vHIT refers to relatively new technology to assess the high-frequency Vestibular Ocular Reflex (VOR). vHIT stands for video head impulse test, and theoretically allows the examiner to find VOR deficits that would not be visible under standard examination techniques. This may allow less experienced examiners to detect obvious deficits, and allow more experienced examiners to detect milder deficits.
The basis of the test is that it provides objective eye movement data, in the same way that recording of nystagmus through videonystagmography allows for documentation and measurement of pathologic or physiologic nystagmus. Those results can then be compared to established normative data.
Unlike the recording of nystagmus, vHIT records the latency and gain of the VOR in response to impulsive head movement. It is basically a computerized Halmagyi Head Thrust exam. At high speeds, the VOR should have a gain very close to 1, and be 180 degrees out of phase with head movement. In layman’s terms, this means that the amount of eye movement should be equal to and opposite the amount of head movement, and that the eye should react almost immediately when the head moves.
Keep in mind that the goal of the VOR is to visually offset head movement. In order for you to keep your eyes stable (therefore focused) on an object of interest while you move your head, the eyes have to do exactly opposite what the head is doing.
A simple demonstration of the VOR can be done while reading. Unfortunately, unless you are reading this on a tablet, this demonstration may require you go pick up a book or magazine. Hold the book at normal distance, and move your head back and forth horizontally as fast as you can and still be able to make out the words (or maintain focus). Then, hold your head still, and move the book back and forth at approximately the same speed that you moved your head. If you have a normal functioning VOR, you will see that your focus and visual stability breaks down when the book is moving at high speeds, while your focus is stable with similar speed head movement viewing a stationary object. If you don’t see a difference, you may have a VOR deficit. Visual blurring with head movement is a common complaint of vestibular patients.
Next week, I will show you an example of vHIT equipment and test results.
Photo courtesy of scienceopen.com.
