Caloric Test
The caloric test assesses the function of each horizontal semicircular canal separately by warming and cooling the inner ear. Warm or cool water or air is delivered into the ear canal, which sets up a gentle flow of fluid in the horizontal canal, and this produces a reflex eye movement (nystagmus). It is the only routine test that compares the two labyrinths one at a time, and it tests the low-frequency range of the vestibulo-ocular reflex.
Cold water makes the fluid in the canal flow away from the utricle (toward the ear), producing nystagmus that beats away from the cooled ear. Warm water gives the opposite, with nystagmus beating toward the warmed ear. The common memory aid is COWS: Cold Opposite, Warm Same.
Four irrigations are done (each ear with warm and cool stimuli) at 44 and 30 degrees Celsius, with water at 7 degrees higher and lower than body temperature, for 30 to 40 seconds, or with air at 50 and 24 degrees Celsius. Eye movements are recorded with infrared video goggles, and the peak speed of the slow phase of nystagmus is measured for each irrigation.
The test takes 30 to 60 minutes, because a pause of 5 minutes is needed between irrigations. A canal paresis (reduced response in one ear) is defined as a difference of 20 to 25% between the two ears. A response difference between the ears is found in up to 50 to 70% of patients with peripheral vestibular disease. The test is more sensitive than the head impulse test for mild losses, but it assesses only the lowest frequencies and the horizontal canals.
Indications.
- Vertigo and imbalance, to find whether the cause is in the inner ear or the vestibular nerve and to find which side.
- Unilateral vestibular loss after vestibular neuritis, labyrinthitis, trauma, or surgery.
- Suspected Meniere disease, vestibular schwannoma, or other cerebellopontine angle lesion.
- Bilateral vestibular loss (ototoxic drugs, autoimmune disease, idiopathic), shown by reduced responses in both ears.
- Assessment before ablative treatment (intratympanic gentamicin, labyrinthectomy), and monitoring after it.
- Unexplained dizziness with normal imaging.
- Part of brain death evaluation, in the form of a cold caloric test with ice water.
- Assessment of vestibular function recovery and compensation. Not used for acute vertigo with severe vomiting, unless needed, and not a test for benign positional vertigo.
Contraindications and precautions.
- Perforated eardrum, grommet, or a mastoid cavity: water is avoided because it can cause infection and severe vertigo, and air or a closed-loop water system is used.
- Active ear infection, discharge, or recent ear surgery.
- Wax that blocks the canal, which is removed first, since it dampens the response.
- Recent severe head or neck injury, or suspected cervical spine instability when the head must be positioned.
- Severe acute vertigo, unstable cardiac disease, or severe nausea, in which the test is delayed.
- Sedatives, alcohol, vestibular suppressants (antihistamines, benzodiazepines), and anticonvulsants reduce responses, and are stopped 48 hours before when safe.
- The patient needs to be warned about vertigo and vomiting, and a bowl and a helper are on hand. Fasting for 2 to 4 hours before reduces vomiting.
Preparation and technique.
Otoscopy is done to check the drum and canal. The patient lies supine with the head raised 30 degrees, so that the horizontal canal is vertical. In a darkened room with video goggles (to prevent visual fixation), spontaneous nystagmus is recorded first. Each ear is irrigated in turn, usually starting with the right ear warm, then the left warm, the right cool, and the left cool, with 5 minutes between. The maximum slow phase velocity during the response (reached at about 60 to 90 seconds) is measured.
The patient is kept alert by mental tasks, such as naming items, because drowsiness reduces nystagmus. Fixation suppression is checked by asking the patient to look at a light, and the response should fall by at least 50%.
