A human temporal bone study of changes in the basilar membrane of the apical turn in endolymphatic hydrops

We observed that some temporal bones with endolymphatic hydrops (EH) showed varying degrees of basalward displacement (towards the scala tympani) of the basilar membrane (BM) in the apical turn of the cochlea. In some, the BM was adherent to the bony wall of the scala tympani (i.e., the interscalar septum). Such mechanical distortion of the BM could conceivably alter cochlear mechanics and lead to sensorineural hearing loss. The results of a systematic evaluation of 234 temporal bones to characterize, quantify and determine the functional significance of this observation are presented. Four groups of bones were evaluated: normal (N = 78), presbycusis (N = 96), Ménière's disease (N = 23), and EH secondary to labyrinthitis (N = 37). The incidence of extreme displacement of the BM in the apical turn such that it adhered to the interscalar septum was 52% in Ménière's disease, 57% in EH secondary to labyrinthitis, 10% in presbycusis, and 1% in normals. These differences were significant and could not be explained on the basis of age, sex, postmortem time, or artifact of technique or processing. Displacement of the BM was not observed in other turns of the cochlea. Its pathogenesis is not known, but may be related to atrophy of the spiral ligament. It is likely that such BM displacement results in sensorineural hearing loss. However, our data and theoretical analyses both indicate that such a loss will be restricted to frequencies below 100 Hz and that this pathologic change alone is not likely to cause appreciable hearing loss at clinically tested frequencies of 250 HZ and higher. Hence, even though this pathologic finding is common in endolymphatic hydrops, it cannot explain the low-frequency hearing lost observed in Ménière's disease.