Guidelines on clinical presentation and management of nondystrophic myotonias

Muscle Nerve. 2020 Oct;62(4):430-444. doi: 10.1002/mus.26887. Epub 2020 May 27.


The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications.

Keywords: management; myotonia congenita; nondystrophic myotonias; paramyotonia congenita; skeletal muscle channelopathies.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Acetazolamide / therapeutic use
  • Age of Onset
  • Carbonic Anhydrase Inhibitors / therapeutic use
  • Chloride Channels / genetics
  • Electrodiagnosis
  • Electromyography
  • Fatigue / physiopathology*
  • Genetic Testing
  • Humans
  • Lamotrigine / therapeutic use
  • Mexiletine / therapeutic use
  • Muscle Weakness / physiopathology*
  • Muscle, Skeletal / physiopathology*
  • Myalgia / physiopathology*
  • Myotonia Congenita / drug therapy
  • Myotonia Congenita / genetics
  • Myotonia Congenita / physiopathology
  • Myotonic Disorders / genetics
  • Myotonic Disorders / physiopathology*
  • NAV1.4 Voltage-Gated Sodium Channel / genetics
  • Practice Guidelines as Topic
  • Ranolazine / therapeutic use
  • Sodium Channel Blockers / therapeutic use
  • Voltage-Gated Sodium Channel Blockers / therapeutic use


  • CLC-1 channel
  • Carbonic Anhydrase Inhibitors
  • Chloride Channels
  • NAV1.4 Voltage-Gated Sodium Channel
  • SCN4A protein, human
  • Sodium Channel Blockers
  • Voltage-Gated Sodium Channel Blockers
  • Mexiletine
  • Ranolazine
  • Acetazolamide
  • Lamotrigine

Supplementary concepts

  • Potassium aggravated myotonia