This video by Jack Bennett - International Clinical Trainer at the Interacoustics Academy - describes the clinical process, interpretation and clinical utility of the Sinusoidal Harmonic Acceleration test on the Rotary Chair.
You can read the full transcript below.
Sinusoidal harmonic acceleration is performed in darkness. So with the goggle cover on, and the patient will be given an alerting task throughout the test. The purpose of this test is to assess the patient's vestibular ocular reflex by rotating the patient in a pendulum or oscillating pattern at various frequencies, ranging from 0.01 hertz to 0.64 hertz.
This test is considered the gold standard test for identifying a bilateral vestibular weakness. The SHA test can also be used to aid in the diagnosis of a unilateral vestibular loss and can be used to monitor vestibular compensation over time.
We can now see the chair has come to rest before accelerating in the opposite direction. By plotting the chair velocity, we get the characteristic sinusoidal pattern. The first half cycle of each frequency tested is excluded from analysis for improved reliability. In the eye position graph, we can see the nystagmus that's generated by the vestibular ocular reflex and in the eye velocity graph, we can see the speed of that nystagmus in relation to the speed of rotation of the chair.
The chair velocity is the smooth the yellow line we can see plotted on the eye velocity graph. We should expect the eye velocity to be out of phase with the chair velocity. This is due to the compensatory movements generated by the vestibular ocular reflex. Phase is one of the parameters that we will analyze at the end of the test. Real time analysis of gain and phase occurs during the measurements.
At the end of each measurement, you'll be able to review the results before moving on to the next frequency. If we skip now, towards the end of the test to the higher frequency accelerations, you can see that the rotations are much shorter in distance and the measurement is shorter in time. But we are getting through more oscillations. This higher frequency testing stimulates the vestibular system more and allows us to assess the integrity of the vestibular system to different frequencies or intensities of movement.
Having completed all of the steps in this protocol, we can review the results.
First of all, we can assess gain. Gain is the amount of eye movement as a percentage of the amount of chair movement. Abnormally low gain across all frequencies is a strong indicator for bilateral peripheral vestibular loss. In rare cases, central pathologies will also cause reduced gain across all frequencies, but they will usually be associated with strong central indicators.
Partial injury to the peripheral vestibular system will cause low frequency gain loss. As a peripheral condition progresses, the mid and higher frequencies may also show low gain. Abnormally high gain is a very rare occurrence. If all measurement and technical errors have been ruled out, then this could point to a central lesion in the cerebellum.
The Phase graph describes how well the eye movements are timed with the chair movements. Different result patterns have significant clinical indicators. Abnormal phase in the presence of normal gain is the most common pattern of results you may see. And this indicates central compensation of a peripheral vestibular injury.
The symmetry graph can be considered in a similar way to directional preponderance. This means that the vestibular system may have a biased preference towards the rotation in one direction. Abnormal symmetry provides evidence of a potential unilateral vestibular pathology.
The final parameter is spectral purity. A high percentage of spectral purity indicates a more reliable result. The closer the data fits a sine wave, the higher the spectral purity. When spectral purity falls below 60%, it provides evidence that the response may not be of high quality, and the clinician should consider re-testing that frequency.
In summary, the SHA test provides the clinician with a number of very helpful applications including but not limited to:
SHA should also be used in conjunction with other tests. Most notably ocular motor tests in VNG, and the velocity step test in rotary chair testing.