Test-Retest Reliability of oVEMP's Across Different Electrode Montages

Specific Aim: Compare the difference in the contralateral peak-to-peak N1-P1 amplitude and N1 latency between the belly tendon electrode montage with the conventional infraorbital electrode montage in otologically normal patients grouped by decade with the goal of developing normative values to use clinically.

The oVEMP is a short latency (~10 ms), negative polarity evoked myogenic potential that is recorded from the extraocular muscles. The oVEMP test is well-tolerated by patients and is simple to administer. Whereas the recording of the cVEMP to air-conducted stimuli represents a means for evaluating the function of the saccule and inferior vestibular nerve, the oVEMP in response to mechanical, and probably air conducted stimuli, is now felt to represent a method for assessing the functional integrity of the utricle and superior vestibular nerve.

The oVEMP response is recorded from beneath both the ipsilateral and contralateral eyes, but the largest response is typically seen in the contralateral recording. The contralateral utricular-ocular pathway terminates on the inferior oblique muscle, the electrical field of which can be recorded by an electrode placed at the midline of the lower lid and having the subject gaze upward (i.e. which results in the inferior oblique muscle becoming more superficial to the location of the surface electrode.

The the absolute oVEMP N1 peak-to-peak amplitude has become the most important measurement for diagnostic purposes. Typical oVEMP latencies are ~11 ms for the negative peak (N1) and ~15 ms for the following positive peak (P1) .

Variables that may confound recording oVEMPs include body position and electrode montages. Regarding electrode placement, there are three different electrode montages that have been studied and implemented clinically. The vast majority of clinics favor the electrode montage reported in the early papers describing the oVEMP. The authors reported that the non-inverting electrode was placed infraorbitally at the margin of the lower eyelid. The inverting electrode was placed 2 cm inferior to the location of the non-inverting electrode. Two additional electrode montages that have been reported include: 1) the noninverting electrodes placed at the margin of the lower eyelid slightly lateral to midline and the inverting electrode placed rostral to the inner canthus of the eye, and, 2) the noninverting electrodes placed at the middle of the lmargin of the lower eyelid and a single common inverting electrode placed on the chin. A report by Sandhu et al. (2013) indicated that the maximal oVEMP amplitude was recorded with the former electrode montage. In this montage, the inverting electrode is placed over a tendon, which is believed to be (relatively) electrically neutral.

Skin surface electrodes are sensitive to the desired muscle activity (i.e. signal) recorded locally as well as non-stimulus-related electrical activity (i.e. noise) that can be both endogenous and exogenous. Where the signal is common to both the inverting and non-inverting electrodes this activity routed into a differential electrode can result in cancellation of some or all of the desired signal.

Recently published data from our laboratory has shown that the Sandhu montage (i.e. the "belly-tendon electrode montage) in the sitting position is the optimal method for testing patients who are otologically and neurologically normal. This montage is associated with a larger N1 amplitude and a larger peak-to-peak amplitude for oVEMP measurements for young, normal patients. Due to the knowledge that the amplitude of the oVEMP response decreases with age, however, normative values based on decade would be necessary to have the oVEMP response be as clinically useful as possible.

Significance and Potential of the Research:

Establishing normative values based on decade will allow for higher sensitivity and specificity to vestibular impairments affecting the utricle and superior portion of the vestibular nerve, as well as increase the sensitivity and specificity to disorders such as Superior Semicircular Canal Dehiscence Syndrome.

Methods:

Subjects: Participants will include groups of patients between 20-90 years of age. The groups will be composed of 10-15 subjects per group to allow for normative data to be calculated. Participants will be recruited by phone using databases housed in the Odess Otolaryngology Department at Vanderbilt Bill Wilkerson Center, via flyers posted at Vanderbilt University Medical Center, and through Research Match email blasts. The study protocol will undergo review by the Vanderbilt Institutional Review Board (submitted-see Appendix).

oVEMP Recording: For the oVEMP recording (utricular assessment), subjects will be seated in a comfortable reclining will be compared during this study. First, an electrode montage and recording parameters similar to those reported by Chihara et al. (2007) will be used to record the oVEMP. Disposable silver/silverchloride electrodes will be placed infraorbitally beneath each eye (i.e., infraorbital 1 cm) representing the non-inverting amplifier inputs and 3cm infraorbitally will serve as the location for the inverting electrode. A second electrode montage with recording parameters similar those reported by Sandhu et al. (2013) will be used simultaneously to record oVEMP responses. Disposable silver/silverchloride electrodes will be placed on the belly of the inferior oblique muscle representing the non-inverting amplifier inputs and the tendon of the inferior oblique muscle will serve as the location for the inverting electrode. The ground electrode will be placed at Fpz for both electrode montages.

When recording the oVEMP, the subjects will be seated and instructed to keep their head at midline and gaze at a target positioned ~30° upward at midline. During the recording, the investigator will monitor to ensure the subject's chin is parallel to the horizon. Stimuli for the oVEMP recordings will be presented monaurally through Etymotic ER-3A insert earphones or a B81 bone conductor and consist of 500 Hz tone bursts presented at a rate of 5.1/sec. The tone burst will have a 2 cycle rise time, 1 cycle plateau and a 2 cycle fall time. A Neuroscan (Herndon, UA) multi-channel evoked potential recording system will enable us to record simultaneously the oVEMP response from each of the two electrode montages. EMG activity will be amplified by 100,000 times and signal averaged over 100 msec. A minimum of 120 individual samples will be collected for each recording. Each tracing will be replicated at least one time so that the waveform reproducibility can be estimated. Order effects will be eliminated by counterbalancing the order of the starting ear. oVEMPs from the two electrode montages will be recorded simultaneously, so counterbalancing the electrode montage will be unnecessary. A minimum of two amplitude measures will be completed for each electrode montage condition in each position, for a total minimum of 8 recordings per subject. The subject will be given 1-2 minutes to rest their eyes between recordings.

Stimulus level will be calibrated in dBpeak SPL using a 2-cm3 coupler to a sound-level meter (Bruel \& Kjaer). The stimulus waveforms and amplitude spectra will be measured by routing the air conduction output of the sound-level meter to a spectrum analyzer.

Statistical Methods:

Analysis: Summary statistics will be provided for all demographic and clinical variables, along with 95% confidence intervals and side-by-side boxplots. Between-group differences in categorical variables will be assessed via chi-squared tests while differences in continuous variables will be assessed via the Wilcoxin rank-sum test (using significance level alpha = 0.0125 in primary analysis as per the power calculation above). Hotelling's T-squared distribution will also be used as an omnibus or simultaneous test that the groups are the same for all variables in the primary analysis. Amplitude differences obtained using each montage will be calculated as a change expressed in percentage using the formula below:

(n1-p1 amplitude montage2 - n1-p1 amplitude montage 1)

--------------------------------------------------------------- n1-p1 amplitude montage 1

All statistical analyses will be performed with SPSS 23.

Facilities Available: The Vanderbilt Balance Disorders Laboratory is located in the Bill Wilkerson Center. The Balance Disorders Laboratory is equipped with state-of-the art equipment for comprehensive assessment of vestibular and auditory disorders. The facility includes a sound treated room, five diagnostic audiometers, five compact disk players for speech audiometry, five immittance meters, three otoacoustic emissions devices, two rotary vestibular test chairs, three videonystagmography systems, a computerized electronystagmography system, a computerized dynamic posturography system, a Neuroscan evoked response system with capability for 64 channel simultaneous brain signal acquisition, and 4 2-channel clinical evoked potentials systems.