These disorders are characterised by defective skeletal muscle voltage sensitive ion channels channelopathies

Hypokalacmic periodic paralysis

Autosomal dominant. Chromosomal location and defective gene unknown. Onset in second decade. Precipitated by: exercise, carbohydrate load. Commences in proximal lower limb muscles and rapidly becomes generalised. Onset usually in morning on wakening. Bulbar muscles/respiration unaffected.

K* falls as low as 1.5 meq/1. Treatment : Acute - oral K.C1. Prophylactic - acctazolamide; low carbohydrate, high K ' diet. With age, attacks become progressively less frequent.

Hyperkalaemic periodic paralysis

Autosomal dominant or recessive. Chromosome 17 location. Na" channel gene defect Onset in infancy/childhood. Precipitated by: rest after activity or by cold.

Commences in lower limbs and evolves rapidly.

Attacks are of short duration (less than 60 min).

Myotonia is evident in some patients.

K ' rises only slightly. Treatment:

Acute - intravenous calcium gluconate or sodium chloride. Prophylactic - Na ' channel blockers, Tocainide or Mexiletine.

Paramyotonia Congenita

Autosomal dominant. Chromosome 17 location. Na ' channel gene defect Onset in infancy.

Precipitated by: rest after exercise, fasting and cooling. Commences in proximal muscles. Repetitive muscle contractions produce increasing stiffness. EMG findings are specific with marked spontaneous activity in limb cooling.

Treatment. Na ' channel blockers, Tocainide or Mexiletine.

Non-familial hypokalaemic periodic paralysis may occur in patients suffering from hyperthyroidism or on potassium-depleting diuretics.

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