CALORIC TEST

Page last modified: June 18, 2009

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 World War I. 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.

Contemporary ENG system (Courtesy of ICS medical)

Variant caloric tests:

• Bithermal -- hot and cold water are irrigated into each ear sequentially. The gold standard.
• Monothermal test -- a single large bolus of ice water is given rather than two irrigations with hot and cold. We advise against doing this.
• Air bithermal caloric -- air is used instead of water. Not a good idea either.
• Bilateral irrigation -- both sides are irrigated simultaneously. Not a good idea.
• Balloon test -- a water filled balloon is used instead of water. Even worse !
• Ice water caloric -- used to confirm complete loss. This is an excellent idea in selected patients.

Balloon version of caloric (one of the variant caloric tests, see below.) We strongly advise against doing this.

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.

Eye movement tracing during caloric in a patient with a mild bilateral reduction in responses. For the first 35 seconds, little is seen. In the next panel, a left-beating nystagmus gradually builds up. It begins to wane after roughly a minute, and at that point, fixation is attempted. This subject did not suppress the nystagmus very well.

The caloric test is ordinarily performed with the subject reclining, head inclined 30 deg up 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 30-60 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 above.

Decision making during the caloric test -- one cannot go "autopilot".

If no response is detected, or at least none greater than the spontaneous nystagmus, then ice water should be performed. This is done with the head in the standard position on the "dead" side, and then the person is turned prone so that the head is inverted. If there is a true caloric response, the waveform will reverse. If it is just spontaneous nystagmus, the nystagmus will not be affected. A possible pitfall of this methodology is positional nystagmus.

Similarly, if there is one "outlier" response -- the operator should attempt to resolve this -- usually by repeating it.

Computations:

From the peak slow-phase velocity of nystagmus four numbers are obtained -- cold right, cold left, warm right, and warm left. Spontaneous nystagmus (SN) should be subtracted from these, and then the absolute value taken. From these responses, LC, LW, RC, RW, three additional numbers are derived:

• 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.

If spontaneous nystagmus is not subtracted off first, peculiar results may appear, such as greater than 100% paresis (which is obviously impossible).

Normal values:

• Total responses less than 20 are often associated with bilateral vestibular paresis (a rotatory chair test is needed for confirmation). Responses less than 10 are highly suggestive of a bilateral reduction, such as due to gentamicin toxicity. These are very conservative values. The mean TR is 129 while the lower 5th percentile for the TR is 27.48, and upper is 601 (Sills, Baloh et al. 1977). These values do not apply to air calorics as the response is generally less and the variability greater. We have never encountered an individual with more than 600 deg/sec total response.
• Unilateral paresis > 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). Central vestibular lesions rarely cause severe caloric weakness because the root entry zone of the eighth nerve has a good vascular supply.
• Directional preponderance has little diagnostic 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.

Although difficult to decode due to presentation of logarithmically transformed data, the paper by Sills, Baloh et al. 1977 has considerable useful data concerning caloric parameters.

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 temperature differential induced convection 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).

The above is somewhat useless, as it boils down to just saying that it is very complicated. A more pragmatic way of thinking about it is to observe that the peak caloric response is largely proportional to the temperature differential across the lateral canal. The temperature differential depends on several things:

1. Temperature difference between the irrigant and the inner ear (presumably body temperature)
2. Thermal conductivity of irrigant (i.e. water is 10 times more conductive than air), and the part of the ear in contact with the irrigant.
   1. The ear drum (which has air behind it) doesn't account for much of the caloric response unless it happens to be full of fluid (usually not the case)
   2. Bone is the main heat conductor
   3. Wax is a good insulator.
3. Area of bone (not TM) in contact with water. Heat is conducted mainly through the bone of the external ear canal, not the ear drum. This is proportional to the radius of the canal in contact with the irrigant.

So, simple predictions are that bigger temperature differences cause bigger calorics responses, water is more reliable than air, and that wax plugs should greatly decrease caloric responses.

Currently caloric testing technology does not control for most of these variables - -body temperature is not measured, there is no adjustment for air vs water in reporting norms, there is no methodology of documenting that the tip went where it went and that the ear is free of wax, and there is no adjustment for the ear canal diameter. The lack of basic controls causes variability and reduces the value of the caloric test.

Fixation suppression

Fixation suppression is ordinarily evaluated by waiting till the caloric response is near peak, then allowing vision for 10 seconds, with instruction to fixate on a target. This is a close-to worthless test. The reason for this is that it is "all over the map" -- some patients fixate very well, some not at all. It depends on how nauseated they are, how well they can see without their glasses, and how cooperative they are.   A more formal way of saying this is that the scatter in fixation suppression is so large, that practically any value falls within the "normal" range.

Another intrinisic problem with fixation suppression is that it is dependent on the size of the caloric response.  It is much easier to suppress a 10 deg/sec response (such as due to an air caloric) than a 50 deg/sec response.  To do this properly, one would need norms scaled to the caloric response. Thus the values that are produced by conventional caloric equipment have no norms. Also one would need to adjust for visual acuity (which usually is greatly reduced in persons who take off their glasses for the test). As there are so many huge flaws with the fixation suppression test, it is a result that is reported but that knowledgeable clinicians ignore.

The rotatory chair fixation test is far better controlled.  It has the same problem though with visual acuity.

Examples of calorics:

Acute Unilateral Vestibular Loss

This patient has only spontaneous nystagmus on the left (about 6-8 deg/sec). On the right the traces are shifted downward due to the spontaneous nystagmus. The weakness calculation probably underestimates the amount of caloric weakness. (Figure courtesy of Dr. D. Yacovino). The most common cause of this ENG pattern is vestibular neuritis.

Another example -- a complete loss of calorics due to vestibular nerve surgery can be found here.

Recovery from vestibular loss

The same patient as above, one year later. Now there are clearly caloric responses on both sides, and the spontaneous nystagmus is gone . The notch in both traces at about 80 seconds shows that this patient has good fixation suppression (Figure courtesy of Dr. D. Yacovino).

Absent Caloric Responses

This patient has no measurable caloric response. The most common cause of absent caloric responses is poor ENG technique (such as use of air rather than water), and after that, aminoglycoside toxicity.

Nearly absent caloric responses, documented with the ice-water test.

This patient has very little caloric response. She had none at all with conventional temperatures. WHen ice water was used, she has a weak right-beating (similar to her spontaneous nystagmus), but it reverses to left-beating on prone. This shows that she does have a caloric response on the left.

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 modeling 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 modeling 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
• Sills, A. W., R. W. Baloh, et al. (1977). "Caloric testing 2. results in normal subjects." Ann Otol Rhinol Laryngol Suppl 86(5 Pt 3 Suppl 43): 7-23.