The caloric test is a specialized diagnostic procedure used to assess the function of the vestibular system, the balance-sensing organ located within the inner ear. It is often performed as part of a larger battery of tests known as videonystagmography (VNG) or electronystagmography (ENG). The test works by stimulating the balance organs one side at a time, allowing doctors to determine if one inner ear is reacting differently from the other. This targeted stimulation helps pinpoint the location of a balance disorder when a patient reports feelings of unsteadiness or spinning.
How the Caloric Test Works
The caloric test relies on thermal convection within the inner ear’s fluid-filled canals. The test specifically targets the lateral, or horizontal, semicircular canal, which is filled with a fluid called endolymph. When the temperature of the external ear canal is changed, thermal energy is transferred through the temporal bone to the endolymph.
This temperature change creates a density gradient in the fluid, causing the endolymph to move via a convection current. Warm stimulation causes the fluid to rise, while cool stimulation causes it to fall, mimicking the natural fluid movement that occurs during a head turn. This artificial flow deflects the hair cells within the canal, which are the sensory receptors that send signals about head movement.
The resulting signal triggers an involuntary, rhythmic eye movement known as nystagmus, which is the measurable response of the test. The direction of the eye movement is predictable: warm stimulation causes the nystagmus to beat toward the stimulated ear, while cool stimulation causes it to beat away. By observing and measuring this thermally induced nystagmus, clinicians evaluate the integrity and responsiveness of each individual inner ear.
The Steps of the Procedure
The patient typically lies on an examination table with the head elevated approximately 30 degrees. This position aligns the horizontal semicircular canal vertically, maximizing the effect of the thermal convection current. Prior to stimulation, the clinician ensures the ear canals are clear of wax or debris, as obstruction could interfere with temperature transfer and invalidate the results.
The patient is fitted with specialized video goggles or electrodes to record eye movements in a vision-denied state, preventing the suppression of involuntary nystagmus. The procedure involves irrigating the ear canal with a medium, typically warm and cool air or water, for 30 to 60 seconds per stimulus.
The standard protocol, called the bithermal caloric test, involves applying four stimuli: warm and cool temperatures to the right ear, and then warm and cool temperatures to the left ear. During each irrigation, the patient experiences a brief, temporary sensation of dizziness or vertigo, which is a normal reaction to the inner ear stimulation.
To keep the brain engaged and prevent the patient from actively suppressing eye movements, the clinician administers alerting tasks, such as answering simple trivia questions. Following the irrigation, the recording system captures and measures the speed and intensity of the induced nystagmus. A rest period of about five minutes is maintained between each of the four stimulations to allow the vestibular system to fully recover.
What the Results Indicate
Analysis of the caloric test focuses on the speed of the slow component of the induced nystagmus, which reflects the strength of the vestibular nerve signal. The responses from the four individual stimulations are mathematically compared to establish patterns of function. A normal response is characterized by roughly equal and robust reactions from both the right and left ears.
One of the most significant abnormal findings is unilateral weakness, or canal paresis, where the total nystagmus response from one ear is significantly reduced compared to the other. A difference in responsiveness greater than 22 to 25 percent is generally considered abnormal and suggests hypofunction in the inner ear or vestibular nerve on the weaker side. This finding is highly effective for localizing a lesion caused by conditions like vestibular neuritis or Meniere’s disease.
Another parameter assessed is directional preponderance, a tendency for the nystagmus to beat more strongly in one direction regardless of which ear was stimulated. Directional preponderance can be caused by a pre-existing, spontaneous nystagmus that shifts the baseline measurement. While less specific than unilateral weakness, this finding can be associated with certain disorders or central nervous system involvement. The results allow the clinician to determine if the balance system is functioning symmetrically, and if not, where the asymmetry is originating.