neurol. 2008 Jan;38(1):61-3

Last reviewed 01/2018