Vestibular-evoked myogenic potentials (VEMPs) are electrophysiological measures of saccular and utricular reflex pathways. These potentials are currently the only method available to gain information on these important balance organs in the clinic. VEMPs are recorded easily and have been accepted widely due to the novel information they provide to the vestibular diagnostic evaluation. In most clinics in the United States, VEMPs are recorded using air-conducted short-duration tone bursts or clicks (Rosengren et al, 2009). To record these responses, however, high-intensity stimuli between 120 and 140 dB pSPL are required, reaching the upper limit of what is considered safe exposure.
During VEMP testing, the cochlea is exposed to high sound pressure levels (Krause et al, 2013; Mattingly et al, 2015; Stromberg et al, 2015), but unfortunately, there is limited reported information about the possible effects of VEMP stimuli on the cochlea. Several studies have evaluated the effects of this stimulus on cochlear function with results showing decreased distortion product otoacoustic emissions (DPOAEs) after VEMP testing, but with no significant changes in hearing thresholds (Krause et al, 2013; Stromberg et al, 2015).
One report identified a case study of sudden permanent bilateral sensorineural hearing loss after VEMP testing with stimulation intensities ranging between 128-135 dB pSPL (Mattingly et al, 2015). Despite the limited information, these reports demonstrate a concern for those completing VEMP testing, indicating that we must take care to safely measure VEMP responses without possible damage to the cochlea.
How Is Sound Exposure Measured?
Noise exposure standards have been put in place to protect workers from hearing loss due to occupational noise encountered over a work day. We use these standards to describe the possible noise dose obtained from our test stimuli and the associated level of risk. These standards include a recommended exposure level (e.g., U.S. National Institute for Occupational Safety and Health [NIOSH],1998 and European Union [EU], 2003) for a damage-risk criterion (DRC). If a person's exposure exceeds these recommended levels, then he or she is at increased risk for hearing loss.
While several DRCs exist for industrial noise exposure across the world, there is no specific recommendation for patient noise exposure in a health care setting. Considering this, we will use existing occupational standards. When evaluating sound exposure from VEMPs, we must consider both instantaneous sound levels and the actual exposure measured over time. For instantaneous sound levels, both the NIOSH and EU recommendations set upper limits of 140 dBC for impulsive noises (i.e., sounds lasting less than one second).
While the published DRC limits for impulsive noises were developed to prevent hearing loss in the majority of workers (Ward, 1961; Price, 1981; Committee on Hearing and Bio-Acoustics, 1992), they still leave the 25th percentile susceptible to hearing loss (i.e., 75 percent of ears would not be at risk for sudden damage at 140 dB pSPL) and the fifth percentile susceptible to hearing loss with stimuli as low as 132 dB pSPL. Because a small percentage of ears might incur sudden acoustic trauma at stimulation levels as low as 132 dB pSPL, the regulatory standards are not protective of all individuals.
We also must consider the "noise dose" of the total test stimulation. For example, NIOSH establishes the maximum (or 100 percent) noise dose for an eight-hour exposure at 85 dBA. Noise dose is a cumulative measure and sums exposures from various activities throughout the day to calculate the total noise dose. Noise doses exceeding 100 percent put the individual at a higher than normal risk for hearing loss.
How Are VEMP Intensity Levels Measured?
|STRATEGIES TO REDUCE VEMP NOISE EXPOSURE|
|Minimize number of sweeps.|
|Minimize number of repetitions.|
|Minimize stimulus duration.|
|Limit search for threshold to necessary cases (e.g., superior semicircular canal dehiscence).|
Consider starting at a lower intensity level.
|Consider the patient's total daily noise exposure in addition to VEMP exposure.|
Air-conducted VEMPs require high sound pressure levels, making it important to understand the patient's overall sound exposure from the test. First, we need to know the actual output level of the evoked potential equipment. Measurements in dB SPL are not adequate for reporting VEMP output, as dB SPL reflects some degree of averaging, either averages of multiple stimuli over time or root-mean-square averages of a specific stimulus token—these measurements do not reflect instantaneous peaks.
