Tetany (medical sign)

Tetany is a medical sign, the involuntary contraction of muscles, caused by diseases and other conditions that increase the action potential frequency. The muscle cramps caused by the disease tetanus are due to a blocking of the inhibition to the neurons that supply muscles and are not classified as tetany.

Mechanism
When the membrane potential is upset, for instance by low levels of ions (such as calcium) in the blood (hypocalcaemia), neurons will depolarize too easily. In the case of hypocalcaemia, calcium ions are drawn away from their association with the voltage-gated sodium channels thus sensitising them. The upset to membrane potential is therefore caused by an influx of sodium to the cell, not directly by the hypocalcaemia. As a result, too many action potentials are sent to muscles causing spasm.

Causes
The usual cause of tetany is lack of calcium, but excess of phosphate (high phosphate-to-calcium ratio) can also trigger the spasms. Milk-and-alkali tetany is an example of this imbalance.

Underfunction of the parathyroid gland can lead to tetany.

Low levels of carbon dioxide causes tetany by altering the albumin binding of calcium such that the ionised (physiologically influencing) fraction of calcium is reduced; the most common reason for low carbon dioxide levels is hyperventilation.

Diagnosis
The nineteenth-century clinician Professor Armand Trousseau devised the trick of occluding the brachial artery by squeezing to trigger the cramps in the fingers (Trousseau sign).