ENG versus VNG: Part III

VNG gogglesPotential Calibration Errors

As a reminder, Electronystagmogaphy (ENG) is based on recording changes in the corneoretinal potential, with these changes assumed to correspond to eye movements. However, there are other factors that cause a change in corneoretinal potential, putting test accuracy at risk.

First, you cannot start your exam as soon as the patient is registered and the electrodes applied. There is a waiting period after turning out the lights before the corneoretinal potential stabilizes. An analogous situation may be the speed of pupillary reaction to changes in lighting. We know that pupillary reaction is slower in the older population, so it is possible that older patients will require additional time to adapt to lighting changes during vestibular testing.

The corneoretinal potential varies in response to light exposure, increasing with more light, decreasing in the dark. Even after a brief exposure to light, such as someone opening the door connecting to a lighted hallway, an adaptation period follows that can last a few minutes or up to as long as 50 minutes. The variation in amplitude of recordings of eye movements can vary by as much as 20% to 30% in response to only a few seconds of light exposure.

Infrared video can record in any ambient lighting condition as well as in complete darkness. Because no adaptation period is required, testing can begin immediately on placement of the goggles, and introduction of light into the test room will not require recalibration.

A potential source of calibration error when performing Videonystagmography (VNG) occurs whenever the camera is moved. The calibration is based on the distance from and the angle of the camera on the eye. Recalibration is needed when the infrared camera is moved or repositioned.

I will give round one to VNG.

About Alan Desmond

Dr. Alan Desmond is the director of the Balance Disorders Program at Wake Forest Baptist Health Center, and holds an adjunct assistant professor faculty position at the Wake Forest School of Medicine. In 2015, he received the Presidents Award from the American Academy of Audiology.