Time course of hearing loss in an animal model of pneumococcal meningitis

A leading cause of morbidity from bacterial meningitis is an irreversible, usually profound sensorineural hearing loss, with an incidence as high as 3 0% in some studies, Bacterial meningitis remains the most common cause of a cquired postnatal sensorineural deafness. Although several clinical studies have examined the long-term outcome of hearing in meningitis, few studies have examined the time course of hearing loss during the acute course of th e disease. We have developed an animal model of meningogenic hearing loss i n the rat and have plotted the time course of that hearing loss. Serial aud itory brain stem responses (ABRs) were measured in rats inoculated in the c isterna magna (subarachnoid space) with Streptococcus pneumoniae (10(5) to 10(7) colony-forming units). All rats injected developed meningitis as evid enced by increased cerebrospinal fluid (CSF) white cell counts and positive CSF cultures. Serial ABR measurements taken 6, 12, 15, 18, 21, and 24 hour s after inoculation demonstrated significant threshold shifts and eventual loss of the ABR waveform as compared with measurements in control rats inje cted with sterile culture medium. Hearing loss began approximately 12 to 15 hours after inoculation and progressed to complete loss by 24 hours (17 of 18 animals). No correlation was found between the magnitude of hearing los s and CSF white cell count or bacterial titer, Temporal bone histology of r ats with meningitis shows a dense inflammatory cell infiltrate throughout t he subarachnoid space. Labyrinthine inflammatory cells were confined to the scala tympani. The cochlear aqueduct is the proposed route of infection fr om the meninges to the labyrinth (scala tympani). Endolymphatic hydrops was also noted throughout the cochlea. These experiments both establish a repr oducible animal model of meningogenic hearing loss and support the hypothes is that this hearing loss is progressive rather than abrupt in onset and is related to the duration of untreated infection, CSF inflammatory cells app ear to enter the cochlea through the cochlear aqueduct, This reliable anima l model will enable future studies directed toward further understanding th e pathogenesis and pathophysiology of this hearing loss.