There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
In this study 9 human temporal bones from 8 individuals were fixed with Karnovsky
solution by perilymphatic perfusion within 1-3 h after death and examined using the
"block-surface method' (Spoendlin and Brun, 1974; Spoendlin and Schrott, 1987) and
the "micro-dissection method' (Johnsson and Hawkins, 1967). The audiogram of 7 individuals
showed high-tone hearing loss, typical for sensory-neural presbycusis. The inner (IHC)
and outer hair cells (OHC) and the myelinated nerve fibers in the osseous spiral lamina
were counted to correlate audiometric curves with hair-cell and nerve-fibre densities.
The "block-surface' method allows accurate hair-cell and myelinated nerve-fibre enumeration
with maximal preservation of cochlear structures. The most significant change in the
cochlea was not the expected loss of hair cells but an evident loss of nerve fibres
in the spiral lamina along the entire length of the cochlea. This loss of nerve fibres
was found to be age-related. Reductions up to 30-40% in comparison to normal-hearing
middle-aged persons were found in cochleae from persons older than 60 years. In 2
cases only 13% of the fibres remained in some regions of the cochlea. The hair-cell
counts showed a reduction of approximately 80% of the OHCs, mainly in the apical parts
of the cochlea, and only little differences in the number of IHCs as compared with
a group of normal-hearing middle-aged persons. We conclude that neither loss of hair
cells nor primary degeneration of nerve fibres alone can fully explain the high-tone
loss. Probably injuries of hair cells or neuronal elements at the cellular level can
cause threshold elevation.