Age-related hearing loss: the status of Schuknecht’s typology

PURPOSE OF REVIEW: Recent developments in age-related hearing loss (ARHL) are reviewed with an emphasis on their relation to the framework advocated by Schuknecht. More than a classification scheme, Schuknecht's typology incorporates testable hypotheses about the bases of ARHL. Since there is presently no widely accepted competing framework, research in this area should be aimed at supporting, modifying, or replacing Schuknecht scheme. Only recently has our understanding of cellular changes and gene/environment interactions in ARHL achieved the level needed for hypothesis-driven experiments in this area. RECENT FINDINGS: New findings largely support or amplify aspects of Schuknecht's framework. Consideration of the kinds of cells involved in ARHL has broadened to include more nonsensory and supporting cells. This should provide more complete criteria for comparing models, and for diagnosing particular forms of ARHL. Newly discovered genetic effects and more detailed comparisons have imparted mechanistic significance to the often-noted similarity between sensory ARHL and noise injury. Recent comparative studies, and studies of cell replacement in the cochlear lateral wall, suggest variations in the relation between strial and ligament pathology, and indicate why cell loss occurs during aging. Mouse models carrying mutations affecting processes that may give rise to ARHL are receiving increased attention, even as detailed studies bolster support for mice as valid ARHL models. SUMMARY: Using Schuknecht's framework as a guide, basic research can now seek to model specific forms of ARHL by combining genetic defects and appropriate environmental conditions. Identification of distinct risk factors for age-related degeneration of organ of Corti, afferent neurons, and stria would verify a key tenet of Schuknecht's scheme, and point the way to interventions.