'Cochlear-type' hearing loss as part of aquaporin-4 neuromyelitis optica spectrum disorder

Abstract

Neuromyelitis optica spectrum disorder is an inflammatory autoimmune central nervous system condition caused in the majority of cases by aquaporin-4 IgG antibodies. Aquaporin-4 is expressed in the cochlear and vestibular nuclei regions in the brainstem and a handful of cases of retro-cochlear type hearing loss have been documented in the literature. Aquaporin-4 has also been reported within the organ of Corti and the cristae and maculae of the vestibular apparatus. We present a case where there is evidence of peripheral (labyrinthine) rather than central pathology and propose this is due to autoimmune inflammation as part of neuromyelitis optica spectrum disorder. This is the first case in the literature suggesting a 'cochlear-type' hearing loss occurring as part of neuromyelitis optica spectrum disorder. It raises the possibility of peripheral relapses of neuromyelitis optica spectrum disorder going unnoticed, resulting in patient morbidity, and highlights the importance of research within this area.

Keywords: ear; neurootology; nose and throat/otolaryngology.

Figure 1

(A) Pure tone audiogram (PTA) at onset of right-sided tinnitus, showing a moderate low and mid-frequency right-sided sensorineural hearing loss and normal left-sided thresholds other than at 8 kHz (mild dip to 25 dBHL). (B) PTA prior to assessment in neuro-otology clinic, showing new mild low frequency left-sided hearing loss. (C) PTA following steroid treatment, showing improvement in left-sided hearing thresholds back to baseline.

Figure 2

(A) Absent right-sided transient evoked otoacoustic emissions (TEOAEs) suggesting cochlea outer hair cell dysfunction. (B) Absent right-sided distortion product otoacoustic emissions (DPOAEs) other than at 6 and 8 kHz, suggesting outer hair cell dysfunction and in keeping with the low-to-mid-frequency sensorineural hearing loss shown on pure tone audiometry. (C) Present left-sided TEOAEs. (D) Present left-sided DPOAEs.