Trousseau’s sign of latent tetany is a clinical indicator used to identify increased neuromuscular excitability. It is characterized by an involuntary, forceful muscle spasm triggered by a temporary reduction in blood flow to a limb. The sign helps professionals recognize a susceptibility to muscle cramps and spasms before severe, spontaneous symptoms of a mineral imbalance occur.
Defining Trousseau’s Sign and How It Is Observed
The observation of Trousseau’s sign involves a standardized procedure designed to temporarily deprive the nerves in the arm of oxygenated blood. A blood pressure cuff is placed on the patient’s upper arm and then inflated to a pressure greater than the individual’s systolic blood pressure. This pressure is maintained for approximately three to five minutes to occlude the brachial artery and induce localized ischemia in the forearm and hand.
A positive result is the development of a distinct and involuntary muscular contraction known as carpopedal spasm. The hand assumes a characteristic posture often called main d’accoucheur, or the “obstetrician’s hand.” This position is marked by the forceful flexion of the wrist and the metacarpophalangeal joints, along with the adduction of the thumb, drawing it inward towards the palm. Simultaneously, the distal and proximal interphalangeal joints of the fingers remain extended, creating a pointed, claw-like appearance.
The temporary lack of blood flow caused by the inflated cuff creates an environment where the nerves are primed to fire uncontrollably. Ischemia serves as a provocative test, highlighting a pre-existing state of neuromuscular irritability that is otherwise latent under normal circulatory conditions. The carpal spasm indicates that the nerves controlling the hand muscles are unusually sensitive to external stressors like reduced blood supply.
The Underlying Cause of the Spasm
The presence of Trousseau’s sign highly specific and often points to hypocalcemia, which is an abnormally low level of calcium in the blood. Calcium plays a significant role in stabilizing the membranes of nerve cells throughout the body. It functions by helping to regulate the permeability of the membrane to sodium ions, which are integral to generating an action potential.
When the concentration of calcium in the fluid surrounding the nerve cells drops below the normal range, the membrane becomes destabilized and significantly more permeable to sodium. This change effectively lowers the depolarization threshold. The nerves are then described as being hyperexcitable, or excessively responsive to stimulation.
The transient ischemia induced by the blood pressure cuff further aggravates this pre-existing hyperexcitability in the peripheral nerves of the arm. The combination of low calcium and lack of blood flow causes motor nerves to fire spontaneously, resulting in the sustained carpal spasm. Common causes of hypocalcemia include hypoparathyroidism and severe Vitamin D deficiency.
Other Physical Signs of Low Calcium
Another physical finding discussed alongside Trousseau’s sign is Chvostek’s sign, which also reflects increased neuromuscular irritability due to low calcium. It is elicited by lightly tapping the facial nerve just in front of the ear, resulting in a brief twitching of the facial muscles on the same side.
Trousseau’s sign is considered a more sensitive and reliable indicator for latent tetany than Chvostek’s sign. The carpal spasm is present in a high percentage of patients with hypocalcemia, while Chvostek’s sign can be observed in healthy individuals with normal calcium levels.