Effects of Age on the Bone-Conduction Amplitude-Modulated cVEMP Temporal Modulation Transfer Function.

Objectives: Cervical vestibular evoked myogenic potentials (cVEMPs) reflect saccular stimulation that results in an inhibitory muscle reflex recorded over the sternocleidomastoid muscle. These responses are utilized to study basic vestibular functions and are also applied clinically. Traditionally, cVEMPs have utilized transient stimuli such as clicks and tonebursts to evoke onset responses. Recently, amplitude-modulated tones have been used to elicit cVEMPs (AMcVEMPs). These AMcVEMP responses can provide information about the magnitude, phase synchrony, and nonlinearities from the vestibulo-collic reflexes that cannot be captured using other existing testing techniques. Although temporal modulation transfer functions (TMTFs) of AMcVEMPs for young, healthy adults have been established using different analysis techniques, there is currently no information regarding the effects of age on these responses. Thus, the current study aimed to examine the effects of age on AMcVEMPs across a broad range of modulation frequencies (MFs) using various AMcVEMP metrics including amplitude, signal to noise ratio (SNR), and phase coherence (PC).

Design: The study included 16 (aged 20 to 39 years) young, 17 (aged 40 to 59 years) mid-age, and 16 (60 to 75 years) older adults with no history of neurological, vestibular, or middle-ear complaints. The stimuli consisted of amplitude-modulated tones with a carrier frequency of 500 Hz and 10 MFs ranging from 11 to 397 Hz. These stimuli were presented using a B81 transducer at 123 dB FL. AMcVEMPs were recorded from the sternocleidomastoid muscle (ipsilateral to the stimulating mastoid) using surface electrodes. Response analysis was performed using an FFT-based approach with analyses including amplitude, SNR, and PC.

Results: AMcVEMP waveforms exhibited periodicity corresponding to the stimulus MF, consistent with previous observations. Furthermore, significant age-related degradation in AMcVEMP amplitude, SNR, and PC measures were observed across a broad range of MFs. While AMcVEMPs were elicited across a wide range of MFs (11 to 263 Hz) for young adults, in mid-age and older adults, these metrics were robust only across a narrower range of MFs, resulting in a reduced TMTF. In addition, the MF eliciting the most robust AMcVEMP varied across different AMcVEMP analysis metrics and age groups.

Conclusions: A significant decline in the AMcVEMP response is seen as an effect of aging; however, the effect of aging is not uniform across measures or across MFs. The TMTF of AMcVEMP gets flatter, and the overall range is reduced as an effect of aging. Results from this study enhance our understanding of age-related changes in the vestibular system. Expansion of AMcVEMP to clinical populations may lead to a deeper understanding of the pathophysiology of vestibular disorders.