Table 1

Clinical features of the non-dystrophic myotonias

GeneInheritance patternMuscles most commonly affectedSymptomatic triggersRelieving factorsWarm up phenomenon
(myotonia improves with repetition)
Paradoxical myotonia (myotonia worsens with repetition)Episodic weaknessMuscle hypertrophy
Myotonia congenita CLCN1 Autosomal dominant or recessiveLegs>hands‚ÄČor face. Falls are common.Prolonged rest or sitting (especially after activity)
Temperature extremes hot or cold
Warm environments (although extremes can provoke myotonia)
Gentle movement before engaging in activity
Characteristic findingNoTransient weakness improving with repetition of taskYes
Characteristically calf muscle hypertrophy but can be generalised
Paramyotonia congenita SCN4A Autosomal dominantFace and hands>legsCold environment
rest after activity
Warm environmentsNoCharacteristic findingProlonged weakness or paralysis (can have clinical overlap with hyperkalaemic periodic paralysis).Yes can be generalised
Sodium channel myotonia* SCN4A Autosomal dominantEither patternProlonged rest or sitting (especially after activity)
temperature extremes hot or cold
Warm environments (although extremes can provoke myotonia)Can occurCan occurNoYes can be generalised
  • *Before the genetic basis of channelopathies was known many clinical phenotypes were described including potassium aggravated myotonia, acetazolamide responsive myotonia but these were subsequently all shown to be due to mutations in the SCN4A gene and are now collectively called sodium channel myotonia.