Caloric testing is a useful clinical tool that can assess and quantify the functional status of the individual vestibular systems.The test utilizes the mechanics of the vestibular-ocular reflex (VOR) to test for a unilateral peripheral deficit. The vestibular-ocular reflex requires an intact brain stem to function, and its purpose is to maintain eye fixation on a stationary target while the head is in motion, thus keeping the object of attention in the center of the visual field. As described below, caloric testing manipulates the vestibular-ocular reflex to test the lateral semicircular canals and their afferents specifically.
The semicircular canals are the three fluid-filled structures in the inner ear that provide the brain with spatial orientation. Each semicircular canal dilates into a fluid-filled sac called the ampulla which contains the sensory component of the vestibular system. When the endolymph is warmed (by air or water), an artificial current is created which moves the hairs in the lateral (horizontal) semicircular canal, thereby causing an imbalance between the right and left vestibular-ocular reflexes. This results in nystagmus with both fast- and slow-beating components, depending on the current in the ampulla. When a cold temperature is applied, it causes a fast-beating nystagmus in the opposite direction of the side being challenged and a slow-beating nystagmus in the contralateral side. The opposite goes for an applied warm temperature. The mnemonic COWS, cold, opposite, warm, same, makes it easy to remember this fast nystagmus component.
Caloric testing is clinically useful as a bedside test to isolate the peripheral vestibular system and rule out central etiology of vertigo. The advantage it has over other studies, such as the vestibular evoked myogenic potential and video head impulse test, is that it does not require head movement to be conducted, rendering better patient compliance in those patients whose symptoms worsen with movement, as well as in patients with limited cervical mobility.
Another indication for the use of this test is for brain stem testing in comatose patients. As described above, the reflex arc requires an intact brain stem, and therefore lack of nystagmus could indicate a brainstem lesion.
Of note, when there is a high suspicion of a peripheral lesion, bi-thermal caloric testing is typically performed. However, in cases in which there is a low pretest probability, it can be appropriate only to utilize mono thermic caloric testing and stop when the test is negative or in other words, responses are symmetric.
Medical use of pharmaceuticals that can affect the vestibular system (anti-emetics, anxiolytics, and antidepressants) within 48 hours of testing is a relative contraindication.
Water caloric irrigation system with two baths of 250 cc distilled water, heated to 44 C and 30 C, respectively.
Electro-oculography (EOG) or Video-oculography (VOG) (optional)
Catch basin that measures up to 250 cc
Provider plus one to two additional people to hold the catch basin, stopwatch, and/or dry towel.
Use an otoscope to examine the external auditory canal and ensure that there is no obstruction, infection, or cerumen impaction. Elevate the patient’s head to a 30-degree angle. This places the horizontal canal in a vertical plane, optimizing its stimulation. Place the catch basin beneath the patient’s ear. If using EOG or VOG, attach electrodes or place goggles over eyes, respectively. Advise the patient on the procedure and instruct them to perform an “alerting” exercise when the irrigation begins, for example, counting serial sevens out loud, naming animals, or listing words that begin with the same letter.
Using the irrigation system, deliver 250 cc of either the warm water solution over 25 to 30 seconds to the suspected affected ear. Allow an open system in which the water being delivered can freely dribble out of the external auditory canal and be collected in the basin. The nystagmus beats will occur approximately 30 seconds after the onset of the water delivery and will build in intensity over the ensuing 30 to 45 seconds. The alerting exercise that you have instructed the patient to do will prevent any suppression of nystagmus. Wait five minutes and repeat with the other ear. Repeat with cold water, if indicated.
If there is no response to either warm or cool irrigation, or if the bi-caloric irrigation system is not available, an ice-water irrigation can be considered. The patient is instructed to lie in the semi-recumbent position with the suspected affected ear turned up. Approximately 2 cc of ice water is injected into the ear canal via syringe and kept in that position for 30 seconds. The patient’s head is then turned midline and observed for nystagmus while the patient performs alerting tasks.
Nausea and vomiting are the most common side effects of caloric testing. Additionally, patients may experience worsening of vertigo. Long-term sequelae are not commonly associated with caloric testing.
A caloric test is a useful tool that can help a clinician differentiate a central versus peripheral lesion in the patient who complains of dizziness. Mono-caloric testing (MCT) using a warm medium can be used to triage patients in whom there is low clinical suspicion of a peripheral lesion. A negative unilateral response is one in which a SPEV, calculated as (affected - unaffected)/(affected + unaffected), is less than 10% to 15%; and subsequently, cold testing will not need to be performed. A unilateral weakness cutoff value of less than 15% in MCT has been found to be an appropriate cutoff value with a positive predictive value of 95% (Harvey et al.).
Typically, a warm medium is better tolerated by the patient and is the reason why it is tested first in the patient with a low pretest probability. Although warm water tends to be more commonly used, warm-air caloric testing has also been reported to be 87% sensitive with a negative predictive value of 90% when assessing for unilateral vestibular weakness when a cutoff of 25% inter-ear difference is utilized (Bush et al.). Despite this, a cold medium produces a more drastic response in the measured SPEV, and is, therefore, a more specific test for ruling in a peripheral lesion.
The views expressed in this article are those of the author(s) and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, or the United States Government.
I am a military service member. This work was prepared as part of my official duties. Title 17 U.S.C. 105 provides that “Copyright protection under this title is not available for any work of the United States Government.” Title 17 U.S.C. 101 defines a United States Government work as a work prepared by a military service member or employee of the United States Government as part of that person’s official duties.
Kelly A Murphy, LT, MC, NMCP; Daphne Morrisonponce, LCDR, MC, NMCP
Assessing for vestibular dysfunction by caloric testing can be a bedside test performed primarily by the practitioner, however may require additional assistance by nursing staff to properly and effectively perform. In addition, if Electro-oculography (EOG) or Video-oculography (VOG) will be utilized, special skill and training with this equipment is necessary and may require input and consultation of a trained audiologist.