The caloric test is a specialized diagnostic procedure used to assess the function of the vestibulo-ocular reflex (VOR) and the horizontal semicircular canal within the inner ear. It is a component of a larger battery of tests, often called videonystagmography (VNG), that evaluates the balance system. The test stimulates each inner ear separately using temperature changes to determine if a weakness exists in the balance mechanism of one ear compared to the other. By observing the induced eye movements, known as nystagmus, doctors gather information about the health of the peripheral vestibular system.
The Physiological Basis of the Caloric Test
The inner ear contains three semicircular canals filled with endolymph, which move during head rotation, stimulating hair cells to signal motion. The caloric test introduces a thermal stimulus to the ear canal, causing a temperature change that transfers through the middle ear to the endolymph in the horizontal semicircular canal. This temperature change creates a density gradient, initiating a convection current.
When warm water or air is applied, the endolymph becomes warmer and lighter, causing it to rise. This fluid movement displaces the cupula, a gelatinous structure containing hair cells, mimicking the effect of head rotation. Conversely, a cold stimulus causes the endolymph to become denser and sink, resulting in movement in the opposite direction.
This cupula movement stimulates the vestibular nerve, generating an electrical signal that mimics the sensation of turning. This signal travels to the brainstem, triggering the vestibulo-ocular reflex (VOR), which coordinates eye movements for stable vision. The brain generates involuntary eye movement called nystagmus. A warm stimulus causes the fast component to beat toward the stimulated ear, while a cold stimulus causes it to beat away.
Performing the Caloric Test
The patient is positioned lying down with their head elevated at a 30-degree angle. This position places the horizontal semicircular canal in a vertical plane, maximizing the effect of the convection currents. Eye movements are recorded using specialized goggles or electrodes, typically via videonystagmography (VNG).
The procedure, known as the bi-thermal caloric test, stimulates each ear separately with two temperatures. Standard temperatures are 44°C (warm) and 30°C (cool), 7°C above and below normal body temperature. The stimulus is delivered into the ear canal for 40 to 60 seconds.
Stimulation uses either water or air, though water generally produces a stronger response. Water irrigation is preferred unless the patient has a perforated eardrum, requiring air irrigation as a safer alternative. A break of several minutes occurs between each irrigation to allow the inner ear to return to normal temperature. The sequence involves four total irrigations.
During the test, the temperature change causes a temporary sensation of vertigo or dizziness. The technician records the nystagmus intensity, speed, and duration for analysis. The response is quantified by measuring the slow-phase velocity, the speed of the eye as it drifts away before the quick corrective movement.
Interpreting Results and Clinical Significance
The data from the four irrigations are analyzed by comparing responses between the two ears and temperatures. Interpretation focuses on two primary parameters: unilateral weakness and directional preponderance. These calculations help pinpoint if the balance problem stems from the inner ear or the central nervous system.
Unilateral weakness (canal paresis) is the most significant finding, indicating a reduced or absent response in one ear. It is calculated as the relative difference in total response between the ears. A significant weakness (greater than 20% to 25%) suggests a peripheral lesion, such as damage to the inner ear or the vestibular nerve on the weaker side.
Directional preponderance shows a consistent difference in nystagmus strength when beating in one direction, regardless of which ear was stimulated. This finding can suggest a central nervous system issue or be caused by pre-existing spontaneous nystagmus. The caloric test is used to diagnose conditions like vestibular neuritis, labyrinthitis, and Meniere’s disease.