The Chvostek sign is a diagnostic maneuver used to check for heightened nerve sensitivity by mechanically stimulating a specific facial nerve pathway. A positive result is an involuntary twitching of the facial muscles, indicating increased neuromuscular excitability. Physicians use this quick, non-invasive test to screen for potential electrolyte imbalances that affect nerve function.
Understanding the Mechanism of the Facial Nerve Response
The facial nerve (Cranial Nerve VII) controls the muscles of facial expression. Tapping a specific point on the cheek mechanically irritates the nerve. Under normal circumstances, this irritation would not produce a visible reaction.
When the nerve is highly excitable, the mild mechanical stimulus triggers an action potential. This physiological state, known as latent tetany, results in an involuntary muscular contraction or spasm. The mechanism is a direct stimulation of a nerve that is overly sensitive to perturbation.
Precise Location and Technique for Eliciting the Sign
Eliciting the Chvostek sign requires targeting the facial nerve trunk precisely. The correct anatomical landmark is the area approximately two centimeters in front of the earlobe (tragus), situated just below the zygomatic arch (cheekbone).
The technique involves a quick, light tap directly over the nerve using a fingertip or a reflex hammer. The tap must be gentle, as a forceful strike could cause a response regardless of nerve excitability. The goal is to momentarily compress the nerve against the underlying bone, providing minimal mechanical stimulation.
A positive response is the rapid, involuntary contraction of the facial muscles. This twitching ranges from a slight movement of the corner of the mouth to a more pronounced contraction involving the muscles of the nose and eyelid.
What a Positive Chvostek’s Sign Indicates
A positive Chvostek sign strongly indicates neuromuscular hyperexcitability, most frequently manifesting as hypocalcemia. Hypocalcemia refers to an abnormally low concentration of calcium in the blood. Calcium stabilizes the neuronal membrane and regulates nerve function.
When calcium levels fall, the nerve cell membrane becomes less stable. This instability increases the membrane’s permeability to sodium ions, lowering the threshold required for the nerve to fire an impulse. Consequently, a minor stimulus, like the physical tap of the Chvostek test, easily initiates an action potential and the observed muscle twitch.
While hypocalcemia is the primary cause, the sign can also be associated with other conditions. Respiratory alkalosis (from hyperventilation) can cause the sign because altered blood pH increases calcium binding to proteins, reducing the amount of free, active calcium. Magnesium deficiency (hypomagnesemia) may also increase nerve excitability. Since a positive sign can occur in a small percentage of healthy individuals, laboratory confirmation is necessary.
Other Indicators of Neuromuscular Excitability
Assessment of neuromuscular excitability often includes evaluating other physical signs. The most common accompanying test is Trousseau’s sign of latent tetany, which is also associated with hypocalcemia. Trousseau’s sign is elicited on the arm, unlike the Chvostek sign.
The test involves placing a blood pressure cuff on the upper arm and inflating it above the systolic blood pressure for about three minutes. This temporarily restricts blood flow (ischemia). The ischemia, combined with neuromuscular hyperexcitability, provokes a characteristic carpopedal spasm. This involuntary contraction presents as the wrist and fingers flexing inward. Trousseau’s sign is generally considered more sensitive for detecting hypocalcemia than the Chvostek sign.