Phenotypical Variation with Same Genetic Mutation in Familial Hypokalemic Periodic Paralysis

Hypokalemic periodic paralysis is a genetic neuromuscular disorder characterized by episodes of painless muscle paralysis associated with low serum potassium, exclusively, during the attack. This may be precipitated by heavy exercise, fasting, or high-carbohydrate meals. We report two siblings, presenting at different ages with varying symptomatology—older sibling with episodic weakness in the morning associated with reduced physical exercise and consumption of large carbohydrate meal, whereas younger sibling complained of muscle stiffness following large carbohydrate meal and at the end of physical exercise. Molecular genetic study showed both siblings and their father were positive for calcium channel alpha-1S subunit (CACNA1S) C3716G>A; p.Arg1239His mutation. It is important to check serum potassium in a child presenting with muscle stiffness or weakness after a carbohydrate meal or vigorous exercise. This condition responds with potassium supplement. Often relevant family history and trigger factors with clinical correlation and blood results can lead to its diagnosis.