Neonatal conductive hearing loss does not compromise brainstem auditory function and structure in rhesus monkeys

The effect of conductive hearing loss on the maturation of the auditory pathway was evaluated using the auditory brainstem response (ABR) in rhesus monkeys. Ten newborn rhesus monkeys were assigned to control (N = 4), unilateral hearing loss (N = 3), or bilateral hearing loss (N = 3) groups. Hearing loss was created by surgically excising a 3 mm section of the external auditory canal and suturing the canal. Auditory brainstem responses to click stimuli were recorded prior to and after the surgical procedure and bi-monthly or monthly for a 14 month follow-up period. Results showed that after surgery all ears developed an estimated 30-50 dB conductive hearing loss which was retained throughout the follow-up period. Contrary to expectations, the latencies of the ABR component waves decreased with age in all ears. When adjusted for hearing level, there were no differences between ears in maturation of the component waves of the ABR. These data suggest that, in primates, a conductive hearing loss does not affect the maturation of those aspects of the auditory pathway reflected in the ABR. Furthermore, the conductive losses were not accompanied by any discernible change in the neuronal sizes of brainstem auditory neurons or the volume of the cochlear nuclei.