Because VEMP recordings are saccular and utricular responses to the peak intensity of the stimulus, understanding peak sound pressure levels is needed to determine the risk for possible acoustic trauma. Evoked potential equipment can be calibrated by measuring either peak (pSPL) or peak-equivalent sound pressure level (peSPL) in accordance with IEC 60645-3:2007. Some sound level meters can measure transient stimuli using a "peak hold" type of response to obtain a pSPL measurement. Otherwise, pSPL can be measured using a microphone, preamplifier, and conditioning amplifier, then viewing the absolute greatest amplitude of the signal on an oscilloscope. A sound level meter set to a "fast response" will average sound levels measured over a given time (e.g., 125 ms), and will result in measured levels approximately 6 dB below the actual peak of the signal for 500 or 1000 Hz tone bursts (Beattie and Rochverger, 2001).
Another method for calculating sound pressure level is peak-equivalent SPL (peSPL), which is derived from adjusting the level of the output until it is equivalent to the peak-to-peak, baseline-to-peak amplitude, or voltage of the transient. Of note, pSPL is typically 3 dB greater than peSPL depending on which peSPL value is used (Laukli and Burkard, 2015). It is important to note that pSPL and peSPL are two separate references for describing the stimulus output. Knowing how our evoked potential systems are calibrated allows us to understand the actual output of the device.
What Is Your VEMP Stimulus Intensity?
The first step in creating a safe and effective VEMP protocol is to know the output levels produced by the evoked potential system. If equipment output is provided in either dB nHL or dB HL, clinicians should know their equipment-specific conversion factor between those values and dB pSPL or peSPL. This often can be obtained from the manufacturer or during equipment calibration. Each evoked potential system may have equipment specific calibration methods for deriving peak output. Keep in mind that conversion factors can vary between sites and between equipment. For example, a 95 dB nHL 500 Hz stimulus may be equivalent to 126 dB peSPL in one lab, but 122 dB peSPL in another, potentially leading to different outcomes.
Clinicians should evaluate their current VEMP protocols to ensure VEMP stimuli are within accepted limits for safer exposure to sound. For conservative protocols, total exposure should not exceed 100 percent of the NIOSH noise dose, instantaneous sounds should not exceed 132 dB pSPL, and total energy should remain below 132 dB SPL over one second. Exceeding these limits increases the risk of incurring noise-induced hearing loss (NIHL) in highly susceptible individuals (Price, 1981).
How Can I Modify My Protocol for Safer VEMP Testing?
Air-conducted VEMP stimuli are loud and can be uncomfortable to some patients. While we take care to minimize this as much as possible, we should consider any possible medical-legal concerns associated with VEMP test paradigms. To better protect yourself and your clinic, your first step is to establish a safer protocol that reduces the noise dose your patient receives. Your clinic should consistently use your established protocols and should document that each patient's test exposure fell within these safer criteria so that you have information regarding the test protocol if a patient has concerns about hearing loss after testing.
The output of the evoked potential system is key to providing a safer sound exposure, but variations in test protocol variables also can contribute to the overall output. Table 1 provides several strategies to help the clinician maintain exposure below the recommended total energy levels. Reducing stimulus duration and/or the total number of stimuli presented will diminish the patient's total sound exposure to levels that are well within safe limits for exposure. In addition, a small reduction in maximum intensity will reduce a patient's exposure. For example, with the same number of stimuli, a reduction of 3 dB will reduce a patient's overall dose by half. Often this small reduction in intensity does not significantly affect the diagnostic utility of the VEMP data, but does impact the noise dose for that patient.
Clinical protocols for safer VEMP testing should evaluate several stimulus parameters that you can control. The most important considerations are the stimulus intensity, stimulus duration, and number of sweeps. Table 2 provides a protocol that generally does not exceed NIOSH recommended noise exposure values. Keep in mind that variation in these parameters will affect the overall exposure level. For example, if the number of sweeps or intensity is changed, the total noise exposure will vary accordingly. The clinician should not only be mindful of output level for each variation in protocol but also the projected dose for all testing to be completed (e.g., if both cVEMP and oVEMP testing are to be completed). Once you have settled on your protocol, note that your responses may vary from the reported literature. For example, since VEMP testing requires high-intensity stimulation, if you chose to test at a lower-intensity level, the response amplitude will decrease (Colebatch et al, 1994).
