Tetany or tetanic
seizure is a
medical sign consisting of the involuntary contraction of
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s, which may be caused by disorders that increase the
action potential frequency of muscle cells or the nerves that innervate them.
Muscle cramps caused by the disease
tetanus are not classified as tetany; rather, they are due to a lack of inhibition to the neurons that supply muscles.
Tetanic contractions (physiologic tetanus) are a broad range of
muscle contraction types, of which tetany is only one.
Signs and symptoms
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.
Causes

* The usual cause of tetany is a deficiency of
calcium. An excess of
phosphate (high phosphate-to-calcium ratio) can also trigger the spasms.
* 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 ionized (physiologically influencing) fraction of calcium is reduced; one common reason for low carbon dioxide levels is
hyperventilation
Hyperventilation is irregular breathing that occurs when the rate or tidal volume of breathing eliminates more carbon dioxide than the body can produce. This leads to hypocapnia, a reduced concentration of carbon dioxide dissolved in the blood. ...
.
*
Low levels of magnesium can lead to tetany.
* ''
Clostridium tetani'' toxin, via inhibition of
glycine-mediated and
GABA-ergic
neurotransmission, may lead to tetany.
* An excess of
potassium in grass
hay or
pasture can trigger winter tetany, or
grass tetany, in
ruminants.
* Osteomalacia and rickets due to deficiency of vitamin D
Metabolic alkalosis with hypokalemia like Gitelman syndrome and Bartter's syndrome can cause tetany. Vomiting induced alkalosis and hyperventilation induced respiratory alkalosis also cause tetany because of neuronal irritability.
Pathophysiology
Hypocalcemia is the primary cause of tetany. Low ionized calcium levels in the
extracellular fluid increase the permeability of neuronal membranes to sodium ion, causing a progressive depolarization, which increases the possibility of action potentials. This occurs because calcium ions interact with the exterior surface of
sodium channels in the
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
of
nerve cells and hypocalcemia effectively increases
resting potential (rendering the cells more excitable) since less positive charge is present extracellularly. When calcium ions are absent the voltage level required to open voltage gated sodium channels is significantly altered (less excitation is required).
If the plasma Ca
2+ 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. Hypocalcemia is not a term for tetany but is rather a cause of tetany.
Diagnosis
French Professor
Armand Trousseau
Armand Trousseau (14 October 1801 – 23 June 1867) was a French internist. His contributions to medicine include Trousseau sign of malignancy, Trousseau sign of latent tetany, Trousseau–Lallemand bodies (an archaic synonym for Bence Jones ...
(1801–1867) devised the maneuver of occluding the
brachial artery
The brachial artery is the major blood vessel of the (upper) arm. It is the continuation of the axillary artery beyond the lower margin of teres major muscle. It continues down the ventral surface of the arm until it reaches the cubital fossa ...
by squeezing, to trigger cramps in the fingers. This is now known as the
Trousseau sign of latent tetany.
Also, tetany can be demonstrated by tapping anterior to the ear, at the emergence of the facial nerve. A resultant twitch of the nose or lips suggests low calcium levels. This is now known as the
Chvostek sign
The Chvostek sign () is a clinical sign that someone may have a low blood calcium level (a decreased serum calcium, called hypocalcemia). The Chvostek sign is the abnormal twitching of muscles that are activated (innervated) by the facial nerve (a ...
.
EMG studies reveal single or often grouped motor unit discharges at low discharge frequency during tetany episodes.
References
External links
*
*
* Harrison's Principles of Internal Medicine 16th Edition – Dennis L. Kasper, Eugene Braunwald, Stephen Hauser, Dan Longo, J. Larry Jameson, Anthony S. Fauci.
{{Nervous and musculoskeletal system symptoms and signs
Symptoms and signs: Nervous system
Tetanus