Rotary Chair Testing
- Author: Manali S Amin, MD; Chief Editor: Arlen D Meyers, MD, MBA more...
The visual and vestibular systems interact to maintain visual clarity of objects during head movement. The reflex that makes this possible is known as the vestibuloocular reflex (VOR) (see the image below). It permits individuals to perform such routine activities as reading street signs while walking down a sidewalk. Like many other systems in the body, most individuals are unaware of the VOR and its basic functioning until it does not function properly. Vertigo, dizziness, imbalance, nausea, vomiting, and other similar symptoms often characterize dysfunction of the vestibular system.
Patients who present with symptoms of vestibular dysfunction often undergo a detailed diagnostic workup including a standard electronystagmography (ENG) battery, posturography, and audiometry. Caloric testing is routinely performed in most vestibular laboratories as part of a vestibular dysfunction workup. Caloric testing is used to assess the integrity of the peripheral vestibular system through stimulation of the horizontal semicircular canals. The stimulus of warm, cool, or ice water is equivalent to rotational stimulation at a frequency of 0.002-0.004 Hz. These levels of stimulation are significantly lower than those experienced by the VOR system on a daily basis and may not identify dysfunction at higher frequencies.
In addition, caloric results may be abnormal in individuals with congenital abnormalities such as atretic or stenotic external auditory canals, with anatomic variations such as a thickened temporal bone, or with certain acquired disorders such as a severely atelectatic or absent tympanic membrane or fluid in the middle ear. Thus, an additional test is needed to assess the integrity of the VOR.
Rotational chair commonly stimulates frequencies in the 0.01-1.28 Hz range. Head autorotation, an alternative method of testing VOR, stimulates frequencies of 1-6 Hz. This test is discussed in greater detail in the Medscape Reference article Vestibuloocular Reflex Testing.
VOR testing is often performed with the use of a rotary chair. Rotational chair testing was first introduced by Bárány in 1907. He initially designed the chair for VOR testing with impulsive rotation in mind. The test consisted of manual rotation of the chair 10 times over 20 seconds followed by a sudden stop of the chair to analyze the postrotary nystagmus. Rotational chair testing has undergone numerous changes since that time and now has additional applications, including testing of visual-vestibular interaction, optokinetic after-nystagmus (OKAN), high-velocity sinusoidal testing, and off-vertical axis rotation (OVAR). Some of the newer applications require more sophisticated equipment than that developed by Bárány.
As previously stated, the visual and vestibular systems interact to maintain visual clarity of objects during locomotion and other head movement. The fovea (see the image below) is the part of the eye that has the greatest density of photoreceptors and therefore is the area with the best visual acuity. An object is most clearly viewed when it is centered on the fovea. During motion, the image of the viewed object tends to slip from the fovea, causing it to blur. In fact, visual acuity declines to 50% when an object is 2° from the center of the fovea. To maintain an object on the fovea, the eye must make corrective responses. These corrective eye movements, known as nystagmus, have a slow phase and a quick phase. The vestibuloocular reflex (VOR) slow phase keeps the eyes on the foveal vision (opposite direction to head movement).
Many different types of nystagmus exist, some of which are physiologic and others of which are pathologic. During rotational chair testing, the alert patient's eyes move in a direction opposite to the rotation of the chair. As the globe reaches an eccentric position within the orbit, a corrective response attempts to move it back to the center of the orbit. The nystagmus that is observed is a physiologic response and is observed with acceleration and deceleration of rotation. Sustained rotation results in a decline of this nystagmus. Multiple methods of rotation exist, each of which was designed to analyze vestibular responses by observation of the eye movements.
The rotational chair has primarily been used for analyzing horizontal canal VOR. Rotation of the chair is performed with the assumption that the stimulus applied to the whole body is the same as a stimulus that is applied to the head. Therefore, the head should be secured to the chair during rotation. In addition, most commercially available test protocols do not exceed a frequency of 1 Hz because the skin may move relative to the skull at frequencies greater than 1 Hz, thereby nullifying the assumption.
The vestibular system, which is the system of balance, consists of 5 distinct end organs: 3 semicircular canals that are sensitive to angular accelerations (head rotations) and 2 otolith organs that are sensitive to linear (or straight-line) accelerations.