Further, altering the stimulus duration also will influence the response amplitude and latency (Welgampola and Colebatch 2001), but does not impact clinical interpretation of the VEMP. Similarly, increasing or decreasing the number of sweeps minimally alters the response amplitude, however, it significantly influences the daily noise dose. See FIGURE 1 for an example of change in number of sweeps on VEMP response, output, and noise dose. Overall, these modifications are likely to provide minimal changes to your responses and ultimately your clinical findings, while more importantly, providing a safer VEMP stimulus to your patients.
|Intensity||< 126 dB pSPL*|
|Stimulus duration||4 ms (e.g., 2 ms rise/fall, 0 ms plateau)|
|Number of sweeps|
|Number of repetitions||Two trials at high intensity|
|Two trials at low intensity level|
Consider threshold search in 10 dB steps if asymmetry or concern for third window disorder
|*Include corrections for ear canal volume if necessary|
A Tool for Calculating Noise Exposure from VEMP Stimuli
Portnuff et al (2017) includes a supplemental Microsoft Excel (2013) worksheet that can be downloaded to calculate the noise exposure level that takes into account stimulus intensity, duration, number of sweeps per trial, and number of trials. The worksheet provides three metrics that can weigh the potential risk to an individual patient from their VEMP exposure: the NIOSH and EU DRCs, and a total energy over one second exposure as described by Colebatch and Rosengren (2014). A clinician can use these metrics to judge whether the patient may be at increased risk for NIHL due to a certain VEMP stimulus exposure. It is important to note that this tool allows clinicians to calculate VEMP exposure from tone burst stimuli only and cannot be used for click stimuli.
Should VEMP Testing Be Modified for Certain Populations?
While you have taken care to use an appropriate protocol that meets safer sound exposure limits, there are several populations that may be at higher risk and require additional consideration. These populations include children, those with tinnitus or hyperacusis, those with third-window phenomena, and those with high daily noise exposures outside of the clinic (Table 3). For these patients, the clinician should weigh the diagnostic needs of the individual and if sufficient benefit is gained from obtaining VEMP information.
Infants and young children are often evaluated with VEMPs due to ease of administration. For pediatric protocols, modifications can be made to reduce the sound exposure to these smaller ears without compromising diagnostic information. Maes et al (2010) found that for typical children between ages 4 and 12, VEMP responses can be obtained at approximately 120 dB SPL (note, the reference value was not provided). In these younger populations, using a lower intensity level will lead to safer noise exposure without sacrificing clinical diagnostic utility.
Ear canal volume in these young patients can be used to help determine which lower intensity to use. As a rule, for every halving of volume in the ear canal, there is a doubling of sound pressure based on 2cc coupler measurements (Beck et al, 2009). This estimate is conservative but measuring ear canal volume before VEMP testing will allow for adjusting maximum output levels to accommodate for this change (e.g., -6 dB for 1.0 cc, -12 dB for 0.5 cc). This should be obtained easily, as evaluation for middle ear function before VEMP testing is necessary because of the effects of conductive hearing loss on the response (e.g., Zhou et al, 2012). Knowledge of ear canal volume will allow you to quickly modify your stimulus intensity to provide a safer protocol for the individual child.
For example, in a pediatric clinic, if the clinician followed the recommended parameters provided in Table 2, the noise dose may be higher than anticipated due to the expected increase in intensity in smaller ear canals. A child with an ear canal volume of 0.5 cc would have 12 dB higher output than the average adult, resulting in a noise dose that would be unsafe for that child. If in doubt, consider work recently published by Rodriguez and colleagues (2018) which recommends not exceeding 120 dB peSPL.
Patients with Tinnitus or Hyperacusis
Tinnitus and hyperacusis are commonly reported symptoms in the vestibular laboratory and specific care should be taken in these patients. At this point, there is minimal literature evaluating VEMPs in patients specifically with tinnitus; however, the authors' clinical experience has shown that some patients with tinnitus may experience an increase in their tinnitus perception during VEMP testing. Also, patients who report hyperacusis or sound sensitivity may not be comfortable with VEMP stimuli for the duration of the test.
