pediatrics
Familial Periodic Paralysis
Diagnosis of Familial Periodic Paralysis
History of typical episodes Serum potassium level during symptoms Exercise test Genetic testing The best diagnostic indicator is a history of typical episodes. If measured during an episode, serum potassium may be abnormal. Previously, for diagnosis, an attempt was made to provoke episodes by givingdextroseandinsulin(to cause the hypokalemic form) orpotassium chloride(to cause the hyperkalemic form), but because these tests may cause respiratory paralysis or cardiac conduction abnormalities and are not needed to make the diagnosis, they have been replaced by a safer exercise test. In the exercise test, the patient vigorously exercises a single muscle for 2 to 5 minutes in an attempt to cause focal muscle weakness. Weakness is assessed by an electrophysiologic study called the compound muscle action potential (CMAP), which is done before and after exercise. A ≥ 40% decrease postexercise is abnormal and consistent with periodic paralysis. Diagnosis of the hyperkalemic form is based on clinical findings and/or the identification of a heterozygous pathogenic variant in the alpha-subunit of the skeletal muscle sodium channel. An ECG should be performed in all patients to evaluate for a prolonged QT interval, which is suggestive of Andersen-Tawil syndrome. 