Stimulation of the vestibular receptor by means of step temperature changes during continuous aural irrigation

Association Between Saccule and Semicircular Canal Impairments and Cognitive Performance Among Vestibular Patients

OBJECTIVES

Growing evidence suggests that vestibular function impacts higher-order cognitive ability such as visuospatial processing and executive functioning. Despite evidence demonstrating vestibular functional impairment impacting cognitive performance, it is unknown whether cognitive ability is differentially affected according to the type of vestibular impairment (semicircular canal [SCC] versus saccule) among patients with diagnosed vestibular disease.

DESIGN

Fifty-four patients who presented to an academic neurotologic clinic were recruited into the study. All patients received a specific vestibular diagnosis. Forty-one patients had saccule function measured with the cervical vestibular-evoked myogenic potential, and 43 had SCC function measured using caloric irrigation. Cognitive tests were administered to assess cognitive performance among patients. One hundred twenty-five matched controls were recruited from the Baltimore Longitudinal Study of Aging to compare cognitive performance in patients relative to age-matched healthy controls.

RESULTS

Using multivariate linear regression analyses, patients with bilaterally absent cervical vestibular-evoked myogenic potential responses (i.e., bilateral saccular impairments) were found to take longer in completing the Trail-Making test (β = 25.7 sec, 95% confidence interval = 0.3 to 51.6) and to make significantly more errors on the Benton Visual Retention test part-C (β = 4.5 errors, 95% confidence interval [CI] = 1.2 to 7.8). Patients with bilateral SCC impairment were found to make significantly more errors on the Benton Visual Retention test part-C (β = 9.8 errors, 95% CI = 0.2 to 19.4). From case-control analysis, for each SD difference in Trail-Making test part-B time, there was a corresponding 142% increase in odds of having vestibular impairment (odds ratio = 2.42, 95% CI = 1.44 to 4.07).

CONCLUSIONS

These data suggest that bilateral saccule and SCC vestibular impairments may significantly affect various domains of cognitive performance. Notably, the cognitive performance in patients in this study was significantly poorer relative to age-matched healthy adults. Cognitive assessment may be considered in patients with saccule and SCC impairments, and cognitive deficits in vestibular patients may represent an important target for intervention.

Clinical testing of otolith function: perceptual thresholds and myogenic potentials

Cervical and ocular vestibular-evoked myogenic potential (cVEMP/oVEMP) tests are widely used clinical tests of otolith function. However, VEMP testing may not be the ideal measure of otolith function given the significant inter-individual variability in responses and given that the stimuli used to elicit VEMPs are not physiological. We therefore evaluated linear motion perceptual threshold testing compared with cVEMP and oVEMP testing as measures of saccular and utricular function, respectively. A multi-axis motion platform was used to measure horizontal (along the inter-aural and naso-occipital axes) and vertical motion perceptual thresholds. These findings were compared with the vibration-evoked oVEMP as a measure of utricular function and sound-evoked cVEMP as a measure of saccular function. We also considered how perceptual threshold and cVEMP/oVEMP testing are each associated with Dizziness Handicap Inventory (DHI) scores. We enrolled 33 patients with bilateral vestibulopathy of different severities and 42 controls to have sufficient variability in otolith function. Subjects with abnormal oVEMP amplitudes had significantly higher (poorer) perceptual thresholds in the inter-aural and naso-occipital axes in age-adjusted analyses; no significant associations were observed for vertical perceptual thresholds and cVEMP amplitudes. Both oVEMP amplitudes and naso-occipital axis perceptual thresholds were significantly associated with DHI scores. These data suggest that horizontal perceptual thresholds and oVEMPs may estimate the same underlying physiological construct: utricular function.

NON-INVASIVE TECHNIQUE FOR IN VIVO HUMAN EAR CANAL VOLUME MEASUREMENT

This prospective study is to develop a new non-invasive in vivo technique for the measurement of the human external auditory canal (EAC) volume using high resolution computed tomography (HRCT). Eighteen ears of nine male volunteers, with an age range of 20–30, were measured. The EAC volume was measured using three different methods: tympanometry, water injection, and HRCT. Two dimensional image slices of the EAC, in vivo, were obtained from the raw HRCT data. Each 2D CT image slice was composed of 512 × 512 pixels, with a 0.5 mm slice thickness, and a 0.175 × 0.175 × 0.5 mm3 voxel size. HRCT images were processed with Amira® software (Visage Imaging, Inc., San Diego, USA). Three-dimensional images of the EAC were reconstructed using an unconstrained smoothing mode. The resulting volume of the 3D model of the EAC was calculated using a voxel gate stacked tool. Using tympanometry, the mean measured EAC volume, averaged over all ears, was 766.66 mm3 (Standard Deviation, SD = 194.03 mm3). Using water injection, the mean measured EAC volume was 1102.77mm3 (SD = 121.60 mm3). Using HRCT, the mean measured EAC volume was 1082.22 mm3 (SD = 119.63 mm3). As evident from the SD values, variability was greater in the tympanometry group than in the water injection and HRCT groups. This study successfully developed a new technique to measure the EAC volume using HRCT imaging. This non-invasive technique for in vivo EAC volume measurement was less variable than tympanometry, and avoided the intrusive aspect of measurement by water injection. The technique allows effective, non-invasive assessment of the EAC volume pre-canalplasty, providing a predictor of EAC volume post-canalplasty.