When a patient with tinnitus or hyperacusis presents for a vestibular evaluation, VEMP testing may be deferred or perhaps modified to use a lower stimulus intensity level. Modifications to the sound level provide a more comfortable experience for the patient but may limit the diagnostic interpretation. If higher intensity information is needed for appropriate management of the patient, the patient should be counseled about the risk of discomfort. Patients with phonophobia or misophonia (psychological conditions involving fear or hatred of sound) may be wholly unwilling to participate in VEMP testing.
|CONDITION||RATIONALE FOR MODIFYING PROTOCOL|
|Tinnitus/Hyperacusis||Patients with tinnitus/hyperacusis may be bothered by high-level sounds.|
|Known susceptibility to noise exposure (e.g., existing NIHL)||Greater risk for NIHL than typical population.|
|Third window phenomenon (e.g., SSCD, large vestibular aqueduct)||Sound levels within cochlea may be higher than expected.|
|Pediatrics||Possibility of increased susceptibility to NIHL, reduced ear canal volume.|
|Current/recent use of ototoxic agents (e.g., platinum chemotherapy, antibiotics)||Synergistic effect of noise and ototoxin causes greater risk.|
|Patients with additional daily noise exposure (e.g., factory workers)||Addition of VEMP may cause total noise dose to exceed recommended limits.|
Patients with Third-Window Phenomena
Third-window disorders, such as superior semicircular canal dehiscence (SSCD), are disorders with an abnormal opening into the inner ear labyrinth. For SSCD, the opening is in the osseous roof of the superior semicircular canal, which creates a window into the middle cranial fossa. These third-windows create a pathway for sound stimulation through the dehiscent bone and increase intra-labyrinthine pressure. Evidence suggests that ears with SSCD have VEMP thresholds that are approximately 20 dB lower and interpeak amplitudes considerably larger than ears unaffected by SSCD (Welgampola et al, 2003).
For this reason, the stimulus intensity level for VEMP testing in patients suspected to have SSCD may be started at a lower level to determine threshold. A lower intensity level may avoid unnecessary patient discomfort and reduce the potential for over-driving the acoustic signal to the inner ear through the dehiscence.
Patients with High Susceptibility to NIHL
Many of our patients have their own risk factors for NIHL. While we have no direct diagnostic test to evaluate susceptibility to NIHL, we may be able to infer increased susceptibility. We can assume that patients with suspected pre-existing NIHL are likely susceptible to worsening hearing due to overexposure to noise especially if they also receive occupational or recreational exposures. Beyond this, patients who are or were recently undergoing significant medical treatments, including potentially ototoxic medications, may be at higher risk for NIHL (Boettcher et al, 1987). These medications include platinum-based chemotherapy (e.g., cisplatinum, carboplatinum), aminoglycoside antibiotics, and loop diuretics.
A comprehensive case history is advised to determine if patients are on potentially ototoxic medications, have a history of NIHL or who have a significant family history of NIHL. For those patients who may be at increased risk, the clinician can determine the diagnostic usefulness of VEMP information, opting to defer or modify protocols in these cases to ensure safer noise exposures.
Patients with Additional Noise Exposure
While you can control the noise dose acquired during VEMP testing, the overall daily noise exposure of a patient also should be considered. As noise dose is cumulative across all activities in a patient's day, it may be important to avoid additional sound exposure for some patients. For example, consider a patient who works in a noisy environment or the patient who has just completed a series of MRIs. These patients may have received all or part of their noise dose before arriving at the vestibular laboratory. The diagnostic need for VEMP testing should be considered, weighing the possible risks and benefits before testing. You may recommend that patients remain isolated from noise before the testing. Depending on the dose accrued from the VEMP testing, you may need to advise patients to remain isolated from noise following the testing.
VEMP testing can be a safe and effective tool in the vestibular test battery and provides information on important reflex pathways. As clinicians, we should consider the overall noise dose provided to our patients from VEMP stimuli, taking into account each patient's unique risk insomuch as this is possible. This is especially important for the pediatric population, patients with a history or increased risk of NIHL, and patients with certain disorders, including tinnitus, hyperacusis, and third-window phenomena. By following the suggestions reviewed in this article, you can provide safer VEMP testing to the patients in your vestibular laboratory.
The sample protocol provided reduces sound exposure to levels acceptable to NIOSH recommendations while providing adequate diagnostic information. This protocol may not be ideal for every clinic, but it provides a framework for establishing a VEMP test battery that limits noise exposure to the inner ear. Using this or a similar protocol can help to provide consistent results that can be reported and compared across clinics resulting in improved clinical and research outcomes for VEMP testing.