Medial olivocochlear reflex dysfunction in multiple sclerosis: The influence of brainstem lesion localization and its clinical implications.

Abstract

Background: Multiple sclerosis (MS) is known to affect many sensory systems, yet most auditory research in MS has focused on the afferent pathways, with relatively few studies examining efferent function. The brainstem is a common site for MS plaques, and the medial olivocochlear (MOC) system is located in the superior olivary complex (SOC) of the brainstem. The cochlear nuclei are also involved in the MOC reflex arc. Additionally, the temporal cortex can modulate the SOC and cochlear nucleus, so lesions in the brainstem or temporal cortex may affect the MOC reflex in MS.

Aim: To investigate efferent auditory system activity in patients with multiple sclerosis via the MOC reflex.

Methods: The study included 50 patients with MS and 50 healthy controls. Patients with MS were divided into three subgroups according to cranial magnetic resonance imaging findings: Patients with brainstem lesions (Group 1, n = 20); patients with temporal cortex lesions without brainstem involvement (Group 2, n = 20); and patients without any lesions in the brainstem or temporal cortex (Group 3, n = 10). Tympanometry, acoustic stapedial reflex thresholds, pure-tone audiometry, and transient-evoked otoacoustic emission (TEOAE) tests (with and without contralateral noise) were performed for all participants.

Results: There was no significant difference in pure-tone hearing thresholds or baseline TEOAE amplitudes between the MS and control groups, indicating normal cochlear function in patients with MS; however, MOC reflex suppression was significantly reduced in patients with MS compared to controls (P = 0.021). In particular, Group 1 (MS with brainstem lesions) showed the lowest mean suppression values, which was significantly lower than that of Group 2 and the control group (P = 0.002). By contrast, Group 2 and Group 3 did not significantly differ from controls. Additionally, patients with MS exhibited a sex difference in MOC function: Male patients had significantly lower suppression compared to female patients both within Group 1 and in the MS group as a whole.

Conclusion: The findings indicate that the efferent auditory system (specifically the MOC reflex) is affected by MS. MOC reflex activity was most significantly decreased in patients with MS with brainstem lesions, while temporal cortex lesions alone did not appear to notably impair the MOC reflex. Diminished MOC activity may underlie various auditory difficulties in patients with MS (e.g., hearing in noise), and loss of efferent suppression could contribute to symptoms such as hyperacusis or tinnitus in this population. Further studies are needed to better understand the relationship between MOC dysfunction and auditory symptoms in MS, as well as the potential diagnostic value of MOC testing in MS.