8 Caloric Tests
8.1 Physiology
The semicircular canals are paired sensory structures responsible for the detection of angular acceleration. Each canal possesses a dilation at one end called the ampulla. Within the ampulla exists a saddle-shaped crista upon which sits a gelatinous cupula; the membranous canal is filled with endolymph. The inertia of the endolymphatic fluid with head movement means that there is a relative difference in the velocity of the canal and the fluid. This results in fluid being forced through the gap between crista and cupula and a deflection of the stereocilia. In turn, this causes either an increase or decrease in the resting tonic discharge, depending on the direction of deflection. The two labyrinths work in conjunction so that an increase in neural signals from one canal will be associated with a decreased discharge rate from the corresponding canal on the opposite side. As the three canals are mutually at right angles, complex three-dimensional movement information is provided.
8.2 Background
The caloric response can be used to test the integrity of this part of the balance system. It was first described by Robert Barany in 1906, for which he was awarded a Nobel prize in 1914. He postulated that altering the temperature of the endolymph sets up thermally induced convection currents. This fluid movement leads to stimulation of the stereocilia and consequent nystagmus and vertigo. Caloric testing was further refined in 1942 by Fitzgerald and Hallpike when they described a standardised bithermal caloric test, which remains an essential vestibular investigation to this day.
It has become apparent in recent years that thermal convection currents are not the only component in the caloric response. Positional alterations and the presence of a caloric response in microgravity have led to suggestions that a direct thermal effect on the sensory organs may account for as much as one-third of the response, although this in no way reduces the value of the test.
8.3 Procedure
The classic bithermal caloric utilises water at 30 and 44°C, kept at these temperatures in two heated tanks about 1 m above the test couch. The patient reclines on the couch at 30 degrees above the horizontal so as to bring the lateral semicircular canal into the vertical position. After checking that the external canals are clear of wax and debris and that the tympanic membranes are intact, cold water (30°C) is run into the left ear, via a siphon tube and 14-gauge cannula, for 40 seconds. A stopwatch is used to time the period from the start of this manoeuvre to the point at which the nystagmus stops with the patient fixating on a point on the ceiling. This procedure is repeated for the right ear and then for both ears with the warm water (44°C). Five minutes is usually left between each stimulus to allow for normalisation of the temperature in the ear.
Several variations on this basic theme exist. Very cold water (4°C) can be used as a single-temperature screening test and is used as part of the battery of testing for brainstem death. In patients with perforated eardrums, or other significant middle ear pathology, air may be used to supply the thermal stimulus, or water circulated through a closed system (typically at 50 and 24°C). This is not quite so reliable, or effective as standard water irrigation. Further refinements can be added using Frenzel’s glasses to remove optic fixation or measuring the nystagmus electrically (electronystagmography). In this case, the slow-phase velocity (SPV) of the nystagmus is measured.
Normally, optic fixation will suppress the nystagmus, such that it will reappear and persist for typically 20 to 60 seconds once optic fixation (and therefore vestibulo-ocular reflex [VOR] suppression) is removed. In those individuals who are frequently exposed to significant vestibular stimulation and who have developed a strong ability to use optic fixation to suppress their nystagmus (fighter pilots, ballet dancers, acrobats, etc.) they may show a shortened duration of nystagmus with optic fixation, but normal total duration without optic fixation. An alternative technique is to perform the procedure without optic fixation, but then introduce it during the test for 10 to 20 seconds. A normal response would be a reduction in the speed of the slow phase of the nystagmus at least 50%.
8.4 Interpretation
By definition, the direction of the nystagmus is described by its fast phase. Cold stimulation leads to nystagmus with the fast phase to the opposite side, while warm stimulation leads to nystagmus with the fast phase to the same side. This is easily remembered by the mnemonic COWS (cold opposite, warm same).
There are three parameters measured by caloric testing: canal paresis or weakness, directional preponderance and fixation index/fixation suppression.
Various formulae exist, based on the recorded times (or SPV) for each part of the standard caloric test, to predict the degree of vestibular activity. The results are given as a percentage figure.
Canal paresis is calculated using Jongkees’ formula:
