A new parameter of vestibular sensitivity

Caloric irrigators: air, open-loop water and closed-loop water

Selective stimulation of the semicircular canal system of one ear by establishing a temperature gradient is still widely used in the clinic in the caloric test. The results obtained are noted for their imprecision and variability. This is, in part, due to a lack of uniformly applied standardized test protocols, as well as difficulties in applying a precise stimulus. The relative merit of the three types of irrigation are reviewed. Whilst some authors argue that both the closed-loop and air irrigators produce even more variability of response than open water systems, the present author believes that intratest replication studies indicate that the closed-loop irrigation stimulus is the least variable and best tolerated by the patient. In perhaps one-fifth of patients, response to the first irrigation is much stronger than subsequent stimulus applications. It is suggested that there may therefore be a case for routinely performing five irrigations in the bithermal caloric test, including a replication of the first irrigation. It is assumed that psychological factors, especially anxiety, may explain the phenomenon of enhanced initial response, and some of the variability of caloric test responses in general. Use of the closed-loop irrigator is advocated for ease of use, reduced routine maintenance and to minimize patient distress, thereby reducing anxiety and result variability.

Post caloric nystagmus: an analysis of culmination frequency and maximum slow phase velocity

Neurons of the vestibular nuclei receive substantial afferent inputs from various parts of the cerebellum and brainstem. The efferent output of the vestibular nuclei thus reflects the interaction of these connections. This study investigates the influence of lesions of the posterior neuro-axis (PNA) on thermally induced nystagmus in 70 such patients. The caloric responses of all these patients were analyzed automatically with respect to culmination frequency (CF), but an automated analysis of SPVMax was possible in only 36 of these patients. Analysis of both sets of data showed that the coefficient of variation of SPVMax was significantly greater than that of CF. Vestibular decruitment, which correlates well with PNA lesions, was noted in 83% of cases when CF was used. In contrast, when SPVMax was used, vestibular decruitment was noted in only 53% of patients. Considering the low coefficient of variation of CF and considering that vestibular decruitment is noted with greater reliability when this variable is used, it is recommended that caloric nystagmus should be analyzed with respect to frequency rather than SPVMax.

Air caloric test with continuous thermal change

A new technique was designed for vestibular testing with an air caloric stimulator. With this technique, the temperature threshold necessary to induce caloric nystagmus was measured as air temperature decreased at a constant rate (from 37 degrees C). As a pilot study, an air caloric test with continuous thermal change was done at 6 different rates of decrease: 0.01, 0.03, 0.05, 0.1, 0.15 and 0.2 degrees C/s. The rate of 0.05 degrees C/s gave the smallest standard deviation for temperature threshold in normal subjects. This deviation had the narrowest normal limits of all ordinary caloric tests when the coefficient of variation was compared (standard deviation/mean x 100). No discomfort was observed during or after the air caloric test with continuous thermal change at this rate.

Clinical evaluation of the monothermal caloric test

The Torok Monothermal Caloric Test has been described as offering a strong advantage over bithermal caloric testing because it can distinguish labyrinthine from retrolabyrinthine lesions. Weak and strong irrigations of room-temperature water are administered to each ear and the ratios of strong to weak nystagmus responses compared to those of normal individuals. Ratios that fall below normal are said to represent decruitment and to denote retrolabyrinthine disease. Ratios that exceed normal are said to represent recruitment and to denote labyrinthine lesions. We attempted to verify the clinical value of the Monothermal Caloric Test by examining patients with known labyrinthine (N = 9) and retrolabyrinthine (N = 25) lesions. Ten patients with dysequilibrium and nonlocalizing testing were also examined. The test was performed exactly as described by the originator. Decruitment was found either unilaterally or bilaterally in 56% of patients with retrolabyrinthine lesions. However, it occurred on the side of the lesion in only 42%. Decruitment was also found in 47% of patients without retrolabyrinthine lesions, including 63% of those with unilateral Meniere's disease. Recruitment was never found on the side of a labyrinthine lesion. A description of the test and a summary of our results are presented. We conclude that the Monothermal Caloric Test is moderately sensitive to the presence of retrolabyrinthine lesions, but lateralizes poorly and is not highly selective. The test was found to be insensitive to the presence of labyrinthine disease.

Vestibular recruitment in Meniere's disease

The aim of this study was to search for vestibular recruitment in a sample of patients with Meniere's disease. Recruitment was defined as an abnormal growth of response with increasing stimulus intensity. Twenty-nine patients were tested with sinusoidal rotation of three different magnitudes at four different frequencies. We also searched for auditory recruitment in each patient via tests of auditory brain stem responses, acoustic reflexes, and loudness balance and discomfort level. Analysis of vestibular responses indicated, on average, a linear relationship between stimulus magnitude and response magnitude, ie, doubling the stimulus magnitude resulted in twice the response magnitude. Meniere's patients did not yield results significantly different (although they were more variable) from those of the normal subjects. The vestibular responses of patients with auditory recruitment did not differ systematically from those of patients without auditory recruitment. We conclude that vestibular recruitment, if it exists in patients with Meniere's disease, is not demonstrated by sinusoidal rotational testing.

Central vestibular signs, posterior fossa pathology and computerized tomography-regional blood brain circulation

Central vestibular signs are sensitive indicators of lesions in the posterior fossa. With the availability of computerized tomography and computerized tomography-regional blood brain circulation, the identification of suspected retrolabyrinthine lesions has improved considerably. This is a report about the reliability of central vestibular signs in predicting posterior fossa lesions.

Vestibular recruitment and decruitment

Serial Thermal Vestibulometry with the application of increasing caloric stimuli (corresponding to decreasing temperature of water at successive irrigration of the ears) was performed on normal subjects and patients with lesions in the central nervous system involving the statoacoustic subsystem. The normal values and 'slope' values (i.e.increase in degrees of velocity of slow component of nystagmus in response to each 4 degrees C change in water temperature) were computed to yield values of the normal vestibulogram. The values obtained in patients were compared with those of the Normograms and the following diagnostic patterns observed: (a) Hypograms, (b) Hypergrams, (c) Central Vestibular Decruitment. Central Vestibular Recruitment has been explained on the basis of the locus of the lesion leading to involvement of the nystagmogenic area in the lower part of the reticular formation, suprasegmental neurones in the vicinity of the vestibular nuclear complex, and partial destruction of the complex itself, each being responsible alone or in combination. Hypergrams presumably result from suprasegmental lesions, e.g. in cerebrum and stem components. Central decrittment follows neural lesions in the vestibular subsystem. Hypograms are found in vestibular nuclear pathology. Furthermore, it helps in charting a plan for therapy, viz. head and balance exercise, in particular, in the sense that his exercise is not helpful in cases with central decruitment but is in peripheral one. An explanation for this has been put forward.

Vestibular decruitment in central nervous system disease

The differential caloric test consisting of a weak and of a strong stimulation may provide evidence of certain vestibulo-oculomotor abnormality. The phenomenon of vestibular decruitment serves as an indication of central nervous system or intracranial pathology. A classification of the degree of decruitment was attempted in 139 patients exhibiting this phenomenon. Ample evidence is provided that vestibular decruitment is indicative of a variety of intracranial lesions predominantly in the posterior fossa. Decruitment was encountered with hypo- and hyperactive vestibular responses alike and also in patients in whom there was no deficit in end organ sensitivity. The armamentarium of neurotologic testing is enriched by the differential caloric stimulations providing an indication of the "site of lesion."