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Tetany or tetany seizure is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disease or other conditions that increase the action potential frequency. Muscle cramps that are 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.
Low calcium levels in the bloodstream increase the permeability of neuronal membranes to sodium ions, causing a progressive depolarization, which increases the possibility of action potentials. If the plasma Ca2+ decreases to less than 50% of the normal value of 9.4 mg/dl, action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscles. Tetany is also known as hypocalcemia because it is the medical term for tetany 
French Professor Armand Trousseau (1801-1867) devised the trick (now known as the Trousseau sign of latent tetany) of occluding the brachial artery by squeezing, to trigger cramps in the fingers. Also, tetany can be demonstrated by tapping anterior to the ear, at the emergence of the facial nerve. This is now known as the Chvostek sign.
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.
EMG studies reveal single or often grouped motor unit discharges at low discharge frequency during tetany episodes.
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