Is the round window membrane a portal for bacterial toxins? A putative mechanism for sensorineural hearing loss following acute otitis media

Abstract

Aim: To investigate the hypothesis that bacterial toxins produced in the middle ear during acute otitis media (AOM), travel via the round window membrane (RWM) to cause cochlear damage and hence sensorineural hearing loss (SNHL). Methods: We investigated toxins produced by the two most common pathogens isolated from middle ear effusions in children. These were Pneumolysin, a product of S. pneumoniae and a cytoplasmic extract of Haemophilus influenza. Both electrophysiological measurements and morphological investigations were made after these toxins were placed on the RWM of guinea pigs. Control experiments were done using artificial perilymph as a substitute for the toxins. In each experiment one of the cochleas served as an internal control. Results: Losses in both the Cochlear Microphonics (CM) and Compound Action Potential (CAP) were observed indicating impaired hair cell function resulting in SNHL. Morphological studies showed hair cell destruction. Examination of the RWM exposed to pneumolysin showed disruption of this three-layered structure. Conclusions: The RWM is a probable portal for access to the inner ear by substances produced during AOM. Sensorineural hearing loss could result from the type of damage observed. More experiments are needed to verify this hypothesis and to determine the extent of damage that can be caused by these substances, especially with regard to the position of the damaged hair cells. Most of the damage observed occurred in the basal turn. Careful audiological examination (especially at high frequencies) of children who suffer with AOM might reveal a population of children with sensorineural hearing loss which might otherwise go undetected.