The semicircular canals (see the image below) are arranged as a set of 3 mutually orthogonal sensors; that is, each canal is at a right angle to the other 2. This is similar to the way 3 sides of a box meet at each corner and are at a right angle to one another. Furthermore, each canal is maximally sensitive to rotations that lie in the plane of the canal. The result of this arrangement is that 3 canals can uniquely specify the direction and amplitude of any arbitrary head rotation. The canals are organized into functional pairs wherein both members of the pair lie in the same plane. Any rotation in that plane is excitatory to one of the members of the pair and inhibitory to the other.
For more information about the relevant anatomy, see Vestibular System Anatomy, Visual System Anatomy, Extraocular Muscle Actions, Extraocular Muscle Anatomy, and Inner Ear Anatomy. Also see Vestibuloocular Reflex Testing.
The rotational chair has primarily been used for analyzing horizontal canal vestibuloocular reflex (VOR).
See the list below:
Electrodes for electrooculography monitoring
The patient is prepared for electronystagmography (ENG) testing by the placement of electrodes for electrooculography monitoring. The skin is first cleansed of any oil or debris with alcohol and is then prepped with an electrode prep pad.
Electrodes are placed on the skin at the lateral canthus of each eye for horizontal channel recording. An additional electrode is placed on the forehead or sometimes on the earlobe as a ground.
The patient is then asked to sit in the rotational chair and the electrodes are connected to a computer for analysis of nystagmus. In some paradigms, eye movement recordings are made using infrared oculography, rather than electrodes.
Although variability exists in equipment made by different manufacturers, most chairs share similar features. The rotational chairs are usually placed in a lightproof booth and contain some form of a head restraint, an infrared camera, and a 2-way communication system.
Prior to testing, the patient's head is restrained and the camera is positioned such that the examiner may monitor the patient's eye movements outside of the booth. Once the set-up is completed, testing is begun. Frequently, the head restraint device tilts the patient's head forward at 30°, placing the horizontal canals perpendicular to the plane of gravity.
The patient is instructed to keep his or her eyes open and is given mental alerting tasks during the testing.
Preparation for rotational chair testing
Prior to rotational chair testing, the patient is asked to refrain from use of alcohol or caffeine for 48 hours. In addition, certain medications, including antihistamines, antihypertensives, sedative/hypnotics, and anxiolytics, are withheld if permissible by the patient's physician. Eating for 2 hours prior to the examination is discouraged because this may exacerbate nausea and emesis. Similar to pre-ENG instructions, patients are asked not to wear makeup.
Position the patient as described above in the Positioning section.
Vestibuloocular testing paradigms
Traditionally, angular acceleration stimulus to test VOR has been delivered in one of four manners: constant, impulsive, step testing, or sinusoidal.
Constant angular acceleration
The constant acceleration test is one in which the patient's rotational velocity is increased at a constant acceleration to a set angular velocity, which is then maintained for a period of time, after which the chair is slowly decelerated to a velocity of 0°/s.
The patient is observed for a nystagmus during acceleration and deceleration, and a nystagmus threshold is determined by the rate at which nystagmus is first observed with acceleration, then again later with deceleration. Thresholds are determined for different rates of acceleration. Montandon determined that the threshold is 1°/s2 in healthy individuals but greater than 6-7°/s2 in patients with vestibular dysfunction.
Impulse angular acceleration
Impulse testing involves rotating a patient for a period of time at a constant velocity and then suddenly stopping the chair and analyzing the postrotary nystagmus. The main advantage of this stimulus, similar to constant acceleration, is that it allows assessment of the VOR independently in each direction. However, the stimulus is very brief and therefore more subject to inaccuracies resulting from patient inattention.
Therefore, several tests in each direction should be performed and the postrotary nystagmus results averaged. The velocity of the slow component of the nystagmus is plotted against time. The duration of nystagmus is proportional to the log of the impulse intensity. Gain (ie, the ratio of the slow-component velocity to the chair velocity) and time-constant velocity (ie, the time required for the slow-component velocity to fall to 37% of its initial value) can be determined from these plots. In 1984, Baloh, Honrubia, et al determined that gains of 0.63 ± 0.18 seconds and time constants of 12.2 ±3.6 seconds are within the reference range.
Step test angular acceleration
Variations of the impulse test include step testing. Step testing involves rotating a patient at an acceleration impulse of 100°/s2 to a new fixed chair velocity. Between steps the patient rotates at a constant velocity. Step changes of different magnitudes to the right and left can be delivered in a semi-random fashion to prevent patient anticipation of the next step. Step changes of small to large magnitudes can be achieved in each direction. At a constant velocity with no acceleration, the patient incorrectly perceives that the chair is slowing down, and the velocity of the slow component of the nystagmus decays over time. After 45-60 seconds, the patient is given a second stimulus. Ideally, the time constant and gain for both impulses (acceleration and deceleration) should be equal.
