Pathology of hair cells and sensory neurones
Overview / Aminoglycosides / Noise Trauma / Excitotoxicity / Presbycusis / Tinnitus
Pictures : M. Lenoir, M. Lavigne-Rebillard , R. Pujol

Hair cell pathology

Three main non-genetic factors for loss of hair cells and thus its related acquired deafness are ototoxic drugs, noise trauma and ageing.
Among drugs known to be ototoxic by inducing hair cell loss, the most common are aminoglycoside antibiotics, and antimitotic agents (chemotherapeutic drugs) such as cis-platinum.
Acoustic trauma affects both hair cells and neurones.
In ageing,
presbycusis is also characterised by a loss of both hair cells and neurones.

Loss of OHCs: incomplete deafness and hearing aids
M. Lenoir
When the lesion affects OHCs only, as in these pictures showing intact IHCs, deafness is characterized by a 50 to 60 dB loss and a dramatic impairment of frequency discrimination.
See below a corresponding speech audiogram.
This test is based upon the ability of the subject to correctly repeat words presented to him at different loudnesses. A normally-hearing subject (blue curve) is able to repeat correctly 100% of words heard at 20 dB. A subject whose OHCs have been damaged (red curve) needs a 50 dB amplification to begin understanding the list of words, and he never gets 100% of correct responses no matter what the sound intensity (i.e. his speech discrimination is altered).

N.B. A hearing aid will give back the 50 dB loss of sensitivity by amplificaion, but not 100% discrimination.

Loss of IHCs and OHCs: complete deafness

M. Lavigne-Rebillard

When both IHCs and OHCs have disappeared, the organ of Corti is no longer recognisable and deafness is total.

However, as in this pictures, nerve endings (asterisk) often remain.

This is how cochlear implants are able to function.

M. Lenoir

In this scanning electron micrograph, a surface view of the organ of Corti (soon after hair cells have disappeared) shows no stereocilia, but there are remnants (red arrow).

The head of pillar (p) and Deiters' cells (d) are still recognizable.

scale bar: 10 µm

N.B. In these situations, an hearing aid will have no effect. Only direct electrical stimulation of these fibres may help (i.e. a cochlear implant).

M. Lavigne-Rebillard

Another example of organ of Corti degeneration.

But in this case, due to the very low number of remaining fibres (asterisk) a cochlear implant would not work very well.

Cochlear neuron pathology

A major factor in cochlear neurone pathology is glutamate excitotoxicity. This occurs in two main situations: local ischaemia and acoustic trauma. Excitotoxicity is also involved in the loss of neurones in ageing (neural presbycusis).

Before neurones are actually lost, the first step of excitotoxicity may alter the auditory nerve activity and be the starting point of peripheral tinnitus.

Genetic pathology (cf. refs)
Many gene mutations may cause genetic deafness (which accounts for almost 75% of deafness at birth or within the first few years of life).
This topic is currently being intensively researched. About 60 genes responsible for deafness have been localised so far, and one of them which codes for an adhesion molecule (connexin) seems to be involved in 50% of cases of genetic deafness (see: ref. a4).

Note that a genetic factor is often involved in acquired deafness: this accounts for the large inter-individual differences in susceptibility to ototoxicity, noise trauma, or presbycusis.

An example of genetic IHC pathology: the bronx waltzer (bv/bv) mutant mouse cochlea

Mouse mutants offer a variety of models for genetic deafness. Here, the bv/bv mouse cochlea demonstrates normal OHCs but very sparse and abnormal IHCs (red arrows).

The mouse is deaf, only oto-acoustic emissions may be recorded.

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