Rotational chair testing in patients with unilateral peripheral vestibular disorders

Objective The aim of the study was to evaluate the rotational chair (RC) sinusoidal harmonic acceleration (SHA) test and the rotational velocity step (RVS) test in patients with unilateral peripheral vestibular disorders. Study design This study was retrospective in nature. Setting It was conducted in a clinical tertiary care vestibular function test center. Patients A total of 119 patients (63 men and 56 women) with documented unilateral peripheral vestibular lesions were evaluated with bithermal binaural caloric and sinusoidal and step velocity RC tests. Intervention Clinical, caloric, and RC tests were carried out. Patients and methods The study group was selected by including patients with a clinical diagnosis suggestive of peripheral vestibular lesions and with documented unilateral caloric weakness matching the clinical diagnosis, and by excluding patients with a clinical diagnosis suggestive of a peripheral vestibular lesion but not documented by unilateral caloric weakness, as well as patients with incomplete tests or uninterpretable data (due to poor recordings or eye movement artifacts). Caloric testing was performed using an infrared video-oculographic system and a Brookler-Grams closed-loop irrigation unit with standard bithermal irrigations of 30 and 44°C for 45 s each in the following order: L30°C, R30°C, R44°C, and L44°C. The RC testing paradigms used in this study were the rotational SHA test and the RVS test. Results When patients were classified on the basis of etiology, the most common cause of dizziness in the study group was found to be Mιniθre′s disease (34.9%), followed by vestibular neuritis (31.1%). Seventy-two patients in the study group demonstrated right caloric weakness and 47 patients demonstrated left caloric weakness, whereas only 18 of 119 patients demonstrated directional preponderance. When the RC SHA test parameters (gain, phase, and symmetry) in the study group were compared with the manufacturer′s normal values, statistically significant difference was seen in vestibulo-ocular reflex (VOR) gain at low frequencies (0.01-0.08 Hz), in VOR phase at all frequencies, and in VOR symmetry at low frequencies (0.01-0.04 Hz). Conclusion In summary, despite the encouraging results of the discriminating power of caloric testing, it should be emphasized that RC testing has its own unique capabilities, such as the following: (i) it is a physiologic stimulus whose frequency and amplitude can be varied precisely; (ii) the stimulus is unrelated to physical features of the external ear or temporal bone; (iii) it is useful in children who may not tolerate caloric testing; and (iv) it is very useful in assessing patients receiving vestibulotoxic drugs. Multiple stimuli of varying intensities can be applied to the vestibular system within a relatively short period of time.


Introduction
Balance is a complex sensorimotor task involving three steps: (1) Accurate and redundant sensory input from the visual, vestibular, and proprioceptive systems; (2) Central nervous system integration of the sensory signals and the generation of appropriate motor commands; and (3) Adequate musculoskeletal capabilities to perform the motor tasks involved in oculomotor and posture control.

Rotational chair testing in patients with unilateral peripheral vestibular disorders
Mohamed F. M. Ahmed a,b

Objective
The aim of the study was to evaluate the rotational chair (RC) sinusoidal harmonic acceleration ( SHA) test and the rotational velocity step ( RVS) test in patients with unilateral peripheral vestibular disorders.

Study design
This study was retrospective in nature.

Setting
It was conducted in a clinical tertiary care vestibular function test center.

Patients
A total of 119 patients (63 men and 56 women) with documented unilateral peripheral vestibular lesions were evaluated with bithermal binaural caloric and sinusoidal and step velocity RC tests.

Intervention
Clinical, caloric, and RC tests were carried out.

