OBJECTIVE: The incidence of COM without cholesteatoma in Australian aboriginals is quoted as being between 25 and 47%. The incidence of cholesteatomatous OM is very low in comparison (between 1 and 3%). A comprehensive literature search found no studies offering an explanation for the large discrepancy between these two types of OM. We offer a theory to account for the disproportionate rates of the two types of OM together with an observational study on aboriginals with COM to substantiate our argument.
METHOD/RESULTS: The crux of our hypothesis centres on the anatomy of the posterior attic. We found that in a study of 40 aboriginal patients undergoing tympanoplasty with audiometric evidence of an intact ossicular chain that, after posterior tympanomeatal flap elevation, no part of the incudostapedial assembly was visible in 95% of cases denoting its location behind the posterior attic wall.
CONCLUSION: We hypothesise that in aboriginal ears the resulting 'crowding' of the posterior attic by the incudostapedial assembly results in narrowing of the epitympanic space which compromises atticoantral drainage, thus leading to mucosal COM, however supporting the posterior-superior segment of the tympanic membrane, impeding the formation of a retraction pocket, and cholesteatoma formation migrating into the mastoid antrum.