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Show Rotary chair testing Rotary chair testing includes rotation around a vertical axis, and evaluates the horizontal semicircular canal vestibulo-ocular reflex (VOR). The patient sits in a mechanized chair with the head secured in a neutral position or in 30 degree forward flexion (to better isolate the horizontal canals). The chair is located within a cylindrical chamber, which excludes outside light (Figure 1). Eye movements are recorded with either videonystagmography (VNG)/videooculography (VOG) or electronystagmography (ENG). The chair is then rotated, commonly in one of two paradigms.1 During the step-velocity testing, a rapid steady acceleration brings the patient to predetermined velocity which is maintained for about one minute before a rapid deceleration stops the rotation. Since the semicircular canals are sensitive to acceleration, there is an initial deflection of the cupula. Each acceleration will cause eye movement in the opposite direction if there is a functioning VOR. There is also a reinforcement of these eye movements by the velocity storage system, which supplements low frequency vestibular inputs. This is seen as slow phases of nystagmus in either ENG or VNG. As the cupula rebounds with constant chair velocity, the slow phase velocity of the induced nystagmus decreases. After this is maintained, the chair is brought to a stop. The decreasing chair velocity is an acceleration in the opposite direction and causes stimulation of the opposite horizontal semicircular canal. Testing can be repeated at different rotational velocities. Three important pieces of data can be obtained from the step-velocity testing. First is the gain of the VOR, which is the ratio of peak slow phase velocity (output) to peak chair velocity (input - gain is a generic term used in dynamic systems to describe the ratio of output to input). Second, the rate at which the slow phases decay can be determined and this is reported as the time constant, which is the time that it takes for slow phase velocities to decrease to 37% of the maximum. This is a measurement of the function of velocity storage. Decreased velocity storage (and therefore decreased time constants) can be seen in either unilateral vestibular loss or disorders of the medial or superior vestibular nuclei the commissure between vestibular nuclei (see Figure 2 for a more thorough explanation). Prolonged time constants can be seen in disorders of the nodulus/uvula, as well as in patients with vestibular migraine. Another way to assess for nodular dysfunction, is through dumping. In normal patients and in patients with migraine, pitching the head forward/downward after the end of rotation will truncate the velocity storage-related nystagmus. In cases of nodular dysfunction, the time constant will be preserved despite head pitch. This makes sense given our understanding of the role of the nodulus (and its connections with the vestibular nuclei) in the velocity storage mechanism, and findings that can be appreciated in pathologic conditions involving these structures - e.g., "central" patterns of head-shaking nystagmus, and periodic alternating nystagmus. Directional preponderance can also be seen in the nystagmus recordings, and indicates tonic vestibular imbalance and is poorly localizing, similar to as discussed in the section on caloric testing. Laboratory or vendor specific norms are established as appropriate. #12;The other commonly used protocol is sinusoidal rotation. This can be tested with various frequencies of rotation, from 0.01 to 1 Hz. The data reported from this testing again include the gain and directional preponderance. Gains determined from sinusoidal acceleration are more accurate than from stepvelocity testing, but the testing takes more time. In addition, the phase of the VOR at low frequency rotations can be determined. Phase is the timing of the eye velocity relative to chair velocity. At low frequencies, eye velocities typically lead chair velocities during sinusoidal rotations. Phase is mathematically related to the time constant. In cases without directional preponderance, comparing the timing of zero or peak velocities can directly reveal this information, otherwise more advanced analysis is required. Other VNG/ENG components can be incorporated into the rotary chair set up, including saccades, pursuits, OKN, VOR suppression and visually enhanced VOR. There are several limitations to rotary chair testing. There are practical limitations, particularly that it is not a widely available test. Patients, especially those with vestibular migraine, may find it uncomfortable. There are also physiologic limitations. During head rotations, the semicircular canals on either side are stimulated. The ipsilateral semicircular canal increases its firing rate, and the contralateral semicircular canal decreases its firing rate. At high accelerations, the inhibitory effect on the contralateral canal is saturated and the VOR is more precisely a measurement of ipsilateral function. Video head impulse testing (vHIT) uses this phenomenon to lateralize peripheral vestibular disease. Due to mechanical limitations, rotary chairs do not have the ability to produce a high enough acceleration. Because of these factors, rotary chair testing is not as effective in measuring unilateral vestibular loss. On the other hand, rotary chair testing is able to provide additional information in specific central vestibular disorders, for which caloric testing and vHIT cannot - e.g., high time constants in vestibular migraine or nodulus pathology, and the ability to assess for dumping with nodulus lesions. #12;Figure 1. Example of rotary chair apparatus. Figure 2. Examples of VOR tracings and data for step-velocity rotations. Above, a plot of slow phase velocity (vertical axis) vs time during a sustained rotation at 60 deg/s (left) and 240 deg/s (right). Time 0 is when steady velocity is reached and acceleration is stopped. As expected there is a robust initial response followed by an exponential decay. The gain is the ratio of the initial response to chair velocity immediately after the acceleration. The time constant is the time at which the eye velocity drops to 37% of its maximum. Rightward eye movements are by convention displayed as being positive and above the horizontal axis, and conversely leftward eye movements are negative. Gain data at our lab are reported as gR (gain for rightward eye movements/leftward rotation) and gL (leftward eye movements/rightward rotation). Similarly, time constant data is displayed as tcR and tcL for leftward and rightward rotations respectively. For our laboratory, the most pertinent values for assessment of the horizontal semicircular canal VOR are 1) gains at 240 deg/s and 2) time constants at 60 deg/s. Overall, this is a normal rotary chair test report, without clear evidence of unilateral or bilateral vestibular loss. #12;1. Furman JM. Rotational testing. Handb Clin Neurol 2016;137:177-186. Tony Brune, DO Department of Neurology The Johns Hopkins School of Medicine Daniel R. Gold, D.O. Departments of Neurology, Ophthalmology, Neurosurgery, Otolaryngology - Head & Neck Surgery The Johns Hopkins School of Medicine |
