CALORIC TEST

Timothy C. Hain, MD Page last modified: January 21, 2008

The caloric test is a part of the ENG. It reflects an attempt to discover the degree to which the vestibular system is responsive and also how symmetric the responses are, between left and right. It is a test of the lateral semicircular canals alone -- it does not assess vertical canal function or otolithic function. While not as good as we would like, the caloric test is the best that we have.

History:

The caloric response was first described in by Robert Barany in 1906, and more completely in a book on the vestibular system in 1907. His findings were immediately considered pivotal and Barany received the Nobel Prize. However, his colleagues at the Vienna University Clinic were less enthusiastic and suggested that he took the ideas of others and presented them as his own. Barany worked with Alexander, Heinrich Neumann, and Politzer -- all otologists at the time. Alexander suggested that he rather than Barany first described the fistula test, and also that he had jointly described with Barany, ocular counter-roll. Alexander is now best known for his description of the effects of gaze on spontaneous nystagmus, Alexander's law. Neuman claimed that he had been critical in discovering the mechanism of the caloric response. (Baloh, 2002). Neuman became head of the Vienna Otology clinic after Politzer retired. Despite being awarded the Nobel prize, Robert Barany lived a quiet and even lonely existence in Uppsala Sweden after WW1. He died of cerebral hemorrhages due to malignant hypertension.

Practical method

Most caloric tests nowadays are done using a computerized system as shown below. The computer analyzes the caloric data, computing peak slow-phase velocity over time.

Contemporary ENG system (Courtesy of ICS medical)	Balloon version of caloric (one of the variant caloric tests, see below)

Variant caloric tests:

• Bithermal test (the standard)
• Monothermal test -- a single large bolus of ice water is given rather than two irrigations with hot and cold
• Air caloric -- air is used instead of water.
• Bilateral irrigation -- both sides are irrigated simultaneously
• Balloon test -- a water filled balloon is used instead of water.
• Ice water caloric -- used to confirm complete loss

There are several variant caloric tests, but at present, the standard is the bithermal caloric test. This is based on simple common sense. All of these variants have the advantage of greater convenience, but the disadvantage of lower accuracy than the bithermal test. When air is used, there is greater variability than water. When water is not used directly, as in the balloon test, heat or cold is not conducted as efficiently to the ear. When fewer irrigations are used, as in the monothermal test, there is less averaging of data and lower accuracy. Bilateral irrigation sacrifices knowledge of each ear's individual response in favor of speed. Because of the considerable inaccuracy even for the best of the lot, the bithermal water method, we think it is imprudent to adopt more convenient but less accurate methodology.

The caloric test is ordinarily performed with the subject reclining, head inclined 30 deg from horizontal so as to make the lateral canal horizontal. Water is introduced into the ear canal on one side, either 7 deg centigrade above or below assumed body temperature. The flow rate is such that the ear rapidly equilibrates with the water. The water is stopped after 30 seconds, and nystagmus is observed, while the subject is distracted (usually with tasks such as naming of animals, counting backwards, etc.). This is sometimes called "tasking", see following. Nystagmus commonly builds for about 30 seconds, then gradually decays away over roughly 2 minutes. After a rest of at least 5 minutes, the procedure is repeated with either the opposite temperature water, or on the other side.

Eye movements are usually recorded with either EOG or a video method, such as is shown on the graphic to the right. From the peak slow-phase velocity of nystagmus four numbers are obtained -- cold right, cold left, warm right, and warm left. These four numbers are used to compute three additional numbers:

Computations:

• Total response -- absolute sum of all appropriately directed responses. TR= (RC+LC+RW+LW). The goal is to detect bilateral weakness. The total response should be 20 or greater. When responses are corrected for spontaneous nystagmus, the procedure here is to sum the absolute value of responses that go in the right direction - -i.e. left cold and right warm should be right-beating, right cold and left warm should be left beating. One should NOT sum the signed responses, as this would not be a good measure of bilateral weakness -- a perfectly symmetrical recording of very strong responses would work out to a TR of 0. It is usually enough to simply add the absolute value of all responses, and to change procedure only when a response goes the wrong direction.
• Directional preponderance -- right-beating - left-beating/total. It should be 35% or less. There is little if any clinical value in DP.
• Unilateral paresis or RVR (relative vestibular reduction), RVR = (RC+RW-LC+LW)/TR. This is called "Jongkee's formula". It should be 25% or less. If all responses are appropriately directed (see commentary on TR above), one can simply take the sum of the absolute values of responses due to the right ear, subtract the sum of the absolute responses due to the left, and normalize to the sum of the absolute values of all responses.

Normal values:

• Total responses (TR) less than 20 are often associated with bilateral vestibular paresis (a rotatory chair test is needed). Responses less than 10 are highly suggestive of a bilateral reduction, such as due to gentamicin toxicity.
• relative vestibular reduction (RVR) > 40% is often associated with unilateral vestibular paresis, such as due to vestibular neuritis (there is no reasonable way of confirming a mild one). Paresis between 25 and 40% is suspect for the reasons outlined below and should be considered valid only if confirmed through other tests (i.e. Rotatory chair testing, clinical data).
• Directional preponderance (DP) has little if any meaning. A value > 35% is abnormal using the 5th percentile criterion.

There is considerable test-retest variability. The upper limits for test-retest variation for paresis were found by Proctor et al to be 24%. The upper limits for DP was 22%. This suggests that upper limits of normal should be set at greater than these values, which do not take into account variability of the normal population, and are therefore inappropriately low.

Tasking

There are many methods of distracting persons during the process of recording their nystagmus. Without distraction, responses can be suppressed which reduces validity. Tasks in which the subject produces a listing of items from memory seem reasonable and effective.

Mathematical Modeling:

There have been several attempts made to model the caloric response. The response is theoretically a combination, possibly nonlinear, of convection induced stimulation of the canal, a direct effect of temperature on the nerve, transduction responses in the mechanics of the cupula, adaptation responses in the nerve and brainstem, and other central processing effects, mainly including velocity storage. A descriptive curve-fitting approach to the response is exemplified by that of Formby et al (1992, 2000).

References:

• Baloh, RW. Robert Barany and the controversy surrounding his discovery of the caloric reaction. Neurology 2002: 58: 1094-1099
• Formby C, Carter RL, Hansen CA, Kuntz, LA. Measurement, analysis and modelling of the caloric response. 1. A descriptive mathematical model of the caloric response over time. Acta Otolaryngol (Stock) 1992:Suppl 498, 4-18
• Formby C, Kuntz, LA, Rivera-Taylor IM, Rivera-Mraz N, Weesner DR, Butler-young NE, Ahlers AE. . Measurement, analysis and modelling of the caloric response. Evaluation of mental alerting tasks for measurement of caloric-induced nystagmus. Acta Otolaryngol (Stock) 1992:Suppl 498, 19-29
• Formby C, Robinson DA. Measurement of vestibular ocular reflex (VOR) time constants with a caloric step stimulus. Journal of Vestibular Research 10 (2000) 25-39
• Proctor et al. Reference values for serial vestibular testing. Ann ORL 1996, 95, 83-9