Patients and methods
The study group was selected by including patients with a clinical diagnosis suggestive of peripheral vestibular lesions and with documented unilateral caloric weakness matching the clinical diagnosis, and by excluding patients with a clinical diagnosis suggestive of a peripheral vestibular lesion but not documented by unilateral caloric weakness, as well as patients with incomplete tests or uninterpretable data (due to poor recordings or eye movement artifacts). Caloric testing was performed using an infrared videooculographic system and a Brookler-Grams closed-loop irrigation unit with standard bithermal irrigations of 30 and 44°C for 45 s each in the following order: L30°C, R30°C, R44°C, and L44°C. The RC testing paradigms used in this study were the rotational SHA test and the RVS test.
A defect in any one or more sensory integration and motor functions can lead to inappropriate muscle activity and imbalance [1].
Peripheral vestibular disorders are considered one of the most common causes of dizziness and vertigo. Peripheral vestibular defi cit is defi ned as any disease process that results in damage, either partial or complete, to only one side or both sides of the peripheral vestibular system involving the vestibular end organs and/or the vestibular nerve [1]. Th e pathophysiology of peripheral vestibular disorders may be classifi ed into the following: (1) Abnormalities of coupling (conduction) of head motions to the canal or macular receptor hair cells; (2) Abnormalities of the canal or macular hair cell receptors (sensory) and peripheral nerve supply (neural); (3) Combination of the fi rst two categories (mixed) [2].
Th e ability to evaluate the range of physiological functions using electronystagmography is limited, given that the use of caloric irrigation stimulates the system in a manner equivalent to a frequency between 0 .002 and 0.004 Hz and accelerations of less than 10°/s. Th ese values are below the level within which the vestibulo-ocular refl ex (VOR) generally functions in daily activities [3]. Th erefore, rotational chair (RC) testing has been used to expand the evaluation of the peripheral vestibular system as it can stimulate frequencies in the 0.01-1.28 Hz frequencies, which are considered more physiologic frequencies [4]. RC testing is a test in which sequences of sinusoidal angular velocity signals at several test frequencies are applied for evaluation of the VOR function [5].
Zee [6] reported that RC testing may provide valuable information for the diagnosis and subsequent management of patients with vestibular disorders. It completes the spectrum of tests necessary for diagnosing vestibular abnormalities, and assists in identifi cation of peripheral vestibular defi cits not detectable by other vestibular tests [6]. In contrast, Cass [7] mentioned that the major clinical advantage of computerized rotational testing is the ability to produce angular accelerations that can be precisely controlled and repeated. Multiple stimuli of varying intensities can be applied to the vestibular system within a relatively short period of time [7].
RC testing is ideal in the assessment of patients suff ering from peripheral vestibular disorders because, unlike caloric testing, higher frequencies are also tested and both labyrinths are stimulated simultaneously. Th is allows for accurate determination of the remaining vestibular function. Th us, it can diff erentiate central from peripheral and compensated from uncompensated vestibular disorders through abnormalities detected in gain, phase, symmetry, and time constants [8]. Th e aim of this study was to demonstrate the RC sinusoidal harmonic acceleration (SHA) test and the rotational velocity step (RVS) test in patients with unilateral peripheral vestibular disorders.

Patients and methods
After obtaining approval from the institutional review board, 250 charts of patients with complaints of dizziness and balance problems were randomly selected for retrospective review; all patients were referred for caloric and RC testing. A total of 119 charts were selected out of the 250 patient charts on the basis of the following criteria: Th e clinical diagnosis was made independently of the caloric and RC test results on the basis of a combination of detailed history taking, physical examination, bedside neuro-otologic examination, audiologic evaluation, and radiologic studies as clinically indicated. Th e entire study group was subjected to the following tests.

Caloric stimulation
Caloric testing was performed using an infrared videooculographic system (Micromedical Technologies, Chatham, Illinois, USA) and a Brookler-Grams closed-loop irrigation unit with standard bithermal irrigations of 30 and 44°C for 45 s each in the following order: L30°C, R30°C, R44°C, and L44°C.
Th e caloric test variables used in this study were the percentage of caloric weakness (CW) as calculated using the Jongkees Index formula.

Rotational stimulation
RC testing was performed using Micromedical Technologies System 2000 with the standard commercially available software (4 Channel Spectrum S1.0.3, Micromedical Technologies , Inc., 10 Kemp . Th e entire procedure was performed with both clockwise and counter-clockwise initial rotation. Th ere were two bursts of nystagmus for each per-rotatory: (1) Prerotatory nystagmus, which starts with the initial acceleration of the chair; and (2) Postrotatory nystagmus, which starts with the complete stop of the chair.
Th e derived parameter is the time constant (Tc), which represents the time in seconds for the nystagmus slowcomponent velocity to decay to 37% of its peak value. Overall, four time constants were identifi ed for each patient.

