The vestibulo-ocular reflex (VOR) can be tested using the following methods:
1. Head impulse testing (HIT):
- Head impulse can be safely and quickly completed by the patient’s bedside. It is used to diagnose a defective VOR that may occur in cases of vertigo.
- To complete the test, the patient is asked to fixate on a target while the examiner rotates the head.
- The starting position should be such that the patient’s head is turned slightly past the midline (15-20°) and then should be thrust to the opposite side using abrupt movements.
- The movements should remain small (the patient’s head should not move very far either to the left or right), and the movement direction should be unpredictable.
- An abnormal result is indicated when the patient is unable to maintain a steady gaze on the target during a head thrust.
- If the patient has a unilateral loss, the abnormal head impulse tilting will occur with a thrust toward the side of the lesion.
- In a bilateral loss, the abnormal result will occur with head thrusts in both the directions.
2. Rotational chair testing:
- Another method of testing VOR is computerized rotational chair testing. This test causes stimulation of both canals at once, which provides information about the function of the vestibular system as a whole during head rotation. Several rotational chair tests are used clinically to assess VOR function:
- Sinusoidal harmonic acceleration testing chair: The chair is moved in a sinusoidal fashion with successive increases in frequency, usually from 0.01-2 Hz. This allows for the analysis of eye movements in response to various angular accelerations. This test provides information about VOR at lower velocities.
- Constant angular acceleration testing chair: For this test, the chair must be able to rotate 360°. Rotational velocity of the chair is increased at a constant acceleration until a maximum velocity is reached. The chair rotates at a steady velocity for a time, after which deceleration occurs gradually. This test can be completed in both directions.
- Impulse angular acceleration testing chair: For this test, the chair rotates 360° at a constant rate until the chair rotation is abruptly stopped. The test is completed several times in both directions, and the results are plotted across time.
- Velocity step testing chair: In this test, the chair is accelerated very rapidly (100-200°/s), and this velocity is maintained for 60 seconds, after which rapid deceleration occurs. This deceleration is perceived as rotation in the opposite direction. Gain and time constant values are measured. Results from this test can also help identify unilateral peripheral loss.
3. VOR testing in pediatric patients:
- VOR is generally tested by turning the patient's head on their long axis and observing for the doll's eyes response (DOLL). The eyes deviate opposite to the direction of head rotation.
- A possibly related maneuver has been used in infants as a neurologic test.
- The examiner turns 90-180° in 1-2 seconds on their own long axis, while holding the upright infant at arm's length, face-to-face. The response to this test is usually described as the deviation of the eyes opposite to the direction of rotation.
- In using this maneuver (both as a neurologic test and to elicit eye opening), it has been observed that deviation of the eyes in the direction of the rotation, rather than opposite to it, is an abnormal response.
- Patients with reduced VOR function cannot generate sufficient compensatory eye movement, meaning their eye movements are not large enough to compensate for their head movements. A common complaint from these patients is the experience of visual blurring while in motion (walking and driving). This is called oscillopsia. There is an infinite list of examples demonstrating the necessity of our VOR. Head and body movements are constantly occurring, and a focused foveal image during those movements will prevent a blurred image being presented to the brain.