Rotary Tests

What is the vestibular ocular reflex? The short answer is “reflexive eye movement in response to head movement.” It only makes sense then, that tests of the VOR would involve recording eye movement in response to head movement. Unfortunately, the ENG battery only evaluates the eyes response to head movement during the Dix-Hallpike test. The caloric test could be called simulated head movement, but why settle for a substitute?

The two widely used rotary tests: Rotary Chair (RC) and Active Head Rotation (AHR) involve recording the slow phase response of eye movement in response to head movements across a wide range of frequencies (head speeds). In RC, the fast phase of nystagmus is eliminated from the recording. In AHR there is no fast phase.

During normal to and fro head movements we encounter in real life, there is no nystagmus. The VOR responds to head movement, allowing for visual stability and clarity as we move about. The prolonged rotation used in RC is not a typical stimulus encountered in day to day life. Additionally, the VOR is only as efficient as it has to be. Since the visual pursuit system is pretty accurate up to about 1 Hz, the VOR does not have to be efficient (therefore is not) for slower speed head movements. Above one to two Hertz head movements, only the VOR can provide visual stability. You will note on pages 100 and 101 of my book that gain of the VOR for low frequencies can be a low as .3 and still be considered normal. Normal values approach a gain of 1 with increasing head speed. You will also note the AHR recording on page 104, showing that normal gain for higher speed head movements should be about 1. Remember, a gain of one means that for every one degree of head movement, there is an equal one degree movement of the eye in the opposite direction.

I am sure that by now you are wondering why we even bother to do low frequency testing, when it seems that we really need to know how well the VOR works at higher speeds than are typically tested with RC. The answer is simple. Low frequency RC tests are repeatable, reliable, and largely unaffected by artifact (eye blinks, etc.). High frequency AHR testing, while very useful, has pretty poor test-retest reliability. Ideally, an evaluation of the VOR through the entire frequency range would be done on patients with suspected vestibular pathology. If I had to choose only one of the two tests, I would do RC on most patients. The expense of a rotary chair ($60 to 90K) makes it cost prohibitive for all but a few clinics specializing in vestibular disorders. AHR is a lot cheaper, and can provide useful information over and above that which can be learned from ENG alone. To get a better feel for the importance of rotary testing, read my article for VEDA “Common Misconceptions about ENG”. The main point of the article is that you can identify many vestibular problems through ENG, but you can not rule out a vestibular problem based on a normal ENG exam. Several balance clinics perform RC as an initial screening exam for possible vestibular dysfunction. While there is no one test to separate vestibular from non-vestibular patients, RC is by far the most sensitive. RC is also very helpful in the diagnosis of bilateral vestibular hypofunction, most commonly seen in ototoxicity. If you get a very weak response to calorics at the standard temperatures, you will want to know what kind of affect that is having on the VOR as the patient moves around.