Statistical anal ysis
SPSS (version 13.0; SPSS Inc., Chicago, Illinois, USA) for Windows was used. Results are presented as percentage, mean, and SD. Th e paired t-test was used to compare results between the study group and the manufacturer's normal values. Level of signifi cance was se t at P-values less than or equal to 0.05. Table 1 demonstrates the demographic characteristics of the study group (119 patients): the age range was 22-79 years, and there were 63 men and 56 women. When patients were categorized according to etiology, the most common cause of dizziness in the study group was found to be Ménière's disease (34.9%), followed by vestibular neuritis (31.1%) (Fig. 1).     Tables 4 and 5 show the correlation between the side of caloric weakness and the clockwise and counterclockwise stimulation of the step velocity test.

Results
When RC step velocity test parameters (gain and time constant) in patients with right caloric weakness were compared with the manufacturer's normal values we found a statistically signifi cant reduction in the gain and time constants of the clockwise prerotatory and counter-clockwise postrotatory parts of the step velocity test (Table 4).
When RC step velocity test parameters (gain and time constants) in patients with left caloric weakness were compared with the manufacturer's normal values we found a statistically signifi cant reduction in the gain and time constants of the clockwise postrotatory and counter-clockwise prerotatory parts of the step velocity test (Table 5).   tilted toward men (53% of men vs. 47% of women) ( Table 1). Kamal et al. [9,10] noticed light increase in peripheral vestibular disorders among men. However, Stahle and Arenberg [11] and Tokumaso et al. [12] reported that peripheral vestibular disorders are more common in women than in men.
Th e present study demonstrated that the most common cause of peripheral vestibular disorders in the study group was Ménière's disease (34.9%), followed by vestibular neuritis (31.1%) (Fig. 1). El-Gohary et al. [13] reported another observation in a similar study on patients with peripheral vestibular disorders in which the most common was benign paroxysmal positional vertigo (35%), followed by vestibular neuritis (30%), ototoxicity (20%), and Ménière's disease (15%). Th is discrepancy between the two studies could be attributed to the selection criteria of the study group [13].
In this study, one of the selection criteria was presence of unilateral caloric weakness; therefore, 100% of the study group had unilateral caloric weakness, 60% had right caloric weakness, and 40% had left caloric weakness. Unilateral caloric weakness was considered when the diff erence in the mean maximum slowphase velocity exceeded 20%. In contrast, other studies demonstrated normal caloric response in patients with peripheral vestibular lesions. For example, Haid et al. [14] reported abnormal caloric test results in only 54% of patients with peripheral vestibular lesions, similar to the report of Stahle [15]. Diagnosis of peripheral vestibular lesions with normal caloric response and abnormal RC testing in terms of reduced time constant, low gain, and asymmetrical responses to the rotational stimuli was also reported by Shepard and Telian [17] and Takahashi et al. [16], who reported such an observation in 10 of 47 dizzy patients with normal caloric responses.
Th e observation of normal caloric response and abnormal RC testing in patients with suspected peripheral vestibular lesions could be attributed to the fact that the caloric test assesses the integrity of the peripheral vestibular system through stimulation of the horizontal semicircular canals. Th e stimulus of warm, cool, or ice water is equivalent to rotational stimulation at a frequency of 0.002-0.004 Hz. Th ese levels of stimulation are signifi cantly lower than those experienced by the VOR system on a daily basis and may not identify dysfunction at higher frequencies in the 0.01-1.28 Hz range, which are considered more higher and physiological frequencies [4].
Fourteen of those 18 patients showed asymmetry in RC testing. Such consistent association of directional preponderance (caloric testing) and asymmetry of the SHA test was noticed by Shepard and Telian [17]. Th is observation could be explained by the presence of response bias within the system, favoring larger slow-component velocities in one direction versus the other. A bias usually results from a peripheral lesion with incomplete dynamic compensation in the central nervous system [17].
