Tetany

Tetany or tetanic seizure is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disorders that increase the action potential frequency of muscle cells or the nerves that innervate them. Muscle cramps caused by the disease tetanus are not classified as tetany; rather, they are due to a lack of inhibition to the neurons that supply muscles. Tetanic contractions (physiologic tetanus) are a broad range of muscle contraction types, of which tetany is only one. Tetany is characterized by contraction of distal muscles of the hands (carpal spasm with extension of interphalangeal joints and adduction and flexion of the metacarpophalangeal joints) and feet (pedal spasm) and is associated with tingling around the mouth and distally in the limbs. The usual cause of tetany is a deficiency of calcium. An excess of phosphate (high phosphate-to-calcium ratio) can also trigger the spasms. Underfunction of the parathyroid gland can lead to tetany. Low levels of carbon dioxide cause tetany by altering the albumin binding of calcium such that the ionized (physiologically influencing) fraction of calcium is reduced; one common reason for low carbon dioxide levels is hyperventilation. Low levels of magnesium can lead to tetany. Clostridium tetani toxin, via inhibition of glycine-mediated and GABA-ergic neurotransmission, may lead to tetany. An excess of potassium in grass hay or pasture can trigger winter tetany, or grass tetany, in ruminants. Osteomalacia and rickets due to deficiency of vitamin D Metabolic alkalosis with hypokalemia like Gitelman syndrome and Bartter's syndrome can cause tetany. Vomiting induced alkalosis and hyperventilation induced respiratory alkalosis also cause tetany because of neuronal irritability. Hypocalcemia is the primary cause of tetany. Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to sodium ion, causing a progressive depolarization, which increases the possibility of action potentials.