Hidden Hearing Loss

It is thought that the predominant cause of sensorineural hearing loss, especially when it is acquired, is damage to the cochlear hair cells. This initial damage is followed by degeneration of the cochlear nerve fibers. The process begins with hair cell damage and progresses to spiral ganglion cell loss, a progression that can take weeks, months, or even years. Notably, damage to hair cells and their synaptic connections is not accurately reflected in audiometric thresholds, indicating a gap between physiological damage and its clinical measurement. Recent studies have shed light on the vulnerability of cochlear nerve synapses and their peripheral synaptic connections, particularly in situations of noise exposure and aging, and have shown that they are often more at risk than the hair cells themselves. In some cases, these synapses can be destroyed before the hair cells are damaged. This leads to a situation where damage to the synapses between inner hair cells and cochlear nerve fibers may have little effect on behavioral hearing thresholds until significant nerve loss has occurred. This phenomenon has been termed “hidden hearing loss,” in which individuals experience hearing difficulties despite hair cell preservation and normal audiogram results. This topic is discussed in this chapter.