Synonym: Adynamic Episodica Hereditaria
Chromosome location: 17q22
Gene affected. Skeletal muscle sodium channel SCN4A at Nav1.4
Mutations: T704M or M1592V
Inheritance: autosomal dominant, although sporadic cases may occur
Clinical features:
(1) Onset is typically as a child (age < 10 years of age).
(2) Episodes may be frequent (can be daily) but are of short duration (< 24 hours).
(3) Episodes may be triggered by exercise (occurs afterwards while resting), fasting or ingestion of potassium-rich foods.
(4) Exposure to cold usually does not cause weakness but this may occur in some kindreds (overlap with paramyotonia congenital).
(5) An episode features muscle weakness with intermittent myotonia (muscle stiffness).
(6) Some patients experience paresthesias.
(7) The patient's serum potassium is usually elevated but may be normal. If the serum potassium is normal before an episode then it may increase as one evolves.
(7) Administration of potassium causes muscle weakness.
Diagnostic testing:
(1) A skeletal muscle biopsy shows a vacuolar myopathy.
(2) An EMG during an episode will confirm myotonia.
