This week, I am using an excerpt from my book, Vestibular Function: Clinical and Practice Management (Thieme, 2011) to describe the process involved in Videonystagmography.
Infrared Video (as used in VNG) involves the use of a conventional black and white video camera; however, the eyes are illuminated with infrared light. Because the eyes are not reactive to infrared light, the eye can be viewed while the patient’s eyes are in total darkness, eliminating the possibility of visual fixation, which is known to suppress peripheral vestibular-generated nystagmus.
The camera is able to take rapid, sequential pictures of the eye, so even very fast eye movement can be recorded accurately. Because the picture is black and white, the eye is represented by black, white, and gray dots (or pixels). The pupil is a hole in the front of the eye that allows light to pass into the eye. As a result, it is not as reflective as the iris (the eye color portion) or the sclera (the “white” of the eye). Because the infrared light is not reflected from the pupil, it appears as a black spot on the screen, and this “dark spot” can be tracked for recording. A caveat of infrared recording is that it will track the darkest spot in the camera’s field. Dark-colored mascara, as well as the sides of the camera goggles, can sometimes be darker than the pupil. These problems can be overcome by removing mascara, adjusting the recording threshold of the computer, or packing white gauze into the sides of the goggles.
Several factors must be considered when making the decision to purchase ENG or VNG equipment. Significant differences exist between infrared video and EOG systems in the following areas: calibration, artifact, recording of rotary nystagmus, the presence of Bell’s phenomenon, and cost-effectiveness.
Each of these is discussed in detail in the next few blogs.
Photo courtesy of Micromedical Technologies, and my thanks to Rick Miles for his help in writing this passage.