Question
Could you review the VEMP and its correlation to caloric testing? Also, does a normal VEMP in the presence of a unilateral caloric weakness indicate saccule involvement only?Answer
The caloric test evaluates the lateral (horizontal) canal and its afferent pathways, which travel through the superior portion of the vestibular nerve. VEMPs evaluate primarily the saccule and its afferent pathways, which travel through the inferior portion of the vestibular nerve. VEMP responses are typically quantified by Asymmetry Ratio, defined as the difference between the right-left VEMP amplitudes divided by the sum of the right-left VEMP amplitudes. The norms for VEMP responses are still emerging but most laboratories consider Asymmetry Ratio of greater than 35% or 40% to be abnormal.
When the caloric test is abnormal and VEMPs are normal, it indicates a focal lesion involving the lateral canal and/or superior portion of the vestibular nerve. When VEMPs are abnormal but the caloric test is normal, it indicates a focal lesion involving the saccule or the inferior portion of the vestibular nerve. The only stipulation in this case is that if both the unilateral caloric weakness and the VEMP asymmetry ratio are borderline, it may not reflect one test being normal and the other being abnormal. It may simply reflect the difference between the normative limits for the two tests.
More information on this topic can be found in Dr. Barin's course, Correlations in Vestibular Test Findings: www.audiologyonline.com/ceus/recordedcoursedetails.asp?class_id=13040
Kamran Barin, Ph.D. is the Director of Balance Disorders Clinic at the Ohio State University Medical Center and Assistant Professor, Department of Otolaryngology, Department of Speech and Hearing Sciences, and Biomedical Engineering Program. He received his Master's and Doctorate degrees in Electrical/Biomedical Engineering from The Ohio State University. He has taught national and international courses and seminars in different areas of vestibular assessment and rehabilitation. He has served on NIH and NASA panels and was an appointed member of the Equilibrium Subcommittee of the American Neurotology Society. He works as a consultant to GN Otometrics and provides courses and other educational material to the company.
