Mechanisms of hearing impairment

Hearing depends upon the middle ear for collecting, amplifying and delivering the vibration of sound to the highly specialised cochlea, where sensory hair cells in the spiral organ of Corti detect these mechanical signals and convert them to electrical changes within the cell (auditory transduction). Depolarization of the cell allows synaptic release, triggering action potentials in cochlear neurons. Hair cells have an array of stereocilia (modified microvilli) at their upper surface, with extracellular

18-50 31-40 41-50 51-60 61-70 71-80 81+ Age group (years)

Figure 33.1 Prevalence of hearing loss (HL) with age, for different severities in decibels (db).

18-50 31-40 41-50 51-60 61-70 71-80 81+ Age group (years)

Figure 33.1 Prevalence of hearing loss (HL) with age, for different severities in decibels (db).

Figure 33.2 Anatomy of the cochlea. Cross section through the cochlear canal showing the organ of Corti (OC) on the floor of the endolymph (el) filled cochlear duct with the stria vascularis (SV) on the lateral wall and perilymph (pl) filled scala vestibuli and scala tympani (upper figures). Cross section of cochlear duct (scala media) showing the tectorial membrane (tm), inner hair cells (ihc), pillar cells (p); outer hair cells (ohc), Dieter's (d), Hensen's (h) and Claudius' (c) cells, outer sulcus (os), inner sulcus (is), spiral ligament (sl), spiral limbus (slm), interdental cells (i), marginal (m), basal (b) and intermediate (i) cells (lower figure).

Figure 33.2 Anatomy of the cochlea. Cross section through the cochlear canal showing the organ of Corti (OC) on the floor of the endolymph (el) filled cochlear duct with the stria vascularis (SV) on the lateral wall and perilymph (pl) filled scala vestibuli and scala tympani (upper figures). Cross section of cochlear duct (scala media) showing the tectorial membrane (tm), inner hair cells (ihc), pillar cells (p); outer hair cells (ohc), Dieter's (d), Hensen's (h) and Claudius' (c) cells, outer sulcus (os), inner sulcus (is), spiral ligament (sl), spiral limbus (slm), interdental cells (i), marginal (m), basal (b) and intermediate (i) cells (lower figure).

links between them. When the stereocilia (or hair) bundle is deflected by incoming vibration, one type of link, the tip link, mechanically opens the trans-duction channel allowing rapid influx of cations into the cell. Hair cell function depends upon precise control of the environment, and specialized supporting cells within the organ of Corti and cellular structures around the cochlear duct serve this function. For example, the stria vascularis on the lateral wall pumps out potassium into the fluid bathing the tops of the hair cells, generating a high potassium concentration and a high resting potential, the endocochlear potential, which are both essential for hair cell function (Figure 33.2).

It is frequently stated that age-related hearing loss is due to degeneration of the sensory hair cells in the organ of Corti of the cochlea. However, there is no evidence for this contention from animal studies. Rather, it appears that hair cell degeneration is a correlate or a consequence of some primary dysfunction, either of hair cells or of some other part of the auditory system.

Figure 33.3 Typical audiograms of 70-80 years old, sensory type. Hearing loss (HL) is shown in decibels (db).

These sensory hair cells seem to be particularly sensitive to any disturbance of their homeostasis, resulting in their degeneration. However, findings in animal models suggest that hearing impairment correlates with hair cell function rather than with hair cell death, as there can be plenty of surviving but dysfunctional hair cells in a mouse with a Tmcl or a Cdh23 mutation, or in a mouse with a mutation affecting cochlear homeostasis. Even with noise-induced damage, cochlear responses correlate better with stereocilia bundle damage than with hair cell death (e.g. Holme and Steel, 2004).

Mature mammalian hair cells in the cochlea never regenerate, so hair cell loss accumulates with age and this can be observed in human temporal bone specimens from people with hearing impairment. However, donated human inner ears generally represent the end-stage of a long disease process. Schuknecht and Gacek (1993) proposed several different fundamental types of hearing loss based on observations of many temporal bones and audiograms of people with progressive hearing loss with age (also known as presbyacusis). The most common type was sensory presbyacusis, characterised by the steeply sloping audiogram with poor thresholds at high frequencies, as illustrated in Figure 33.3, with associated hair cell loss at the basal end of the cochlear duct (the region normally involved in high frequency detection). The second type was strial presbyacusis, with more equal hearing loss across the frequency range and atrophy of the stria vascularis, a structure on the lateral wall of the cochlear duct that pumps high levels of potassium into the fluid bathing the upper surface of the hair cells and generates a high resting potential in this fluid. A third type, neural presbyacusis, was rare but showed loss of cochlear neurons leading to limited threshold increases but difficulty in speech discrimination. This is a useful first step in grouping pathological mechanisms. However, as we discover more of the genes underlying hearing impairment (see later), and understand more about their involvement in the pathological process, it is becoming clear that there are dozens, maybe hundreds, of different ways that hearing can be compromised. Furthermore, other parts of the auditory system can be involved in progressive hearing loss, such as the middle ear (Browning and Gatehouse, 1992; Rosowski et al., 2003). Cognitive decline with age may interact with hearing impairment to exacerbate the functional disability in the elderly population, but the vast majority of cases of hearing loss are the result of a pathological process affecting the inner or middle ear (mainly the inner ear) rather than an effect of the central auditory system alone.

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