Familial periodic paralysis is a rare autosomal dominant condition with considerable variation in penetrance characterized by episodes of flaccid paralysis with loss of deep tendon reflexes and failure of muscle to respond to electrical stimulation. There are 4 forms: hypokalemic, hyperkalemic, thyrotoxic, and Andersen-Tawil syndrome. 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.

Each form of familial periodic paralysis involves a different gene and electrolyte channel. The hypokalemic form is due to a mutation in the dihydropyridine receptor–associated Ca channel gene; this is the most common form of familial periodic paralysis but is nonetheless quite rare, with a prevalence of 1/100,000. The hyperkalemic form is due to mutations in the gene that encodes the α-subunit of the skeletal muscle Na channel (SCN4A). The mutations and affected electrolyte channels in the thyrotoxic form are unknown, but this form usually involves hypokalemia and is associated with symptoms of thyrotoxicosis. Incidence of the thyrotoxic form is highest in Asian men. Andersen-Tawil syndrome is due to an autosomal dominant defect of the inward-rectifying K channel; patients can have a high, low, or normal serum K level.

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, emotional or physical stress, alcohol ingestion, and cold exposure. Ocular, bulbar, and respiratory muscles are spared. Consciousness is not altered. Serum and urine K are decreased. Weakness may last 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 rest after exercise, exercise after meals, or fasting. Myotonia (delayed relaxation after muscle contraction) is common. Eyelid myotonia may be the only symptom.

Thyrotoxic: Episodes last hours to days and are usually precipitated by exercise, stress, or a carbohydrate load, similar to the hypokalemic form. Symptoms of thyrotoxicosis (eg, anxiety, emotional lability, weakness, tremor, palpitations, heat intolerance, increased perspiration, weight loss) are typically present. Clinical features of hyperthyroidism often precede the onset of periodic paralysis by months or years; however, features have been noted to occur at the same time as (in up to 60% of patients) or after the development of (in up to 17% of patients) periodic paralysis.

Andersen-Tawil syndrome: Episodes usually begin before age 20 and are precipitated by rest after exercise. Episodes may last for days and occur monthly. Associated manifestations include dysmorphic features (eg, scoliosis, syndactyly, clinodactyly, micrognathia, low-set ears), and cardiac abnormalities that may cause significant dysrhythmias and increased risk of sudden death.

Diagnosis

• Clinical evaluation
• Serum K level during symptoms
• Sometimes provocative testing

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

• Varies with type and severity

Hypokalemic: Episodes of paralysis are managed by giving KCl 2 to 10 g in an unsweetened oral solution or giving KCl IV. Following a low-carbohydrate, low-Na diet, avoiding strenuous activity, avoiding alcohol after periods of rest, and taking acetazolamideSome Trade Names
DIAMOX
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250 mg po bid 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, acetazolamideSome Trade Names
DIAMOX
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, or inhaled β-agonists. Severe episodes require Ca gluconate or insulinSome Trade Names
HUMULIN
NOVOLIN
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and dextrose IV. Regularly ingesting carbohydrate-rich, low-K meals and avoiding fasting, strenuous activity after meals, and cold exposure help prevent hyperkalemic episodes.

Thyrotoxic: Acute episodes are treated with KCl (see Inherited Muscular Disorders: Hypokalemic) and serum K levels are closely monitored. Episodes are prevented by maintaining a euthyroid state (see Thyroid Disorders: Treatment) and giving β-blockers (eg, propranololSome Trade Names
INDERAL
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).

Andersen-Tawil syndrome: In addition to lifestyle changes including tightly controlled levels of exercise or activity, episodes may be prevented by giving a carbonic anhydrase inhibitor (eg, acetazolamideSome Trade Names
DIAMOX
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). A cardiac pacemaker or implantable cardioverter-defibrillator may be required to control cardiac symptoms.

Key Points

• There are 4 types of familial periodic paralysis, which are caused by rare mutations of membrane electrolyte channels.
• Serum K is usually but not always abnormal but may be low or high.
• Patients have intermittent episodes of weakness, typically precipitated by exercise and sometimes meals (particularly containing carbohydrates) or alcohol.
• Diagnose by typical symptoms and measuring serum K during symptoms.
• Treat episodes by correcting serum K and prevent episodes by recommending lifestyle changes.

Last full review/revision March 2013 by Michael Rubin, MDCM

Content last modified March 2013