Step testing can help to distinguish patients with a unilateral vestibular deficit from those without a unilateral vestibular deficit. One study used step testing at multiple frequencies, ranging from 16°/s- to 256°/s in clockwise and counterclockwise directions. The percentage difference of maximum velocity of the nystagmus slow phase components between clockwise and counterclockwise stimulation in a given person was then calculated. At a rotation of 256°/s, patients with a unilateral deficit were over 2 standard deviations from the mean of healthy patients. Furthermore, diminished responses at 256°/s were always in the direction of the ampullofugal stimulation of the functioning horizontal semicircular canal.
Based on these results, impulse testing can be used to identify patients with a unilateral deficit and identify the site of the lesion. Additional qualitative observations made in this study included the ability of impulse testing to provide measurable responses in patients with bilateral vestibular losses and greatly increased responses in patients with pure cerebellar degeneration.
A variation of step testing, referred to as pulse-step-sine testing, was developed to further diagnose patients with unilateral vestibular deficits. This testing paradigm involves rotating patients at a low frequency and high amplitude while simultaneously superimposing a higher frequency sinusoidal rotation. The low frequency, high amplitude rotation is known as the bias component and is created by pairing a pulse and a step of rotation acceleration. This selectively inhibits signals from one of the paired semicircular canals in the plane of rotation.
The higher frequency sinusoidal rotation, referred to as the probe component, stimulates the other canal, resulting in afferent neural discharge. Afferent neural activity is decreased when rotating toward the dysfunctional canal. This testing paradigm may help to diagnose patients with unilateral vestibular deficits. Additional research is needed to fully understand the potential of step testing in clinical diagnosis.
Sinusoidal harmonic angular acceleration
A fourth type of angular acceleration stimulus applied to rotational chair testing is sinusoidal. Sinusoidal testing involves observing a nystagmus when rotating a patient in alternating directions. This may be performed one frequency at a time or simultaneously with a combination of frequencies. The latter is also known as the sum of sinusoidal stimuli and, although faster to perform than the former, is not routinely used because of the complex mathematics required for analysis.
Sinusoidal testing at multiple individual frequencies is also known as sinusoidal harmonic acceleration (SHA). SHA is routinely used in clinical testing and involves rotating a patient in alternating directions (cycles) at a given frequency. Peak chair velocities are typically fixed at 50-60°/s, and frequencies generally range from 0.01-1.28 Hz. The lower frequencies tend to produce the weakest VOR responses and are associated with a greater likelihood of unpleasant side effects such as nausea. Frequencies higher than 1.28 Hz may cause the head to slip relative to the chair and are therefore inaccurate.
As with other angular acceleration stimuli, analysis of SHA consists of gain and a time constant and can be assessed for symmetry of the response in each direction. The velocity of the slow component of the nystagmus observed during each cycle of rotation is averaged with subsequent cycles of rotations, and an average response is reported for each frequency. The main disadvantage to SHA is the time required to test patients.
Applications of the Rotational Chair
Clinical applications of the rotational chair
Any of the above-mentioned angular acceleration stimuli may be used to diagnose vestibular disorders. Parameters that are commonly examined include gain, phase, and symmetry. Gain is the ratio of the amplitude of eye movement to the amplitude of head movement (stimulus). Phase is a parameter that describes the timing relationship between head movement and reflexive eye response. When the head and eyes are moving at exactly the same velocity in opposite directions, they are said to be exactly out of phase, or 180°. If the reflex eye movement leads the head movement, a phase lead is present, and if the compensatory eye movement trails the head movement, a phase lag is present. Symmetry is a comparison of the slow component of the nystagmus when rotated to the right compared with rotation to the left.
A fourth parameter that may be studied is the time constant. This is a measure of the time (in seconds) required for the VOR gain to exponentially decrease by 63%. It is measured in step-velocity testing after the chair is rapidly stopped and is reduced in patients with vestibular dysfunction. Note that the time constant and gain may be reduced in inattentive patients as well.
Each of these parameters is useful in diagnosing and localizing vestibular lesions. Studies have shown that patients with a unilateral peripheral vestibular lesion may exhibit asymmetric responses to rotation. On the other hand, patients with a compensated unilateral lesion show a characteristic pattern of decreased gain and increased phase lead at low-frequency stimulation. Bilateral peripheral vestibular lesions are characterized by low gain and phase lag with sinusoidal testing. Rotational chair testing is ideal in the assessment of these patients because, unlike caloric testing, higher frequencies are also tested and both labyrinths are stimulated simultaneously.
