Utilizing Video Head Impulse test (vHIT)
Purpose of test:
To assess the patient’s vestibulo-ocular reflex (VOR) for the Left and Right Lateral Semicircular Canals (SCC). The test allows you to view head and eye movement tracings simultaneously in real time. VOR instantaneous gain and velocity regression are calculated for both Left Lateral and Right Lateral canals during this test.
Two calibrations will be completed prior to beginning the test. Standard calibration is completed to calibrate the patient’s eye relative to the laser targets. Place the patient 1.5 meters from the wall on which the laser targets will be projected. Once standard calibration is started, the patient will see 5 red laser dots appear. Instruct the patient to keep his/her head still while moving their eyes between the dots as instructed. Head calibration is completed to calibrate the Inertial Measurement Unit (IMU). With the patient’s eyes fixated on a single target, instruct him/her to gently move the head side to side approximately 5° from center in either direction for 5 seconds. Next, have the patient move the head up and down, following the same procedure.
Beginning the Test:
To begin a lateral vHIT test, select lateral from the session menu, click prepare, then start. The impulses will be performed while standing behind the patient with your hands placed beneath the goggle strap around the patient’s jaw. You will be testing in the plane of Left Lateral/Right Lateral canals, as shown below.
“I will be moving your head side to side in small movements. Keep your eyes focused on the target on the wall the entire time. Please do not try to resist the head movements, as this will negatively impact the test results. Simply keep your neck loose to allow me to perform the small movements.”
A guide is present on the test screen to help you generate lateral impulses of appropriate acceleration and velocity. A green check mark indicates that the impulse was successfully completed and met the criteria to be included in the final report. A red “x” indicates that the impulse did not meet the criteria and will not be allowed into the final report. The counter next to test name allows you to view how many accepted impulses have been collected.
What to Expect:
A patient with normal Left and Right SSC function will produce eye tracings relative to head tracings that are essentially 180° out of phase, resulting in VOR gain that is near 1.0 for each of the SCCs tested. Velocity regression should show little to no asymmetry between the two canals tested. There will be no catch-up saccades present in the tracing, as the patient was able to keep their eyes fixated on the target during the lateral impulses.
Normal Lateral Test Result
Abnormal Test Results:
An “abnormal” lateral result will show reduced gain in one or both of the canals tested. Because the patient was not able to keep his/her eyes fixated on the target during impulses, he/she must produce a “catch-up” saccade to bring the eyes back to the target. Below are examples of “abnormal” lateral vHIT tracings:
Lateral vHIT test showing a Left Lateral Canal weakness in an acute stage of neuritis
Lateral vHIT test showing a bilateral asymmetrical lateral canal weakness in a cochlear impatient patient
Lateral vHIT test can be used by the clinician to help determine the presence of Left/Right Semicircular Canal dysfunction. Lateral vHIT testing should not be relied upon by itself, but rather should be used in conjunction with RALP and LARPl SSC tests, as well as other vestibular tests to diagnose the patient.
Note: This is intended only as a guide, official diagnosis should be deferred to the patient’s physician.