Physiologic effects on the vestibular system in Meniere's disease

Ménière syndrome is an inner ear disorder characterized by spontaneous attacks of vertigo, fluctuating low-frequency sensorineural hearing loss, aural fullness and tinnitus. When the syndrome is idiopathic and cannot be attributed to any other cause (eg, syphilis, immune-mediated inner ear disease, surgical trauma), it is referred to as Ménière disease. This article reviews the physiologic effects of Ménière disease on vestibular function, as measured by caloric, head impulse, and vestibular-evoked myogenic potential testing.

Vestibular function and vertigo control after intratympanic gentamicin for Ménière's disease

The aim of this study was to correlate long-term vertigo control with reduction in vestibular function after intratympanic (IT) gentamicin therapy for unilateral Ménière's disease. IT gentamicin injections were given as needed to control vertigo attacks. Vertigo frequency and changes in angular vestibulo-ocular reflex (AVOR) gain (measured using magnetic search coils and manual head thrusts) and caloric weakness were assessed before and after treatment. Better vertigo control after treatment was found with >or=60% reduction in quantitative ipsilateral horizontal semicircular canal AVOR gain from pre-treatment values and/or with caloric unilateral weakness (UW) >50%. However, no correlations were found between the continuous variables of vertigo control and either gain or gain recovery, nor between gain and UW because of the large variability in vertigo control in subjects with lesser reductions in these measures.

Angular vestibulo-ocular reflex gains correlate with vertigo control after intratympanic gentamicin treatment for Meniere's disease

OBJECTIVES

The objective of our study was to determine whether angular vestibulo-ocular reflex (aVOR) gains correlated with vertigo control after intratympanic gentamicin treatment for Meniere's disease.

METHODS

We conducted a prospective study of 18 subjects with unilateral Meniere's disease treated with intratympanic gentamicin injection and followed all subjects for 1 year. We measured the gain of the aVOR elicited by rapid rotary head thrusts in each of the canal planes for each subject before and after treatment with intratympanic gentamicin by using magnetic search coils to record eye movements.

RESULTS

During the follow-up period, 11 subjects ("single-treatment group"; 61%) had control of their vertigo with a single gentamicin injection. The remaining 7 subjects ("multiple-treatment group"; 39%) experienced recurrent vertigo that required a second injection of gentamicin at a mean of 6 months after the first treatment. The 11 subjects in the single-treatment group had significantly greater reduction of labyrinthine function after the first treatment, as measured by change in ipsilateral horizontal canal gain, than did the 7 subjects with vertigo recurrence. Changes in caloric asymmetry did not correlate with vertigo control.

CONCLUSIONS

Our results suggest that successful treatment of Meniere's disease is closely related to attenuation of semicircular canal function as measured by horizontal canal aVOR gains.

Search-coil head-thrust and caloric tests in Ménière's disease

CONCLUSIONS

Our findings suggest that canal function is substantially preserved in subjects with active vertigo attacks as a result of Ménière's disease (MD). In these subjects, the head-thrust test (HTT) may not be as sensitive to canal dysfunction as traditional caloric testing. MD may differentially affect the low-frequency sensitivity of the canals.

OBJECTIVE

Caloric tests have traditionally been used to characterize semicircular canal function in vestibular disorders, including MD. The quantitative HTT provides an objective measurement of semicircular canal function in the frequency and velocity ranges of normal head movements. The aim of this study was to compare the findings of caloric and HTTs in subjects with unilateral MD.

MATERIAL AND METHODS

The study population consisted of 38 candidates for gentamicin treatment due to a high frequency of vertiginous attacks (25 males, 13 females; mean age 52.9 years; range 30-70 years). The duration of symptoms was 1-30 years (mean 5.3 years). Horizontal canal function was characterized with bithermal aqueous caloric tests and recordings of the angular vestibulo-ocular reflexes (aVORs) using the scleral search-coil technique during HTTs. The main outcomes were unilateral weakness (UW) on caloric testing and aVOR gain asymmetry (GA) during HTTs. A caloric response asymmetry of >20% was considered to be indicative of pathologic UW. A difference in GA during HTTs of >5.8% was considered significant.

RESULTS

Twenty subjects (52.6%) showed abnormal results on one or both tests. Pathologic UW was present in 16 subjects (42.1%). During HTTs, 11 subjects (28.9%) showed pathologic GA. Seven subjects (18.4%) showed abnormal results on both tests. A significant correlation was found between UW and GA. However, pathologic GA during HTTs in subjects with unilateral MD was less frequent and the values smaller than those published for vestibular neuritis patients. Two subjects with unilateral MD had 100% UW, but none had >30% asymmetry on HTTs.

