The sinusoidal vertical-axis rotation test.

The sinusoidal vertical-axis rotation test (formerly the sinusoidal harmonic acceleration test) measures various parameters of normal and pathologic vestibular systems. It provides objective diagnostic information and serves as an adjunct to electronystagmographic positional and caloric testing. It covers a larger portion of the dynamic range of the semicircular canals than the caloric stimuli.

Maximum stimulus is provided by a horizontal oscillation of the chair as the patient's head is tilted forward to bring the plane of the horizontal semicircular canal to a right angle to the axis of rotation. Eye movements are calibrated in a manner similar to that used in electronystagmography.

During the test, a sequence of sinusoidal angular velocity signals is transmitted; peak amplitude is kept constant at approximately 60[degrees]/sec. The frequency ranges from 0.01 to 1.0 Hz. Some commercially available products have smaller ranges, depending on the amount of torque that is exerted by the motor that moves the chair.

The test is usually performed in total darkness as the patient keeps the eyes open. A computer program controls the rotary chair. This program also measures the motion of the chair (input) and the simultaneous eye movement (output). The technician is in constant communication with the patient to maintain maximum patient alertness.

Three primary parameters are calculated during the testing sequence: gain, phase, and symmetry (figure).

* Gain is calculated by dividing the output amplitude by the input amplitude.

* Phase is the timing difference between the output and input sinusoids, expressed in degrees. A phase measurement signifies a lead or lag in relation to the input sinusoid.

* Measurement of the symmetry of the response identifies any central nervous system (CNS) compensation. Abnormal symmetry occurs when there is a significant difference between the responses from each horizontal semicircular canal. As CNS compensation takes place, either with medical or surgical treatment, an asymmetrical response moves toward symmetry, generally paralleling the degree of clinical improvement.

Another parameter that is usually examined is the ocular fixation suppression of rotation-induced nystagmus (figure). During this test, the patient focuses on a laser light that is projected on the wall of the test enclosure as the chair is rotated.

Suggested reading

Wall C. The sinusoidal harmonic acceleration rotary chair test: Theoretical and clinical basis. In: Arenberg IK, ed. Dizziness and Balance Disorders: An Interdisciplinary Approach to Diagnosis, Treatment, and Rehabilitation. New York; Amsterdam: Kugler, 1993:299-313.