Hypocalcemia and Tetany

Ionized calcium is the free, biologically active calcium in plasma. It stabilizes excitable cell membranes. When ionized calcium falls, sodium channels open more easily, so nerves and skeletal muscle fire with minimal stimulation. The result is paresthesia, cramping, carpopedal spasm, seizures, or laryngospasm. Tetany is sustained neuromuscular irritability caused by hypocalcemia. It is not simply “muscle spasms.” A patient with painful hand flexion, wrist flexion, and finger adduction may be progressing from early irritability to clinically significant hypocalcemia. Total serum calcium can be misleading because about half of circulating calcium is bound to protein, chiefly albumin. A patient with low albumin can have a low total calcium concentration while ionized calcium remains normal. Conversely, acute respiratory alkalosis increases calcium binding to albumin and lowers ionized calcium without necessarily changing total calcium. This explains why a patient who is hyperventilating may report perioral tingling and hand cramping. The immediate problem is reduced ionized calcium caused by alkalosis, not necessarily depleted total-body calcium. Parathyroid hormone (PTH) normally raises serum calcium through effects on bone, kidneys, and vitamin D activation. Failure of PTH...