The site of the lesion causing hearing loss in bacterial meningitis: a study of experimental streptococcal meningitis in guinea-pigs.

The pathological basis of hearing loss in bacterial meningitis was investigated using an animal model of Streptococcus suis meningitis. Forty guinea-pigs were infected after their hearing had been assessed by brain stem auditory evoked potentials. In 17 animals, it was possible to repeat the procedure at the onset of meningitis; this included one animal with subclinical disease. Fifteen animals showed evidence of hearing loss, which on subsequent histological examination was found to be associated invariably with suppurative labyrinthitis. The remaining two animals without hearing loss had normal cochleas. It is suggested that cochlear sepsis rather than eighth cranial nerve involvement by meningeal sepsis is primarily responsible for hearing loss in bacterial meningitis, and that bacteria enter the cochlea via the cochlear aqueduct and not the internal auditory canal. The tissue within the lumen of the cochlear aqueduct may act as a barrier against invasion by micro-organisms, and haemolytic streptococci could cause lysis of this barrier by the exotoxins they produce.