Hypocalcemia and Tetany

Calcium stabilizes excitable cell membranes. When the ionized, or free, calcium concentration falls, sodium channels open more easily. Nerves then fire with minimal stimulation and skeletal muscle contracts involuntarily. That is the basis of perioral tingling, muscle cramping, carpopedal spasm, laryngospasm, and seizures. Only ionized calcium is immediately available to tissues. About half of circulating calcium is bound to proteins, chiefly albumin, so a low total calcium does not always mean a low biologically active calcium level. Alkalosis increases calcium binding to albumin and lowers ionized calcium. A patient who is hyperventilating may therefore develop tingling and tetany despite a total calcium that appears acceptable. Acidosis has the opposite effect: less calcium binds albumin, so ionized calcium rises relative to total calcium. Parathyroid hormone (PTH) normally protects against hypocalcemia by increasing calcium release from bone, reducing renal calcium loss, and supporting activation of vitamin D. Active vitamin D then increases intestinal calcium absorption. Loss of PTH effect, inadequate vitamin D, kidney disease, magnesium disorders, or sudden calcium shifts can overwhelm these defences. The absolute calcium value matters, but the speed...