Jacobson et al. [18] stated that directional preponderance exists when the nystagmus response to horizontal semicircular canal stimulation is stronger in one direction than in the other. Th is is particularly apparent in patients with acute uncompensated unilateral peripheral vestibular lesions [18]. Black [2] reported that directional preponderance tended to be directed toward the unimpaired ear in patients with a history of short duration vestibular defi cits. Directional preponderance is clinically diagnosed when there is a 25% or greater diff erence in intensity of maximum slow-phase velocity between right beating and left beating responses [19].
Th e SHA test was conducted at frequencies of 0.01-0.64 Hz. It revealed reduced gain limited to low frequencies (0.01-0.08 Hz) (Table 3), which agreed with the fi ndings of El-Gohary et al. [13]. Th ere was also phase lead at all frequencies and asymmetry was limited mainly to the low frequencies (0.01-0.04 Hz). Th e reduction in VOR gain could be considered as reduced vestibular response due to reduction in the input/output function between eye and chair velocities [Th is refl ects that chair velocity (stimulus) was higher than eye velocity (response)] [20]. Koizuka et al. [21] reported that the abnormal VOR gain in peripheral vestibular disorders is due to loss of velocity storage mechanism, which normally functions to maintain the vestibular response.
Th e RVS test revealed reduced gain and time constants for rotation toward the side of the lesion in the study group (Tables 4 and 5). Similar fi ndings were reported by Baloh et al. [22], who examined 48 patients with unilateral signifi cant reduction in gain and time constants for rotation toward the side of the lesion. In this study, as shown in Tables 4 and 5, there was consistent association between the side of the lesion on caloric testing and the RVS test in which patients with right caloric weakness showed statistically signifi cant reduction in the gain and time constants of the clockwise prerotatory and counter-clockwise postrotatory parts, and patients with left caloric weakness showed statistically signifi cant reduction in the gain and time constants of the clockwise postrotatory and counterclockwise prerotatory parts [22].
Shepard [4] stated that, although both ears are involved in responses to rotatory stimuli, the right periphery (horizontal semicircular canal and superior vestibular nerve) is primarily responsible for responding to accelerations to the right or to decelerations from fi xed velocity rotation leftward. Th e reverse is true for the left labyrinth. Th erefore, prerotatory and postrotatory step tests also allow comparison of the time constant for dominant stimulation to one peripheral system. Table 6 demonstrates the sensitivity, specifi city, and positive and negative predictive values of the RVS time constant in relation to the caloric test and the gain, phase, and asymmetry of the SHA test. Th e highest sensitivity was between the RVS time constant and phase (95.6%), whereas the highest specifi city was with gain (44.5%). Ideally, the RVS test and the SHA test can be employed in parallel to increase the accuracy of estimates of the system time constant and identifi cation of the abnormally functioning system.
Palomar-Asenjo et al. [23] studied the caloric and RC tests in 100 patients with unilateral Ménière's disease and reported that the caloric test was fairly specifi c for patients with unilateral peripheral vestibulopathy, whereas the RC test was more sensitive. Th ey attributed these fi ndings to the fact that the time constant of the VOR is more stable than the canal paresis of the caloric test in test-retest reliability studies [23].

Conclusion
Th e combination of caloric test and RC testing produced the strongest predictive capabilities for identifying peripheral vestibular injury. Th is fi nding reinforces the common belief that the caloric and RC tests are complementary tests in evaluating dizzy patients. One explanation for this fi nding may be that caloric stimulation is equivalent to a very lowfrequency rotation of 0.002-0.004 Hz, which falls substantially below the lowest frequency of the RC test (0.01-1.0 Hz) [4]. Th erefore, RC testing has been used to expand the evaluation of the peripheral vestibular system.
Despite the encouraging results of the discriminating power of caloric testing, it should be emphasized that RC testing has its own unique capabilities, such as the following: (1) It is a physiologic stimulus whose frequency and amplitude can be varied precisely; (2) Th e stimulus is unrelated to physical features of the external ear or temporal bone; (3) It is useful in children who may not tolerate caloric testing; and (4) It is very useful in assessing patients receiving vestibulotoxic drugs [4].

Recommendations
On the basis of the current study, we recommend studying the RC test in patients with bilateral peripheral vestibular disorders and in patients with central vestibular disorders and using the RC test as an objective measure of the outcome of vestibular rehabilitation therapy in these patie nts.