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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: hypokalemic, hyperkalemic, and normokalemic. Diagnosis
is indicated by history and is confirmed by provoking an episode
(eg, by giving dextrose and insulin to cause hypokalemia or KCl
to cause 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
Hypokalemic:
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 four limbs. Episodes are also 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.
Hyperkalemic:
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.
Normokalemic:
Affected patients are sensitive to K ingestion and have episodes of mild weakness that occur without any change in serum K.
Diagnosis
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 giving dextrose and insulin (to cause the hypokalemic form) or KCl (to cause the hyperkalemic form), but only experienced physicians should attempt provocative testing, because respiratory paralysis or cardiac conduction abnormalities may occur with provoked episodes.
Treatment
Hypokalemic:
Episodes of paralysis are managed by giving KCl 2 to 10 g in an unsweetened oral solution or giving 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.
Hyperkalemic:
Episodes of paralysis, if mild, can be aborted at onset by light exercise and a 2 g/kg oral carbohydrate load. Established episodes require 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, strenuous activity after meals, and cold exposure help prevent hyperkalemic episodes.
Normokalemic:
Large doses of Na alleviate the weakness. Dextrose has no effect. Attacks may be prevented by avoiding excess alcohol and cold exposure. People should cool down slowly after strenuous exercise and not immediately rest. Eating some form of carbohydrate (eg, candy bar) may help as well.
Last full review/revision August 2009 by Michael Rubin, MD
Content last modified August 2009
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