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Familial
periodic paralysis is a rare autosomal condition characterized by
episodes of flaccid paralysis with loss of deep tendon reflexes
and failure of muscle to respond to electrical stimulation. There
are 3 forms: hyperkalemic, hypokalemic, and normokalemic. Diagnosis
is indicated by history and confirmed by provoking an episode (by
administering glucose and insulin to produce hypokalemia or KCl
to produce hyperkalemia). Treatment depends on the form.
The hypokalemic form of familial periodic paralysis is due to genetic mutation in the dihydropyridine receptor–associated Ca channel gene. The hyperkalemic form is due to mutations in the gene that encodes the α-subunit of the skeletal muscle Na channel (SCN4A). The cause of the normokalemic form is unclear; in some instances it may result from a mutation in a gene that encodes Na channels.
Symptoms and Signs
In the hypokalemic form, episodes usually begin before age 16. The day after vigorous exercise, the patient often awakens with weakness, which may be mild and limited to certain muscle groups or may affect all 4 limbs. Episodes are precipitated by carbohydrate-rich meals. Ocular, bulbar, and respiratory muscles are spared. Consciousness is not altered. Serum and urine K are decreased. Weakness lasts up to 24 h.
In the hyperkalemic form, episodes often begin at an earlier age and usually are shorter, more frequent, and less severe. Episodes are precipitated by exercise after meals or by fasting. Myotonia (delayed relaxation after muscle contraction) is common. Eyelid myotonia may be the only symptom.
In the normokalemic form, affected patients are sensitive to K ingestion and experience episodes of mild weakness occurring without any change in serum K.
Diagnosis
and Treatment
The best diagnostic indicator is a history of typical episodes. If measured during an episode, serum K may be abnormal. Episodes can sometimes be provoked by administering dextrose and insulin (hypokalemic form) or KCl (hyperkalemic form), but only experienced physicians should attempt this procedure, because respiratory paralysis or cardiac conduction abnormalities may occur with provoked episodes.
Episodes of hypokalemic paralysis are managed by administering KCl 2 to 10 g in an unsweetened oral solution or administering K IV. Following a low-carbohydrate, low-Na diet; avoiding strenuous activity or alcohol after periods of rest; and taking acetazolamide 250 to 2000 mg po once/day may help prevent hypokalemic episodes.
At onset, mild episodes of hyperkalemic paralysis are aborted by light exercise and a 2 g/kg carbohydrate load. An established episode requires thiazides, acetazolamide , or inhaled β-agonists. Severe attacks require Ca gluconate, or insulin and dextrose IV. Regularly ingesting carbohydrate-rich, low-K meals and avoiding fasting and strenuous activity after meals and exposure to cold help prevent hyperkalemic episodes.
In the normokalemic form, large doses of Na improve the weakness. Dextrose has no effect. Attacks may be prevented by avoiding resting after exercise, excess alcohol, and cold exposure.
Last full review/revision November 2005
Content last modified November 2005
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