The caloric test is a part of the ENG. It is an attempt to discover the degree to which the vestibular system is responsive and also how symmetric the responses are, between left and right ears. 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 to deduce the function of each ear independently of the other. The rotatory chair is a better test to discover the function of the both inner ears together. The VHIT test is somewhat of a compromise -- one can often determine the bad ear, but not as well as the ENG.
Caloric testing in the USA is dying out. The reason is that doing caloric testing is generally a financial loss for the outpatient medical facility, because it takes a long time (often more than an hour), it requires an expensive piece of machinery, it requires a highly trained individual to do the test, and because many large insurance companies and Medicare pay very little for the caloric test. If you are fortunate enough to find an outpatient facility that still does good quality calorics, you are lucky !
The caloric response was first described in by Robert Barany in 1906. His findings were immediately considered pivotal and Barany received the Nobel Prize. The key observation that led to his Nobel prize was made while he was irrigating out ear wax, after a patient complained that the water was the wrong temperature. He noticed that the eyes went different directions for warm and cold water irrigation. This shows the value of being mindful.
The general idea of the caloric test is that heat or cold changes the density of the water-like endolymph in the semicircular canals. The change in density then changes the balance of force across the cupula, and this change of force causes neural firing resembling a constant acceleration. This concept took a very long while for people to adopt, as explained by Rey-Martinez et al (2017). Barany thought that the cupula was somewhat like a swinging door. Simple but wrong as the cupula blocks the canal. The obvious explanation is just force. This also goes under the name of buoyancy.
Unusual ideas about the caloric mechanism.
The space lab experiments showed that there is some caloric response in 0 gravity, suggesting that buoyancy cannot be the entire answer. This resulted in the idea that the pressure was created by expansion or contraction of the volume of fluid from heat or cold. (Scherer et al, 1985). Of course, if this were true, then the position of the head should not matter with respect to gravity, which is certainly not the case. It is fairly clear that there is a direct temperature effect on the nerve, but this is not a very large component.
Still, many other ideas have been proposed including "local endolymphatic flow". Some have even proposed that heat/cold causes changes in middle ear muscles. These all seem dubious to us. Valli et al (2002) concluded in frogs that "Only the predictions of the model based on buoyancy were fully consistent with the experimental results whereas those provided by the other models were not."
Recently there have been attempts to explain mismatch between VHIT and caloric tests in Meniere's disease due to "vortices" predicted by mathematical modeling (Rey-Martinez et al, 2017). We find this difficult to follow.. We think that the right approach to this would be to start with a simple mechanical model (perhaps just compare a small garden hose to a large garden hose).
Most caloric tests nowadays are done using a computerized system as shown below. The computer analyzes the caloric data, computing peak slow-phase velocity.
|ENG system as was used in the 1960's when we had CRT monitors (Courtesy of ICS medical). Now ENG systems are smaller. Oddly enough this illustration shows someone sitting behind the console. Practically, people are generally on an exam table with a back portion that tilts. We also no longer use CRTs.|
The water bithermal caloric test consists of 4 sections 2 ears * 2 temperatures (warm and cold). Ideally this is done with warm and cold water. Ice water is a potental added option (this adds about 15 minutes to the procedure). Most patients do not need ice water, which is triggered by no response to warm/cold water. Sometimes tests need to be repeated -- so it is best to build in some "slack" into the caloric schedule.
|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.
Patients should be instructed prior to the caloric test to ensure that they do not take inapppropriate medications.
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 the individual takes medications anyway, the person doing the test should note this on the report so that the persons attempting to use this result for diagnosis will be aware that responses may be depressed.
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.
Follow the link above for the details. Basically, the caloric test has a very wide range of values accepted as normal.
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:
- Temperature difference between the irrigant and the inner ear (presumably body temperature)
- 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.
- 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)
- Bone is the main heat conductor
- Wax is a good insulator.
- 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 ear 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.
While it is unusual to record the caloric response long enough, if one waits long enough (i.e. several minutes), a tiny "reversal" phase to the caloric response can be seen in normal subjects. This is generally thought to be due to adaptation processes in both the hair cells of the inner ear as well as centrally, and follows the same general train of logic of reversal phases seen after rotational stimuli. One can also elicit a similar reversal by removing gravity from affecting the lateral canals, through positioning the person back to upright. This technique has some intrinsic problems involving adding another stimulus (the tilt itself), problems with accurate positioning as well as knowledge of the canal plane in any particular individual, attempting to measure tiny amounts of nystagmus when just a small error in positioning could result in nystagmus, and potential contributions of other canals. In other words, this data is pretty hard to interpret in any rational framework.
Well anyway, Ichijo recently reported a study of 12 healthy humans and used an unusual terminology suggesting that the "second phase" was the nystagmus seen after the patient is repositioned to upright to make the lateral canals truly horizontal, and that the "third phase" was the nystagmus resulting after returning the subjects to supine (2015). We are not at all sure why one would use this nomenclature, that confuses the situation where "secondary nystagmus" is typically used for the nystagmus that occurs if you wait a long time after a stimulus (in the same position). Similar results regarding the repositioning maneuver have been reported by others (Wu et al, 2000; Aoki et al, 2006). Ichijo suggested that this secondary phase was related to perilymph pressure. A far more conventional interpretation was that of Gursel and Oosterveld (1983), who suggested that it was due to adaptation. The interested reader is referred to conventional expositions of the secondary vestibular nystagmus, such as can be found in Wilson/Melville Jones classic textbook, as well as the literature concerning caloric responses in outer space.
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. Schuchman and Uri (1986) concluded that "The results were characterized by large inter-subject as well as intra-subject variability. It is cautioned that diagnostic categories should not be inferred on the basis of absolute numeric values of FS."
Another intrinsic 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 caloric 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.|
|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).|
|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.|
|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.|
Completely absent caloric responses, documented with the ice-water/prone test.
This patient has NO caloric response to regular temperature. With ice water, there is a low-amplitude LB nystagmus that does not reverse direction with prone. This shows that there is a 100% vestibular loss.
The Prone test is helpful when one is attempting to determine whether or not there is any response at all. The mechanism is that caloric nystagmus will reverse prone, while spontaneous nystagmus will usually be unaffected.