This allows for accurate determination of remaining vestibular function. In fact, Arriaga et al determined that rotational chair testing has a sensitivity of 71% for diagnosing peripheral vestibulopathies, as opposed to only 31% sensitivity for caloric testing/ENG. Although it is a more sensitive test for peripheral vestibular disorders, rotational chair testing has a specificity of only 54%, compared with the 86% specificity of ENG. Both tests are therefore complimentary and should be used in the diagnosis of peripheral vestibular dysfunction.
Finally, abnormalities may be observed with central vestibular deficits. Thurston et al showed that gains may be increased in some individuals with cerebellar deficits. Cerebellar atrophy, on the other hand, may result in a disorganized nystagmus pattern with beat-to-beat variabilities in the amplitude.
Additional applications of the rotational chair
Although rotational chair testing has traditionally been used to assess the integrity of the VOR, newer innovations have expanded its applications to include visual-vestibular interaction testing, OKAN assessment, and OVAR.
Visual-vestibular interaction consists of 2 subtests that primarily assess central VOR pathways. The first subtest consists of having the patient focus on an object (eg, thumb, light) that is moving at the same velocity as the chair. This allows an individual to suppress the VOR. Suppression is poorer at higher frequencies of rotation.
The second subtest consists of rotating a patient while simultaneously presenting a stationary optokinetic stimulus. This test enhances the VOR nystagmus response. An inability to suppress the VOR may be an indication of a central pathology, specifically brainstem and/or cerebellar deficits. In addition, failure to suppress the VOR correlates with pursuit and gaze fixation abnormalities.
OKAN is the nystagmus that persists after a patient is removed from an optokinetic field. This nystagmus, which is highly variable, is typically weaker than the nystagmus observed when the patient is immersed in a full field and deteriorates over a minute. Immersion in a full field consists of placing an individual in a drum or field in which the visual stimulus has no beginning or end. This is typically achieved with a series of vertical lines that are rotated around the patient. Asymmetric responses are often observed in patients with unilateral peripheral vestibular lesions, and severely reduced responses are observed in those with bilateral weakness.
Finally, OKAN may be used in the diagnosis of mal de barquement syndrome. This is a disorder individuals may experience upon returning to land following a sea voyage. The individual may perceive a rocking or linear motion of being aboard a ship. For this category of patients, OKAN evaluation may typically be positive when all other vestibular test results are normal.
Off-vertical axis rotation
OVAR is an application of the rotational chair that has remained largely experimental. The patient is placed in a rotational chair, which is then tilted so that the chair is no longer vertical with respect to the earth. The patient and the chair are then sinusoidally rotated. Once a constant angular velocity has been reached, only the otolithic organs, the utricle and the saccule, are stimulated. Thus, OVAR allows for testing of otolithic function independent of the semicircular canals. OVAR is used to assess the otolith-ocular reflex as well as the subjective visual vertical (SVV), which reflects the processing of otolithic information in higher brain centers, predominantly in the thalamus and vestibular cortex.
The nystagmus produced with OVAR is assessed for a modulation component and a bias component. The modulation component is the sinusoidal modulation of the slow-component velocity of the induced nystagmus. The bias component is the nonzero baseline. Both of these parameters are useful in the diagnosis of patients with otolithic lesions. For instance, a patient with a unilateral peripheral vestibular lesion tends to exhibit a decreased bias component upon rotation towards the affected ear. The contralateral bias component and the modulation component are normal.
Although OVAR is currently used mostly for research purposes, its clinical applications are promising because few modalities test for otolith dysfunction and OVAR produces minimal nausea. One potential clinical application may be in the diagnosis of BPPV. Sugita-Kitajima et al used OVAR to study patients with BPPV. They found a reduction in VOR gain with OVAR rotation at 0.8 Hz with the patient in the 30° nose-up position, indicating impaired otolithic function.
Other potential clinical applications
Traditionally, rotational chair testing has been used for 2 major purposes: to diagnose patients with peripheral vestibular lesions and to monitor patients undergoing pharmacologic vestibular ablation for Ménière syndrome.
Recent studies suggest that rotary chair testing's clinical use may be expanded to assist in making career choices (ie, airline pilots), as well as in evaluating the clinical efficacy of vestibular suppressant medications.
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