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Hypokalemic periodic paralysis
Hypokalemic periodic paralysis (hypoPP) is characterised by episodes of muscle paralysis lasting from a few to 24-48 hours and associated with a fall in blood potassium levels.
Prevalence is estimated at around 1 in 100,000.
The paralysis most often affects the four limbs, thus resulting in tetraplegia. Principle triggering factors include meals rich in carbohydrates and rest after exercise. Disease onset usually occurs during the second decade of life. In an undefined number of cases, hypoPP may be associated with a vacuolar myopathy resulting in permanent motor deficit occurring during the fourth to fifth decade of life.
Around 70% of cases are associated with mutations in the muscle calcium channel gene CACNA1S and 10% of cases are linked to mutations in the muscle sodium channel gene SCN4A.
Electromyogram with an exercise stress test reveals inexcitability of the muscle membrane and helps orientate the molecular diagnosis. Molecular diagnosis is feasible through analysis of the causative genes identified so far.
The differential diagnosis should include thyrotoxic periodic paralysis (see this term) which is associated with abnormal thyroid hormone levels.
HypoPP is transmitted as an autosomal dominant disease with incomplete penetrance, especially in females. Sporadic cases and de novo mutations have been reported.
Management and treatment
Potassium supplements and/or acetazolamide treatment lead to a significant decrease in the number of episodes and the resulting motor deficit.
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