The term “hidden hearing loss” has been coined to denote hearing impairments resulting from damage to the inner hair cells (IHC) and/or type I auditory nerve fibers. Carboplatin, an anti-cancer drug, selectively damages the IHC/type I neurons through much of chinchilla cochlea. This type of lesion does not alter distortion product otoacoustic emissions from outer hair cells, but greatly reduces the summating potential from IHC and the compound action potential from the auditory nerve. Despite massive carboplatin-induced IHC/nerve-fiber lesions, pure tone thresholds in quiet remain completely normal. However, these massive lesions can be detected by measuring tone-thresholds-in-noise and gap detection thresholds. The massive loss of IHC/type I neurons reduces the neural output of the cochlea, but the brain compensates for this by turning up its gain, like a hearing aid, so that by the time neural signals reach the auditory cortex, neural responses are equal to or larger than normal; a phenomenon referred to as “enhanced central gain”, which could contribute to hyperacusis. Anti-cancer drugs such as carboplatin, also suppress the birth of new neurons (neurogenesis) in the hippocampus, an important structure involved in memory and spatial navigation. These changes likely contribute to cognitive deficits associated with chemotherapy.
Learning Objectives:
- Articulate why the pure tone audiogram is ineffective at detecting damage to the inner hair cells or auditory nerve fibers.
- Articulate why distortion product otoacoustic emissions can be used to assess outer hair cell function, but not inner hair cell function.
- Apply the principles of electrocochleography to detect damage to the inner hair cells or auditory nerve fibers
- Use tone-in-noise detection methods to detect damage to the inner hair cells and auditory nerve fibers
- Detect damage to inner hair cells and auditory nerve fibers by measuring gap detection thresholds.
- Articulate how neurons in the central auditory pathway amplify weak neural signal coming from the cochlea.
- Articulate how anticancer drugs affects memory.
