tetany

[From TETANUS.]

Tetany refers to a state of increased excitability of nerve and muscle, characterized by muscle spasms. At one end of the spectrum, tetany can occur in otherwise healthy people as a consequence of sustained hyperventilation or following excessive vomiting. At the other extreme, tetany signifies a pathological state, arising most often from an endocrine disorder.

Whatever its cause, the common denominator appears to be a reduced level of ionized calcium in the plasma of the circulating blood. Although present in the relatively low concentration of approximately 0.1 g/litre (2.5 mmol/litre), with only about half of this normally ionized or ‘free’, calcium nevertheless plays a crucial role in controlling the electrical excitability in nerve and muscle membranes, including those of the heart. Hormonal mechanisms normally maintain a near-constant concentration of free calcium, but the constancy depends also on that of the pH (relative acidity/alkalinity) of the blood.

The prevailing pH determines the ratio of the calcium bound to plasma proteins (calcium proteinates) and the calcium in its electrically charged ‘cationic’ form of calcium ions ([Ca⁺⁺]). The more acid — the higher the concentration of hydrogen ions ([H⁺]) — the more [Ca⁺⁺] is ‘freed’ from the proteins, and vice versa. This is important because it is the free [Ca⁺⁺] which crosses capillary walls and determines the concentration in the tissue fluids; this in turn influences the electrical excitability of nerve and muscle membranes. Experiments show that low [Ca⁺⁺] increases excitability and high [Ca⁺⁺] diminishes it.

Hyperventilation (overbreathing), whether deliberate or as an aspect of an anxiety state, removes too much carbon dioxide from the body, resulting in respiratory alkalosis (lower [H⁺], increased pH) ; this in turn increases the calcium proteinate and lowers free [Ca⁺⁺]. The increased excitability causes spontaneous tingling in lips and fingers and, through analogous effects on motor nerve fibres, characteristic muscular ‘carpo-pedal’ spasms, mainly affecting the wrists, hands, and feet. Short of full-blown tetany, in medical conditions with a low blood calcium (hypocalcaemia) the motor axons in the peripheral nerves may show an increased sensitivity to mechanical stimulation. The physician can make diagnostic use of this by tapping the facial nerve near the jaw joint in front of the ear, and looking for a twitch in the facial muscles — the so-called ‘Chvostek's sign’, named after the Austrian physician who first described it. The detection of a low calcium is clinically important, because tetany can be fatal should it be severe and involve laryngeal or pharyngeal muscles causing upper airway obstruction. The hormone parathormone, from the parathyroid glands, is the main regulator of blood [Ca⁺⁺], so tetany is a major sign of the condition of hypoparathyroidism. This rare condition occurs most often from autoimmune destruction of the cells that secrete the hormone; it can also result from the inadvertant or inevitable removal of the parathyroids when the thyroid gland, with which they are intimately associated, is surgically removed for independent medical reasons.

— Tom Sears

See also acid-base homeostasis; calcium.

Over-sensitivity of motor nerves to stimuli; particularly affects face, hands, and feet. Caused by reduction in the level of ionized calcium in the bloodstream and can accompany severe rickets.

Spasm and twitching of a muscle which may be due to lack of calcium. It usually affects the hands and feet.

A disorder characterized by hyperreflexia, muscle spasms and cramps, foot and ankle spasms, choreiform convulsions, and sometimes stridor. Tetany is due to the malfunction of calcium metabolism. Possible causes are vitamin D deficiency, the ingestion of alkaline salts, hyperthyroidism, or alkalosis. There are many types of tetany, some of which are more common in animals.

Spasm and twitching of muscle due to lack of calcium, which increases the excitability of nerves. Tetany usually affects the hands and feet. It may be caused by alkalosis.

Continuous tonic spasm of a muscle; steady contraction of a muscle without distinct twitching. It is manifested clinically by rigidity of limbs, pricking of ears, flaring of nostrils. In extreme tetany as in tetanus, the rigidity is lead-pipe in nature in that the resistance to bending is constant. In less severe tetany, e.g. an upper motor neuron lesion, there is resistance at first but as soon as this is overcome the limb flexes easily.

• grass t. — see lactation tetany (2).
• hyperventilation t. — tetany produced by forced inspiration and expiration continued for a considerable time.
• hypocalcemic t. — see puerperal tetany (below).
• hypomagnesemic t. — see calf hypomagnesemic tetany, lactation tetany (2).
• lactation t. — see lactation tetany.
• latent t. — tetany elicited by the application of electrical and mechanical stimulation.
• magnesium-deficiency t. — a product of experimental nutrition but see also lactation tetany.
• milk t. — see calf hypomagnesemic tetany.
• parathyroid t., parathyroprival t. — tetany due to removal or hypofunctioning of the parathyroid glands.
• t. pastures — lush grass or cereal crop pastures, in the early spring when they have grown rapidly and their magnesium content is low. The weather is also inclined to be inclement with a high wind-chill factor and there is little or no shelter for the recently calved cows.
• puerperal t. — a syndrome of hypocalcemia in bitches, usually of small breeds, typically 2 to 3 weeks after whelping, caused by the loss of extracellular calcium in lactation. Affected bitchs show restlessness, panting and severe muscular spasms, all of which are relieved by the administration of intravenous calcium. Also occurs infrequently in queens. Called also lactational or puerperal tetany.
• recurrent t. — see Scottie cramp.
• transit t. — of cows, see transit tetany; of mares, see lactation tetany.

Tetany has two meanings, though both are related to the muscular system.

• Tetany (action potential summation)
• Tetany (medical sign)

The terms "tetany" and "tetanus" are distinct.

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