Vestibular Evoked Myogenic Potentials (VEMPs)

Vestibular Evoked Myogenic Potentials (VEMP)

OVERVIEW

Bickford, Jacobson, & Cody (1964) first recorded a sound-induced potential from an electrode placed at the inion, which was modulated with neck electromyography (EMG) activity. However, vestibular evoked myogenic potentials (VEMPs) were not fully described until 1992 when Colebatch and Halmagyi elicited a short-latency myogenic potential from the tonically contracted sternocleidomastoid (SCM) in response to loud acoustic stimuli. The biphasic positive and negative waveform was determined to be vestibular in origin since it was abolished after vestibular nerve section, but observed in cases of significant hearing loss (Colebatch, Halmagyi, & Skuse, 1994). Today, VEMPs are produced with sound or vibration stimuli and are commonly recorded from the SCM or inferior oblique extra-ocular muscles. VEMPs provide a quick, noninvasive means of evaluating otolith organ (saccule and utricle) function.

The otolith organs are stimulated with linear translations, causing movement of the otoconia across the sensory hair cells (see Chapter 1); however, some otolith receptors respond to air and bone conduction stimuli (Curthoys et al., 2018). The otoliths have defined frequency tuning, with maximal resonance responses in the low-frequency range (Sheykholeslami, Habiby Kermany, & Kaga 2001; Todd, Cody, & Banks, 2000; Todd, Rosengren, Govender, & Colebatch, 2009; Zhang, Govender, & Colebatch, 2012), due to the biomechanical properties of the structures (Todd et al. 2009).

VEMP RESPONSES AND PATHWAYS

There are two main types of VEMPs: cervical (cVEMP) and ocular (oVEMP). Table 8–1 outlines the VEMP reflex pathways.

• cVEMPs are an ipsilateral measure of saccule function obtained from a tonically contracted ipsilateral SCM. The cVEMP response represents a brief inhibition in motor unit activity (Rosengren & Colebatch, 2018; Rosengren et al., 2019).

○ Response characteristics: Biphasic response, with positive deflection (P1) around 13 ms and negative deflection (N1) around 23 ms (Figure 8–1A).

• oVEMPs are obtained from the contralateral inferior oblique muscle and measure utricle function. The oVEMP response describes the excitation of the translational VOR.

○ Response characteristics: oVEMPs can be multiphasic responses, with the first biphasic response reported as negative deflection (N1) around 10 ms followed by a positive deflection (P1) around 15 ms (Figure 8–1B).

Table 8–1. Cervical and Ocular VEMP Reflex Pathways

Note: Ocular VEMP is a bilateral pathway. Recordings may be obtained from the ipsilateral inferior oblique; however, there is a larger amplitude response from the contralateral inferior oblique (Chihara, Iwasaki, Ushio, & Murofushi, 2007).

Source: Colebatch, Rosengren, & Welgampola, 2016.

Stimulus Parameters

VEMP testing can be performed with most one- or two-channel auditory evoked potential devices; however, Rosengren et al. (2019) recommend that these units require the capacity to produce high acoustic tone burst stimuli, monitor rectified EMG activity, and properly amplify the high power vibration required for bone conduction testing. Appropriate calibration is needed for both air conduction (AC) sound pressure level (SPL) and bone conduction (BC) force level (FL). AC stimuli is most commonly used to elicit VEMP responses (Rosengren et al., 2019); however, BC stimuli may be used with a high-intensity bone transducer (e.g., B-81) or a reflex hammer. Optimal stimulus parameters vary based on type of VEMP (cervical or ocular), and stimulus (AC or BC) (Table 8–2). There are clinical guidelines for cVEMPs (Papathanasiou et al., 2014); guidelines for oVEMPs are underway (Papthanasiou & Straumann, 2019).

Figure 8–1. VEMP waveforms evoked using 125 dB SPL 500 Hz tone bursts. A. Shows left cVEMP waveform.

B. Shows left oVEMP waveform.

VEMP Acoustic Trauma Precautions

To reduce the risk of acoustic trauma while performing VEMPs use the following:

1. Tone burst instead of clicks: Thomas, Fitzpatrick, McCreery, and Janky (2017) identified peak-to-peak equivalent sound pressure levels (peSPL) as ~3 dB larger in normal-hearing children’s ears. Since using click stimulus increases the peSPL, the authors recommend using a 500 Hz tone burst in pediatric patients.

2. Lowest possible stimulus intensity: Ear canal volume (ECV) and ear canal length contribute to increased peSPL during testing. Rodriguez, Thomas, Fitzpatrick, and Janky (2018) recommend using:

• 120 dB SPL when ECV <0.8 mL

• 115 dB SPL when ECVs are <0.4 mL

Standard 125 dB SPL, not exceeding 132 dB SPL (Colebatch & Rosengren, 2014), appears to be safe practice in all other cases.

3. Short duration: Rodriguez et al. (2018) suggested using a 750 Hz (rather than 500 Hz) tone burst due to the shorter duration and found equivalent oVEMPs and cVEMP responses in children, adolescents, and adults. The authors also surmised that BC testing is likely the safest mode to elicit a VEMP response, as it bypasses the ECV and ear canal length which add to peSPL.

Table 8–2. Suggested VEMP Air Conduction (AC) and Bone Conduction (BC) Stimulus Parameters

Note: Blackman gating preferred for both air and bone conduction cVEMP & oVEMPs. Artifact rejection must be applied for all VEMP testing. The number of sweeps is typically 100–200; less sweeps may be necessary with bone conduction or reflex hammer stimuli (Rosengren et al., 2019). Present VEMP responses should be replicated at least one time.

SPL = sound pressure level, FL = force level.

^aFrequency tuning shown to shift from 500 Hz to 1000Hz in older adults and for patient > 60 years of age; better responses may be obtained at 750 or 1000 Hz (Piker et al., 2013).

^bBone conduction stimuli may be the preferred stimulus type for oVEMPs (Rosengren, Govender, & Colebatch, 2011).

Sources: Bogle, 2018; McCaslin & Jacobson, 2021; Papathanasiou et al. 2014; Rosengren, 2019.

Patient Preparation

VEMP testing may be performed in a well-lit room: there is no threat of lighting reducing the response. However, there are both patient and operator steps required to elicit robust responses.

General Patient Setup Considerations

1. Perform otoscopy to confirm unoccluded ear canals if using AC stimuli.

2. Perform tympanometry, if necessary, to examine middle ear status. Even a mild conductive component can impede AC VEMP testing (Papathanasiou & Strauman, 2019), raising the need to perform BC VEMPs.

3. Clean the appropriate electrode sites with an alcohol wipe or other solution (e.g., NuPrep with gauze pad).

4. When performing AC testing, place the appropriate sized (adult or pediatric) insert ear phones in the patient’s ears. An advantage of unilateral VEMP recording is the capability for the patient to hear the operator instructions in the nontest ear.

cVEMP Patient Setup

cVEMP Electrode Montage (Figure 8–2)

• Noninverting: Upper 1/3 of SCM (i.e., belly or midpoint of SCM muscle)

• Inverting (vertex): sternoclavicular junction

• Ground: forehead, nose, or dorsum of hand