The cochlear lesion in experimental bacterial meningitis of the rabbit

Sensorineural hearing loss was studied in a rabbit model of experimental bacterial meningitis using electrophysiological and ultrastructural techniques. Hearing impairment was monitored by auditory brain-stem evoked responses (ABERs) and concomitant structural lesions were identified by both transmission (TEM) and scanning (SEM) electron microscopy. Meningitis was induced by intra-cerebrospinal fluid injection of either Escherichia coli (strain 2073 and type K-12) or Haemophilus influenzae type b. Auditory loss of approximately equal to 10 dB occurred in all rabbits by about 10 hours post infection and progressed in severity until by 20 h following infection, hearing losses up to and > 60 dB were obtained. At levels of hearing loss < 20 dB ultrastructural damage to the organ of Corti was barely detectable. With greater levels of hearing loss, patchy structural damage to hair cells, synaptic nerve terminals, supporting cells and inner spiral sulcus cells and cells of the stria vascularis was clearly evident. Bacteria were found in scala tympani, the basilar membrane, the organ of Corti, scala media, the spiral ligament and at the margin of the stria vascularis. Evidence of bleeding was found in some cochleas; erythrocytes were found in scala tympani, scala media, amongst hair cells and beneath the tectorial membrane. The results show that hearing loss is associated with bacterial invasion and damage to the organ of Corti and that the cause of hearing loss is likely to result from multiple lesions within the cochlea. Lesions to sensory cells almost certainly will produce permanent hearing loss. Lesions to supporting cells, nerve terminals and to stria vascularis may well produce only temporary hearing loss.