Hypocalcemia and Tetany

Calcium stabilizes excitable nerve and muscle membranes. When ionized calcium falls, sodium channels open more easily, so nerves fire with minimal stimulation. The result is tingling, muscle cramping, spasm, seizures, and potentially life-threatening airway or cardiac complications. Tetany is sustained involuntary muscle contraction caused by neuromuscular hyperexcitability. It may appear as carpopedal spasm: flexed wrists, thumb adduction, and tightly contracted fingers. Tetany is a manifestation of hypocalcemia, not a diagnosis by itself. The calcium that matters most is ionized calcium, the free and biologically active fraction. About half of circulating calcium is bound to protein, mainly albumin. A low albumin level can make total calcium appear low even when ionized calcium is adequate and the patient has no symptoms. Acid-base changes alter calcium binding. Alkalosis increases calcium binding to albumin, lowering ionized calcium. A patient who is hyperventilating from pain, panic, or respiratory distress may develop perioral tingling and hand spasm even when total calcium is normal. The immediate problem is reduced ionized calcium, not necessarily depleted body calcium stores.