Hypokalemic Myopathy

In the elderly, hypokalemia is the most common cause of myopathy due to electrolyte imbalance. A common cause of hypokalemia in this age group is long-term diuretic use. Other conditions that cause hypokalemia include dietary deficiency, alcoholic myopathy, intestinal potassium wastage, Bartter's syndrome, aldosteronism, and licorice intoxication. Acute hypokalemic paralysis may occur after amphotericin B treatment and with diabetic ketoacidosis, renal tubular acidosis, chronic diarrhea, and most conditions that produce chronic hypokalemia.

Hypocalcemia and hypomagnesemia can also produce prominent neuromuscular symptoms, including tetany, irritability, myoclonus, and tremor, but they do not typically produce muscle weakness.

Symptoms and Signs

Muscle weakness usually develops slowly over days to weeks. It primarily affects the legs but, in severe cases, affects the arms, trunk, neck, and, occasionally, thoracic and diaphragmatic muscles. Ocular and bulbar muscles are spared. In general, chronic hypokalemic myopathy produces greater weakness in the proximal muscles than in the distal muscles. Patients with diabetic acidosis (in which serum potassium levels fall rapidly over hours) may present with respiratory weakness, and the arms may become weak before the legs.

Tendon reflexes become hypoactive and then vanish. The patient has no pain or other sensory complaints. In severe cases, muscle fiber necrosis with myoglobinuria, muscle pain, tenderness, and swelling can develop.

Diagnosis and Treatment

The serum potassium level is usually < 3 mEq/L. The serum CK level may be normal or elevated. Nerve conduction velocities are normal.

Chronic myopathy usually reverses after 1 to 4 weeks of potassium replacement therapy. If myoglobinuria and rhabdomyolysis have occurred, treatment should include vigorous hydration and alkalinization of the urine to avoid renal tubular necrosis and renal failure. Depending on the cause of hypokalemia, long-term potassium replacement may be needed. Occasionally, when hypokalemia is severe, symptomatic, or unresponsive to oral therapy, potassium must be replaced parenterally.