A HUMAN TEMPORAL BONE STUDY OF CHANGES IN THE BASILAR-MEMBRANE OF THEAPICAL TURN IN ENDOLYMPHATIC HYDROPS

We observed that some temporal bones with endolymphatic hydrops (EH) s howed varying degrees of basalward displacement (towards the scala tym pani) 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 coul d conceivably alter cochlear mechanics and lead to sensorineural heari ng 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), Meniere's disease (N = 23), and EH secondary to labyrinthitis (N = 37). The incidence of extreme disp lacement of the BM in the apical turn such that it adhered to the inte rscalar septum was 52% in Meniere's disease, 57% in EH secondary to la byrinthitis, 10% in presbycusis, and 1% in normals. These differences were significant and could not be explained on the basis of age, sex, post-mortem time, or artifact of technique or processing. Displacement of the BM was not observed in other turns of the cochlea. Its pathoge nesis is not known, but may be related to atrophy of the spiral ligame nt. It is likely that such BM displacement results in sensorineural he aring loss. However, our data and theoretical analyses both indicate t hat such a loss will be restricted to frequencies below 100 Hz and tha t this pathologic change alone is not likely to cause appreciable hear ing loss at clinically tested frequencies of 250 Hz and higher. Hence, even though this pathologic finding is common in endolymphatic hydrop s, it cannot explain the low-frequency hearing loss observed in Menier e's disease.