A2 Sensorineural Hearing Loss: Cochlear and Nerve Damage

Topic

Sound reaching the cochlea via the oval window causes movement in the cochlear fluid, which makes the basilar membrane vibrate. This structure does not respond uniformly; specific points along it are more sensitive to particular frequencies. High-pitched sounds produce maximum vibration near the base of the cochlea—close to the oval window—whereas low-pitched sounds do so toward the opposite end. Hair cells, responsible for converting this movement into the signal ultimately transmitted to the brain, are located on the basilar membrane.

Outer hair cells primarily serve a mechanical amplification function. In response to faint sounds, they boost the vibration of the basilar membrane in the region corresponding to the sound's frequency, whereas they reduce this amplification for intense sounds. This compression allows a very wide range of sound intensities to be represented by a much narrower range of vibration amplitudes. When outer hair cells are lost or cease to function, compression diminishes: faint sounds no longer receive the necessary boost, the hearing threshold rises, and the dynamic range narrows, because intense sounds retain similar strength while faint sounds become imperceptible.

Inner hair cells perform a different function: they directly convert basilar membrane vibration into electrical signals that reach the auditory nerve via their synapses. The fidelity of this transduction depends on the number of functional inner hair cells and the preservation of their nerve connections. If these cells or their synapses are lost, fewer fibers receive an adequate representation of the sound, and the transmitted signal fails to faithfully reproduce the original vibration, resulting in distortion. Their loss also raises the hearing threshold, as even a vibration correctly amplified by outer hair cells may fail to be transformed into a detectable nerve signal.

An elevated hearing threshold, reduced dynamic range, and distortion are, therefore, interrelated consequences of cochlear deterioration. The hearing threshold depends on both the amplification and compression provided by outer hair cells and the transduction performed by inner hair cells; consequently, the threshold rises if either mechanism fails, and rises even further if both fail simultaneously. Dynamic range depends primarily on outer hair cells: when their compression capability is lost, the margin between sounds that are barely audible and those that are uncomfortably loud narrows. Distortion depends particularly on inner hair cells and the fidelity with which they convert vibration into neural activity.

Presbycusis is a common form of this deterioration, beginning primarily at the base of the cochlea—the region specialized for high-pitched sounds. With aging, the outer hair cells in this area are the first to be lost, leading initially to an increased threshold and a reduced dynamic range for high frequencies. If the deterioration progresses to affect the inner hair cells in the same region as well, distortion also increases.

Since the damage is located within the cochlea and at its connection to the auditory nerve, correction does not simply involve increasing the energy reaching the ear, as is the case with conductive hearing loss. A hearing aid amplifies sound before it reaches the cochlea and can be effective as long as sufficient outer and inner hair cells remain capable of processing the signal; however, it does not eliminate the distortion caused by defective transduction.

When hair cell deterioration is severe, a cochlear implant bypasses these damaged structures and directly stimulates the auditory nerve using electrical signals generated from the captured sound. In this way, it can transmit information even when outer and inner hair cells have largely lost their function. Both hearing aid amplification and cochlear implant stimulation aim to convert sound back into a signal that the auditory nerve can transmit with a threshold, dynamic range, and fidelity as close to normal as possible.

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