tetany

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.