It can be carried out and recorded using conventional means. In this section, we do not review the protocols or how the results are presented as per the classic "Freyss butterfly". However, we believe that certain considerations should be taken into account with this test.
If the caloric test shows us the compensation of the RVO with the disappearance of the spontaneous nystagmus and directional preponderance, it enables us to see if and how the healthy labyrinth compensates the deficit of the unhealthy one. Furthermore, if it obvious that with the caloric test, each side should be examined separately and for this we regularly use a couple of sensors that are complementary to each other.
Even after using the videonystagmoscopy as part of the daily routine, for each patient we are still surprised to note the stereotypic nature of the responses of a subject suffering from a vestibular disorder. Conversely, in the case of a normal subject, there is a considerable difference between individuals. One of the explanations might be that: the brain of a normal subject enjoys a great freedom with regards to its choice of information source. All this information becomes redundant, by learning these tasks, so that after one year, the brain is able to choose and use the most relevant, the most suitable, and the most effective information when carrying out a task respecting the proviso that they must use as little energy as possible.
With a patient, things are different, the brain either hides the unsuitable information and only uses that which remains, or uses a copy, a schema that has always worked and the few times that the default system is called into action, there will be little more than some discomfort, or conversely they might find themselves in the midst of a panic attack with complete chaos ensuing. It therefore uses everything available without adhering to any of the existing hierarchy and with a huge expenditure of energy. This hypothesis would explain the wide variety of means of compensation. This closely correlates to the needs, and to the difficultly of the tasks to be carried out. It would also explain a whole series of symptoms that patients expressed, symptoms that all authors of this subject have written about: fatigue, sensation of weakness, anxiety, behavioural changes, less automatic and more deliberate, etc.
All healthcare professionals passionate about balance would like to have a localisation test. It is generally written that kinetic testing on a rotatory chair checks both labyrinths at the same time while caloric testing checks them separately. Is this absolutely true?
As far as caloric testing is concerned, whatever method is used: convection current, the thermal gradient between the two ends of the canal, direct stimulation of the sensory cells, it does not hamper, in situations where @
the response is a left-beating nystagmus. Or in both cases, the warm ear is the left one. The latter is therefore the most active, the most tonic. In both cases, the “bias” expressed earlier, presents towards the right. It is therefore logical for there to be a slow right-beating phase and a quick left-beating phase. The latter, in an attempt to recover its “straight ahead” point of reference.
This theory however contradicted the observations from Conraux’s monothermic caloric test. As a reminder, it is the reversal of the direction of the nystagmus following the irrigation of a single ear at the same temperature while moving the head from a "forward-leaning" to a "backward-leaning" position.
Irrigation makes no difference, the stimulated canal is the same however the nystagmus changes direction as if warm irrigation had been replaced by a cold one. What changes with the position of the head? The effect of the gravity vector on the otolithic system. Take, for example, a warm irrigation of the left ear: according to the above theory, the lean of the “straight ahead” motion is towards the right. When the head is leaned forward, the gravity vector does not change the direction or the “straight ahead” reference point. Conversely, again and again caloric stimulation makes the “straight ahead” reference lean towards the right-hand-side of the head. But as the referential has not changed, we have, if you like, moved from one side of the “equator” to the other.
We therefore find ourselves in a situation comparable to that of water, which when exiting the plughole, rotates in a clockwise direction in the northern hemisphere, but rotates in the opposite direction in the southern hemisphere. The “straight ahead” reference point still leans towards the same side, with regard to the gravity vector, but the head has moved from the “upwards-facing” to the “downwards-facing” position. It is therefore logical for the nystagmus to change direction.
It would therefore not appear to be a convection current issue. This is in keeping with observations made under zero gravity conditions.
It should not be forgotten that no test is perfect. The caloric test does have its shortcomings. It is not physiological, it operates at an excessively low frequency (0.03 Hz), and as far as its symmetry is concerned, it is totally dependent:
- on the ability of the tissues to transmit the temperature to the nerve endings,
- on the symmetry of the temperatures at the bottom of the duct,
- on the lack of an air meniscus between the surface of the water drop and the surface of the tympanum.
Still on the condition that there are no anatomical or morphological differences the two outer ear canals...
So what do we expect from these tests? It would appear that we can expect no more than a quantification of the system’s operational functioning at the reflex arc.
For the response to be unadulterated, we must get rid of all factors that might modify the response physiologically. We believe that the first margin of error is visual feedback.
Can we in fact consider the control of the retinal slip to be standard, constant and identical for all induced eye movements? It is therefore important that all stimulations be carried out in complete darkness. For the response itself, we are gradually coming to the realisation that the frequency measurement is unreliable. Frequency can be the result of a specific strategy for each subject, and presumably dependent, not only on the state of awareness, but also on a series of external cortical factors.
Standardisation when carrying out and measuring a caloric stimulation therefore becomes very delicate. Is the reading of the response for thirty seconds after irrigation has been stopped really relevant?
We soon realise when reading through all of the responses registered during the videographic session that culmination is not always reached after a period of 30 seconds. Furthermore, it should not be forgotten that the formulae for directional preponderance and hypovalence should be used with the maximum eye speed of the slow phase. Is using them with a response at frequency still relevant in this case? It would appear that the true measurement of caloric testing should be observed by recording the eyes in the dark using a Vmax (maximum speed) of the slow phase. What E. Ulmer did with the use of his "radios".