THE COCHLEAR LESION IN EXPERIMENTAL BACTERIAL-MENINGITIS OF THE RABBIT

Sensorineural hearing loss was studied in a rabbit model of experiment al bacterial meningitis using electrophysiological and ultra-structura l techniques. Hearing impairment was monitored by auditory brain-stem evoked responses (ABERs) and concomitant structural lesions were ident ified by both transmission (TEM) and scanning (SEM) electron microscop y. Meningitis was induced by intra-cerebrospinal fluid injection of ei ther Escherichia coli (strain 2073 and type K-12) or Haemophilus influ enzae type b. Auditory loss of congruent to 10dB occurred in all rabbi ts by about 10 hours post infection and progressed in severity until b y 20h following infection, hearing losses up to and >60dB were obtaine d. At levels of hearing loss <20dB 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, supp orting cells and inner spiral sulcus cells and cells of the stria vasc ularis 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 f ound in some cochleas; erythrocytes were found in scala tympani, scala media, amongst hair cells and beneath the tectorial membrane. The res ults show that hearing loss is associated with bacterial invasion and damage to the organ of Corti and that the cause of hearing loss is lik ely to result from multiple lesions within the cochlea. Lesions to sen sory cells almost certainly will produce permanent hearing loss. Lesio ns to supporting cells, nerve terminals and to stria vascularis may we ll produce only temporary hearing loss.