Inheritance: autosomal dominant. Sporadic cases may also occur.
Most cases are associated with a mutation in the calcium channel gene CACNA1S (R528H and R1239H). A small number may be due to mutation of the sodium channel SCN4A gene (at codon 669 or 672).
Clinical features:
(1) The patient experiences infrequent episodes of muscle weakness and flaccidity affecting the arms, legs and/or trunk that may last > 24 hours if untreated.
(2) The serum potassium level is reduced prior to and during the attack.
(3) An attack may be precipitated by exercise or by conditions causing a drop in serum potassium (see previous section).
A skeletal muscle biopsy shows a vacuolar myopathy.
Attacks may be prevented by prophylactic therapy with acetazolamide (induces a metabolic acidosis) or spironolactone (potassium-sparing).
Differential diagnosis:
(1) secondary hypokalemic periodic paralysis
(2) hyperkalemic periodic paralysis
(3) paramyotonia congenita