Adaptation in the oculomotor response to caloric irrigation and the merits of bithermal stimulation

Caloric irrigation of the ear is a familiar clinical technique for the investigation of vestibular function that has the advantage of allowing each inner ear to be examined separately. However, it also has the disadvantage that the heating effect lasts for a period of 10-20 minutes, with the result that it is normally necessary to leave a period of at least 10 minutes between successive irrigations. This prolonged heating effect is not immediately apparent from the induced nystagmus, which normally decays within a period of 3 minutes, even in darkness. In the first part of this article, evidence is presented to show that the premature decay of eye movement can be attributed to the effect of adaptation, which is known to operate during other forms of vestibular stimulation. Experiments in which the subject was repeatedly re-orientated with respect to gravity counteracted this adaptation effect and thus revealed the continuation of the underlying thermal stimulus. In the second part of the article, an irrigation technique is described in which an attempt is made to avoid the prolonged heating effect by administering a bithermal caloric stimulus. The effectiveness of this technique is reviewed on the basis of data from previously published trials.

Reference values for serial vestibular testing

This study defines the normal limits of day-to-day variation in vestibular function. A short-acting caloric test was used and nystagmus intensity was measured by means of an interactive computerized technique. Data were analyzed from 30 normal subjects tested on three successive days and from ten of these subjects who were also tested on 11 additional occasions. Individual caloric test scores generally remained between 55% and 170% of their initial value; narrower ranges resulted when scores were averaged together. There was very little habituation of caloric responses. The upper normal limits of test-retest variation in scores for right-left difference and directional preponderance of caloric responses were 24% and 22%, respectively.

Factors contributing to variability of caloric test scores

Data were analyzed from 30 healthy male subjects, examined on three successive days with caloric, rotatory, optokinetic, gait and posture tests. Data were also analyzed from 11 additional test sessions performed on 10 of these subjects. A short-acting caloric test was used and nystagmus intensity was measured by means of an interactive computerized technique. Large inter-individual differences were noted for all test results and these differences remained consistent as tests were repeated. Factors which may contribute to inter- and intra-subject variability of caloric test scores are identified and discussed.

The stimulus controlled caloric tests: a reliability study

Preliminary studies have shown that a new caloric testing method described by Proctor and Dix offers several advantages over conventional methods. However, it had not been established that the Proctor-Dix method would prove reliable and practical when routinely applied in a clinical setting. Therefore, we administered the test to 60 patients referred for evaluation of vestibular function and compared the test scores with results from the conventional Fitzgerald-Hallpike test applied during the same test session. The analysis was based on the average slow phase eye speed of nystagmus during the most intense 20 seconds of individual responses. A close agreement was found between "right-left difference" and "directional preponderance" scores when results of the Proctor-Dix and Fitzgerald-Hallpike tests were compared in the 54 patients who completed all steps in the experiment. Larger differences between the results of the two tests were confined to subjects who were "normal" or only slightly abnormal. Order effects and additional analysis of correlations are discussed. The present study demonstrates that reliable results, comparable to those obtained from conventional methods, can be obtained when the new Proctor-Dix test is applied in a clinical setting.

Results of new air caloric testing method among normal subjects. I. Biphasic testing

A new air caloric testing method is described in which the temperature of a continuous aural irrigation is switched hot and cold values at times calculated to control the intensity of the resulting vestibular stimulation. Applications of low or high caloric stimulus intensities to normal subjects were well tolerated and reliably produced appropriate low or high intensity nystagmic responses. Nystagmus intensity values obtained from this study were compared with predicted intensity values from a computerized simulation of the actual test conditions, and also with values obtained when using biphasic water irrigations. As a result, further improvements in our methodology have been effected.

Caloric testing 2. results in normal subjects

A large number of variables were examined simultaneously for 43 normal subjects over the four irrigations of a caloric test. Care was used for every step of the testing procedure and data analysis to eliminate as much of the variance in the caloric responses as possible. The normality of each variable's distribution was examined using the Wilk-Shapiro W test and corrected if necessary by the best of several transformations. The means, standard deviation, and 95% confidence intervals of the resultant data were derived. Statistical tests of temperature, sidedness, and directionality were done on the variables and several important sources of variance were found and explained.

Results of new air caloric testing method among normal subjects. II. Monophasic testing

A new caloric irrigation method is described. In the new method, the temperature of a continuous aural irrigation is switched between hot and cold values at precisely specified times. The durations of hot and cold pulses have been calculated to produce specific caloric stimulation intensities based on known heat transmission characteristics of the labyrinth area. A brief "washout" irrigation is used to eliminate promptly all caloric stimulation effects at the conclusion of each test. The present study concentrated upon the question of how long the "washout" pulse should be in order to obtain optimum results. Repetitive application of a uniform stimulus intensity to 6 normal subjects indicated that short inter-test intervals can be used without causing vestibular habituation. Prompt removal of the caloric stimulus can be accomplished by proper timing the "washout" phase of the new technique.