Tetany

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 not classified as tetany; rather, they are due to a blocking of the inhibition to the neurons that supply muscles.

Mechanism

Low plasma calcium increases the permeability of neuronal membranes to sodium ions, causing a progressive depolarization. This increases the ease with which action potentials can be initiated. If the plasma Ca²⁺ decreases to less than 50% of the normal value of 9.4mg/dl; action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscle.^[1]

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 cause 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.

Low levels of magnesium can also lead to tetany symptoms.

Cow grazing on rapidly grown pasture with tetany of the neck suggesting Grass Tetany

An excess of potassium in grass hay or pasture can trigger winter tetany or grass tetany, respectively, in ruminants.

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).

References

External links

• Bartleby

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