(PP504) Amplitude-Modulated cVEMPs (AMcVEMPs) and Their Clinical Implications Versus Transient cVEMPs

Lead Presenter(s)

AT

Andrew P. Thorne, BA

Fourth Year Audiology Doctoral Student
James Madison University
Johnson City, Tennessee, United States

Presenter(s)

CC

Christopher Clinard, PhD

Professor
James Madison University
Harrisonburg, United States
Erin G. Piker, AuD, PhD

Associate Professor
James Madison University
Harrisonburg, Virginia, United States
EJ

Ellen Jones, n/a

James Madison University
Harrisonburg, Virginia, United States
MB

Morgan Brenk, n/a

James Madison University, United States

Abstract: Cervical VEMPs (cVEMPs) elicited by high-intensity tonebursts can damage hearing and are difficult to assess for elderly or weak patients. VEMPs are subjectively detected, convey limited information about vestibular function, and can be asymmetrical even in healthy patients. One solution is to elicit cVEMPs with bone-conducted, amplitude-modulated tones (AMcVEMPs). This method allows signal-to-noise ratio, phase coherence, and harmonic distortion products to be evaluated at much lower stimulus and sternocleidomastoid muscle activation levels. These response measures unique to AMcVEMPs could result in improved vestibular diagnostic capabilities and lead to the development of new clinical tests to evaluate saccular function.

Summary:

RATIONALE: Elderly or weak patients are often unable to reach the levels of sternocleidomastoid muscle (SCM) activation required for the subjective visual detection of cVEMP waveforms. cVEMP amplitudes can also vary considerably between ears even in subjects with normal saccular function. High stimulus levels (up to 140 dB SPL) are also necessary to elicit cVEMPs and can be potentially damaging to hearing.

Methods: Thirty six young healthy subjects with no known history of hearing or vestibular pathologies participated in this study. Amplitude-modulated cVEMPs (AMcVEMPs) were elicited by a 1024 msec, 65 dB HL tone with a carrier frequency of 500 Hz and an amplitude modulation frequency of 37 Hz. Transient cVEMPs were elicited by an 8 msec, Blackman-gated 500 Hz toneburst. Stimuli were delivered via a B81 bone oscillator placed 3 cm posterior and 2 cm superior to the ear canal. Electrode montages and recording parameters were standard for recording cVEMPs bilaterally. Five different levels of SCM activation were used: 10, 30, 50, 70, and 90 uV. Response analysis for AMcVEMPs included amplitude, signal-to-noise ratio (SNR), and phase coherence (PC), assessed via Fast-Fourier Transform (FFT) for the 37 Hz modulation frequency bin, as well its second, third, and fourth harmonics. Response analysis for standard cVEMPs included visual detection of amplitude and latency for all five SCM activation levels.

Results: AMcVEMP amplitude increased with higher levels of SCM activation. SNR and PC measures were remarkably robust and did not significantly increase above 30 µV of SCM activation, indicating that only 30 µV of SCM activation is needed to record robust responses. AMcVEMP amplitude measures showed asymmetries comparable to transient cVEMPs, but SNR and PC measures showed far smaller asymmetries. Nonlinear harmonic distortion products were observed for a majority of participants.

Conclusions: Unlike cVEMPs which are elicited with tonebursts, bone-conduction AMcVEMPs use much lower stimulus intensities and can be objectively detected at much lower SCM activation levels; this could increase the diagnostic utility of cVEMPs in elderly or weak patients. SNR and PC measures could allow for more accurate clinical assessments of asymmetries. In addition, the nonlinear harmonic distortion products of saccular hair cells could potentially be used for the development of new clinical tests of saccular function.

Learning Objectives:

1.) Describe the AMcVEMP response properties of signal-to-noise ratio, phase coherence, and harmonic distortion products. 2.) Evaluate the potential clinical advantages of assessing AMcVEMP response properties versus transient cVEMPs.