Caloric testing is an elegant method for evaluating the integrity of the vestibular apparatus of each ear,
independently. Caloric testing is carried out most simply by irrigating the external auditory canal (observed by otoscope to be unobstructed by wax, not infected, and with no tympanic perforation) with water warmer or colder than body temperature, the presumed resting temperature of the labyrinths.
The differential warming or cooling of the horizontal semicircular canal where it lies closest to the external auditory canal causes a decrease or increase, respectively, in the specific gravity of the endolymph at that point. If the head is positioned so that the horizontal canal is vertical (see the
position of the canal in Fig. 6-3), significant convection currents are caused in the canal by the induced changes in specific gravity (Figs. 6-4 and 6-5). Vertical upward currents are caused by warming because of the decreased specific gravity and, with the patient supine, the current in the horizontal canal is toward the ampulla and crista (see Fig. 6-5). This direction of flow is excitatory to the crista, causing increased firing over the pathways diagrammed in Figures 6-1, 6-2, and 6-5. This results in vertigo (in which the patient feels that they are spinning toward the ear being irrigated with warm water), and there will be VOR-induced reflex movement of the eyes away from that ear. If the patient is awake and alert, the drift of vision that is produced by the VOR will result in a rapid corrective "jerk" of the eyes to try to keep them focused on a target. Nystagmus is the result (remember, nystagmus is named by the direction of the fast phase). Nystagmus "to the right" means nystagmus with the fast component to the right. In order to maintain clarity, many examiners use the term "right-beating" to clarify that they are referring to the direction of the fast phase. Cold-water irrigation will have opposite effects. With the horizontal canal in the vertical position (i.e., patient supine with their head on a slight pillow), cold-water irrigation increases the specific gravity of the endolymph closest to the external auditory canal.
Therefore the fluid sinks and a current is created away from the crista/ampulla (see Fig. 6-5). This decreases the spontaneous firing of the ipsilateral horizontal canal vestibular system and causes an
imbalance with the resting tone of the opposite horizontal canal system becoming dominant. The eyes are thus driven tonically toward the irrigated side and the checking or fast component is opposite in direction. This same nystagmus (and concomitant vertigo) is seen in persons with destructive lesions of the vestibular apparatus (see Fig. 6-2).
In performing caloric tests with warm water, 20 cc of approximately 48 degrees C water (higher temperature is painful) is irrigated into the external auditory canal, which should be clear of wax, uninfected, and with no tympanic membrane perforation (Fig. 6-6). Each auditory canal should be irrigated separately for the same duration (30 seconds is convenient), and the time of onset of
nystagmus from the beginning of irrigation, as well as its duration and direction should be recorded.
The findings from the two sides should be compared; a difference of approximately 20% is considered significantly abnormal. At least five minutes should elapse between irrigations to allow the stimulated canal to return to body temperature. The patient should be asked whether s/he is experiencing spinning sensations or nausea and whether there is a difference between the two sides. If there is less vertigo on one side, you must consider that there is hypofunction of the inner ear on that side.
You may have already surmised that vestibular-oculomotor testing has considerable diagnostic usefulness in the unconscious patient since it is objective and not dependent on patient cooperation.
The vestibular-oculomotor reflex pathway encompasses an expanse of the brain stem (upper medulla through mesencephalon) that contains much of the reticular formation necessary for the maintenance of consciousness. Caloric testing is good at assessing the integrity of the brain stem (see Chap. 24).
There are two basic causes of depression of consciousness: diffuse bilateral hemispheric dysfunction; or dysfunction of the brain stem reticular formation (patients can have both). Caloric testing provides a method for rapidly screening to determine which of these causes is producing the depressed
consciousness.
From our discussion of the mechanisms of the VOR it can be surmised that the patient with an intact reflex has an intact brain stem. Also, since the fast phase of nystagmus is mediated by activity in the cerebral cortex, a vestibulo-ocular reflex with tonic eye deviation but no fast, corrective movement, indicates that the brain stem is intact and that the cause of depressed consciousness is diffuse cortical depression. This is most often related to toxic, metabolic or drug-related effects. This occurs because the brain stem response is more resistant to these effects than is cerebral cortical function (of course, if brain activity is sufficiently depressed by toxic or metabolic upsets, even the brain stem can be
ultimately affected). In cases of encephalopathy (i.e., depressed consciousness due to diffuse cerebral cortical suppression), caloric irrigation thus elicits only tonic deviation of the eyes. Warm caloric irrigation causes tonic conjugate deviation of the eyes to the side opposite the irrigation, and cold irrigation elicits deviation of the eyes toward the irrigated ear (Fig. 6-7A).
An interesting and important observation is the finding of normal oculocephalic test results in the patient who is apparently in "coma". The normal slow component of nystagmus indicates the integrity of the brain stem and the normal rapid phase indicates that the cerebral cortex is awake, alert and functional. Therefore this "coma" is actually fictitious and the patient is more appropriately labeled as
"catatonic".
Brain stem damage produces variable effects on the reflex depending on the location of the reflex. For example, a destructive process (e.g., infarction, hemorrhage or tumor) at the midbrain level involves the oculomotor complex with subsequent loss of the medial rectus portion of conjugate horizontal
deviation, with preserved lateral rectus deviation during irrigation (Fig. 6-7B). A bilateral lesion of the pons, involving the abducens nuclei and the proximate medial longitudinal fasciculi, destroys the
vestibular-oculomotor reflexes entirely (Fig. 6-7C). What effect would be seen after complete transection of the basis pontis sparing the tegmentum (see Figs. 4-5 and 6-1)?
"Doll's eyes"
The poorly named "doll's eye" maneuver is a simple mechanical test that is particularly useful in the patient with depressed consciousness. More appropriately called the oculocephalic maneuver, it is composed of a rapid passive rotation of the head laterally, which causes an inertial flow of the
horizontal canal endolymph in the opposite direction of the head rotation. As seen in Figure 6-8, the eyes are driven in a direction opposite the head rotation.
If the patient is awake, the hemispheric checking component (this has the same substrate as the fast component of the nystagmus) keeps the eyes from deviating from midposition and actually may drive the eyes beyond the midposition toward the direction of turning. If the patient is in a coma due to bilateral hemispheric suppression, such as with toxic or metabolic disease (e.g., sedative overdose or uremia), the checking component (also the fast component of nystagmus) is lost. In this case, the eyes deviate away from the direction of head rotation in an unchecked manner (the reflex response is not inhibited by cerebral cortical input). Of course, if dysconjugate gaze is produced during the maneuver, damage to the brain stem in areas that control brain stem extraocular function must be assumed.