Question
Do you look at waveform morphology to define abnormal VEMP results, amplitude ratio, or a combination of both?
Answer
Generally in the case of VEMP, the waveform morphology falls apart when there is a low amplitude. We interpret this more frequently as being a case of low amplitude rather than poor morphology. Generally speaking, people are simply looking to see if the response is present or not, and then additionally defining if the amplitude is low or of normal size.
There are a couple ways of calculating an amplitude ratio. One is described by Young, Wu and Wu in 2002, where you subtract the amplitude of one side from the other, and divide that by the sum of the two amplitudes, which gives you what they called an "augmented VEMP." It gives us a basis for making that sort of a determination. There have been some other studies since that time that have taken the augmented VEMP calculation and turned it around, using it to show that there is actually a side that is much weaker than normal.
The ratio formula that I use is taking the amplitude of one side over the amplitude of the other side, using that fraction ratio as a comparison of test and retest. This ratio calculation should be used as a quality control of test-retest reliability rather than for independent diagnostic conclusiveness.
Young, Y.H., Wu, C.C., & Wu, C.H. (2002). Augmentation of vestibular evoked myogenic potentials: an indication for distended saccular hydrops. Laryngoscope, 112 (3). 509-512.
Dr. Ackley is Director of Audiology at Gallaudet University and former department chair at the University of Northern Colorado and Associate Professor at Colorado State University prior to that. He had an electrophysiological assessment private practice in Colorado from 1987-2000, and he currently directs the Gallaudet Electrophysiology and Balance Center. He has done extensive research on vestibular evoked myogenic potential (VEMP) assessment of Deaf and hard-of-hearing subjects during his tenure at Gallaudet.