Abstract
The ‘duration’ of induced nystagmus was the first parameter used to quantify vestibular function during caloric testing. However, later work suggested that there was a poor correlation between this measure and the physiological events occurring during caloric testing. This, coupled with the development of electronystagmography and the value of a permanent record of the caloric responses, led to the widespread use of the ‘slow-phase velocity’ measurement of induced nystagmus, as the most commonly used parameter for assessment of the caloric test. Despite a wealth of literature, the ‘best’ parameter for measuring vestibular function using the caloric test remains undefined and the duration and slow-phase velocity parameters appear to provide different information. The strengths and shortcomings of these two parameters and their relative values are reviewed.
There can be little doubt that the slow-phase velocity measurement of the caloric response is the parameter which is more relevant physiologically and bears a closer relationship to the intensity of the thermal stimulus. Furthermore, with the widespread use of electro-nystagmographic techniques, the analysis of slow-component velocity of caloric-induced nystagmus has become increasingly common and offers several obvious advantages over direct observation of the eyes. It provides an objective permanent record of events and enables the examination of the caloric response in the absence of optic fixation. Moreover, the marked enhancement of the response in the absence of optic fixation facilitates computer display and analysis of the slow-component velocity.
Nonetheless, whatever the advantages of electronys-tagmographic recording of the caloric response, certain problems exist that have not been fully resolved. In order to obtain a sufficiently large response for analysis by electronys-tagmography, optic fixation must be removed. The most convenient way of achieving this is by simple eye closure, but this has been shown to be unreliable. In some subjects, Bell's phenomenon (the elevation of the eyes behind closed lids) may result in inhibition of the nystagmic response. In addition, sleep mechanisms may be brought into play via the reticular formation; consequently, it is vital to maintain alertness during the caloric test if accurate nystagmic responses are to be obtained. The technique of carrying out mental arithmetic, therefore, is commonly employed.
Recording with eyes open in the dark is more reliable, but in practice it is also difficult to carry out. The corneo-retinal potential is heavily dependent upon light intensity reaching the eye and in darkness may show appreciable variations. Measurement of the slow-component velocity is of course dependent upon stable calibration and such variations in the corneo-retinal potential impose serious limitations upon recording accuracy. This in part may explain Henriksson's (1955) findings that the slow-component velocity is a more variable parameter than duration, and may also explain reports of an unacceptably high test/re-test variability. In addition, it should be stressed that Hood and Dix (1973) have emphasized that electronystagmographic measurement of the duration parameter of caloric-induced nystagmus in the absence of optic fixation is unreliable, as may readily be demonstrated in Fig. 3. For many years, virtually all studies of duration have used electronystagmographic recording (Hamersma, 1957; Stahle, 1958; Uemura et al., 1977), and if the above reservations are accepted then the value of the duration parameter using this technique must be questioned.
The vexed question of whether the direct observation technique using duration or the ENG technique using slow-component velocity is the better method of performing the caloric test remains unanswered. In Fig. 4 from Hood and Korres's work (1979), it is quite clear that the slow-component velocity and duration vary in quite different ways. Thus, it is not surprising that in the study conducted by Vesterhauge et al. (1982) of 50 normal subjects and 130 patients with a variety of vestibular disorders, there was no definite correlation between the duration parameter using the direct observation method with the Fitzgerald-Hallpike technique and the slow-component velocity parameter measured using electronystagmography.
In conclusion, it is clear that measurement of these two parameters provide entirely different information and the need for a definitive study to establish the advantages and disadvantages of each parameter using each technique is paramount. It may transpire that, owing to the complex nature of the generating mechanisms of thermally-induced vestibular nystagmus, information obtained from both parameters may prove to be of more value than either parameter alone.