PP1403 - A Study of Absolute Amplitude (N1P1) vs. Asymmetry Ratio for the Ocular Vestibular Evoked Myogenic Potential (oVEMP) Across Three Signal Averagers

Abstract:
Controversy exists regarding the diagnostic use of the oVEMP absolute amplitude (N1P1) versus asymmetry ratio. For example, oVEMP absolute amplitude is commonly used to diagnose Superior Semicircular Canal Dehiscence (SSCD)¹. oVEMP asymmetry ratios greater than or equal to 34 percent are used clinically to denote an abnormal utricle and/or superior vestibular lesion². In this normative prospective study (N=40), the oVEMP absolute amplitudes for N1P1 and asymmetry ratios were recorded and compared across three commercially available signal averagers. N1P1 oVEMP absolute amplitudes were shown to be the least reliable parameter with the most variability across equipment.

Summary:
Purpose
This study evaluated the reliability of oVEMP N1P1 absolute amplitude versus asymmetry ratio across three commercially available signal averagers: Interacoustics Eclipse EP25, Otometrics ICS Chartr EP200, and Natus Bio-Logic Navigator Pro. This equipment-specific normative data can be used to help determine the best measure to contribute to the clinical diagnosis of vestibular disorders. Currently, decisions about abnormality have been based on data from laboratory equipment that is not available to clinicians¹.   

Methods
Forty healthy subjects (16 males, 24 females) ranging in age from 20 to 55 years were enrolled. All subjects had normal hearing and no vestibular dysfunction. All subjects underwent randomized oVEMP testing for the three signal averagers. A 500 Hz tone burst with manufacturer-specified oVEMP parameters (typically used clinically) for each signal averager was used. For 16 of the 40 subjects, oVEMPs were collected a second time using identical recording parameters across the signal averagers. Absolute N1P1 amplitudes, N1 and P1 latencies, and inter-aural asymmetry ratios were measured for each participant and compared across the signal averagers.   

Results
A significant difference for absolute N1P1 amplitudes across the three signal averagers was demonstrated (p < 0.001, df=2,76, F=78.885). The highest absolute N1P1 amplitudes recorded were for the Bio-Logic Navigator Pro, and the lowest absolute N1P1 amplitudes recorded were for the Interacoustics Eclipse. When tested on the NavPro, >50% of participants exhibited an absolute N1P1 amplitude of ≥17.1 uV in the absence of SSCD. This cut-off (17.1 uV) has been commonly used in the diagnosis of SSCD¹. Note that absolute N1P1 amplitudes ≥17.1 uV were only seen in < 10 percent of participants when tested on the Interacoustics Eclipse. The Eclipse demonstrated the least amount of variability for asymmetry ratios, and no significant difference was demonstrated for asymmetry ratios across equipment (p=.514, df=2, 76, F=0.6).  Latencies for N1 and P1 demonstrated a statistically significant difference across signal averagers (N1- p=0.0, df =2,76, F=46.162; P1 - p=0.0, df=2,76, F=18.380). 

Conclusions
Across commercially available signal averagers, this study suggested that the absolute N1P1 amplitude for oVEMPs is the least reliable measure when compared to asymmetry ratios. Caution should be given when using absolute N1P1 amplitudes or latencies for oVEMPs in the diagnosis of vestibular disorders. Equipment-specific normative data must be established when relying on oVEMP absolute amplitudes and latencies to make clinical decisions.