The aging-related decrease in cVEMP amplitude cannot be accounted for by normalization

BACKGROUND: Cervical Vestibular Evoked Myogenic Potentials (cVEMP) is an electromyogenic measure commonly used in clinic to assess saccule function. The main parameters are peak-peak amplitude and interaural asymmetry ratio (IAR). Several non-vestibular factors may influence these parameters. Notably, a greater EMG contraction level prior to stimulation leads to an increased amplitude. As aging impacts both vestibular structures and muscle propreties, it is still a matter of debate whether the decrease in cVEMP amplitude observed in normal aging is due to EMG differences prior to stimulation or to the effect of aging on the sacculo-collic reflex pathway. At the clinical level, understanding the effect of aging on the relationship between EMG activity and cVEMP response (amplitude, asymmetry ratio) and the effect of normalization is crucial to improving the categorization of healthy versus pathological responses. OBJECTIVE: To investigate whether normalization modifies cVEMP amplitude and asymmetry ratios differently in younger and older heatlhy adults. METHOD: cVEMP recordings were conducted in 42 normal healthy participants divided in two age groups: younger (n = 29): mean = 22.79 years old SD = 1.66; and older (n = 13): mean = 69.00 years old SD = 3.61. Air-conducted cVEMP were recorded using Eclipse (Interacoustics, Denmark). The stimulus was a 95 dBnHL tone burst (500 Hz) with rise, plateau and fall time of 1 ms. cVEMP were recorded only when EMG levels were between 50 mu V and 150 mu V, using the Eclipse (Interacoustic, Denmark) monitoring system. RESULTS: No significant differences were observed for prestimulus EMG levels between younger and older participants (F(1,83) = 1.13, p = 0.291). However, significant differences between groups were observed for raw cVEMP amplitude (F(1,83) = 14.78; p < 0.001) and corrected cVEMP amplitude (F(1,83) = 21.85; p < 0.0001). A significant positive linear relationship between prestimulus EMG contraction level (RMS) and raw cVEMP amplitude was observed in younger participants (r(2) = 0.234; p < 0.001), but not in older adults (r(2) = -0.0144; p = 0.056). Finally, no significant differences between younger and older participants were observed for raw amplitude asymmetry ratios (F(1,41) = 0.124, p = 0.726) or normalized asymmetry ratios (F(1,41) = 0.726, p = 0.508). CONCLUSION: Our results suggest that when EMG is monitored and activation of the SCM is sufficient, the observed decline in cVEMP amplitude with normal aging does not seem to be caused by EMG differences and is therefore likely due to the known histopathological modifications of the vestibular system that occurs with normal aging.