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.
The caloric response was first described in by Robert Barany in 1906.
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)|
|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.
Ideally subjects undergoing caloric tests should have no sedating medications for the last 24 hours. Sometimes this is difficult, as for example, when persons are addicted to medications in the Valium family. In this situation, usually 12 hours is sufficient. More data about medication effects is found here.
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.
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:
If spontaneous nystagmus is not subtracted off first, peculiar results may appear, such as greater than 100% paresis (which is obviously impossible).
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.
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.
|Quiz -- i.e. "what is your age", what is your favorite color.||Less effective||Fomby et al, 1992|
|Hand-motor task, clinician directed. Touch the thumb to finger as directed by clinician.|
|Alphabet task -- third letter of alphabet following a given letter randomly selected by clinician|
|Math task, Add or subtract a number given by clinician from a running total|
|Quiz task: Name colors, states in USA, cities in ...||Best task|
|Hand-motor task -- touch thumb to first finger once, 2nd twice, third three times, etc.|
|Alphabet task #2 -- every third letter in the alphabet|
|Math task #2 -- count backwards by 3's or 7's.||Less effective|
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:
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 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.
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.|
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