Sensorineural hearing loss, often shortened to simply 'sensory hearing loss', is hearing loss caused by damage to the nerves that react to sound waves in the inner ear, the vestibulocochlear nerve (CN VIII), or any part of the central auditory system that carry and process the sensation of sound on its way to the brain. Sensorineural hearing loss is generally a greater problem than conductive hearing loss, as it is much more likely to be irreversible.

The most common cause of sensorineural hearing loss is damage to the hair cells in the cochlea; the cochlea has rows of hair cells, which are stubby fingers of nerve cells that are sensitive to movement; when the sound waves move through the liquid in the cochlea it causes these cells to sway, and the hair cells in the area most affected by a given frequency sway the most -- sending a message to the brain that we are hearing a certain complex of sounds. The louder the sound we hear, the stronger the sound waves, and the more the hair cells move1. If they are 'moved' too much and too forcefully they may either 'fall down' and lay flat on the floor of the cochlea, leading to temporary hearing loss (such as you experience after attending a very loud concert), or they may break, leading to permanent hearing loss (such as you experience after attending a number of very loud concerts).

Damage to the hair cells (AKA 'cochlear trauma') can come about through a number of different causes; an incomplete list includes:

  • Loud noise: Various studies have found different levels of damage depending on age, number of times exposed, and volume, but a general general rule of thumb is that sounds of 85 dB are safe for about 8 hours, 88 dB for four hours, 91 dB over two hours, and so on (an increase of each 3 dB cuts the safe exposer time by half). This rule of thumb puts the recommended time for many loud activities, such as rock concerts and motorcycling, at about 7-15 minutes. In these situations, ear protection is recommended. A common characteristic of hearing loss due to loud noise is that hearing loss will generally be the most severe around 4000 Hz2.
  • Ototoxic drugs: Some antibiotics, namely the aminoglycoside antibiotics (AKA mycin drugs) including streptomycin, neomycin, kanamycin, and gentamicin, can damage the hair cells depending on the dosage, the patient, and other drugs that the patient may be taking. These drugs tend to produce hearing loss of the higher frequencies of both ears. A few drugs, such as aspirin taken in large doses, can cause temporary hearing loss.
  • Presbycusis: Which is to say, hearing loss that has no obvious cause other than old age. It generally starts to be noticeable after 50 years of age, and there's not much you can do about it other than take care of your hearing. And then hearing aids. Presbycusis primarily affects the cochlea, but can also affect the auditory nerves.

Sensorineural hearing loss caused by damage to the cranial nerve or other parts of the central nervous system may be referred to as retrocochlear hearing loss. Because the neural pathways for processing auditory signals have some redundancy built in to them, the pure tones used in audiometric testing may be heard clearly, but more complex signals such as speech may not be heard clearly. Causes of retrocochlear hearing loss do not generally target the auditory nerves specifically, so having any one of the following conditions does not guarantee hearing loss, but possible causes include:

Sensorineural hearing loss is usually detected by testing hearing through both air conduction (i.e., headphones) and bone conduction (a special 'headphone' that transmits sound vibrations directly to the bones of the skull). If hearing loss is due to damage to the middle ear, the air conduction results will be below average, but the bone conduction results will be normal. If hearing loss is due to damage to the nerves, both tests will be below average. It is also possible (and not too uncommon) to have mixed hearing loss, in which air conduction results are low, and the bone conduction results are better, but still low. In this case there are multiple causes for the hearing loss, some affecting the middle (or outer) ear, and some affecting the nerves.

Sensorineural hearing loss can also be detected through a procedure known as auditory brainstem response (ABR), in which electrodes are used to detect the change in action potentials in the nerves of the cochlea and brainstem. This is an easy test and can be done on a patient that cannot respond verbally, so it is frequently used to test infants (it can even be done while they are asleep), however, it does not actually tell you that the patient can hear, only that the signal reaches the brainstem. Moreover, it tends to detect high-frequency hearing loss but ignore low-frequency, and the equipment used is not cheap.

A slightly more limited testing method is otoacoustic emissions (OAEs), in which a very sensitive device measures the waves resulting from the constriction of the outer hair cells1.p2; this will not test the functioning of any nerves other than those in the cochlea, but is very quick and easy, even more so than ABR, and is also popular for use on infants. OAE devices are generally small, portable, and can be used with very little training, so they are currently the most popular portable test for sensory hearing loss.

Treatment of sensory hearing loss is generally limited to removing the underlying cause and the use of hearing aids. It is nearly always permanent, and even slight hearing loss can make it difficult to understand speech, particularly in noisy environments. It is better to avoid the issue altogether by avoiding noisy environments, wearing ear protection, and, of course, getting a full health screening when you become pregnant.



Footnotes:

1. Hair cells are ridiculously complex, but here's an overview. A hair cell refers to cells that have a 50-100 hair-like stereocilia protruding into the liquid of the fluid-filled canal of the cochlea. Inner hair cells have a bunch of short, freestanding cilia which move freely when sound waves pass through the fluid in the cochlea. The intensity of stimulation is coded by the brain as loudness. The structure of the cochlea causes various wave frequencies to be strongest at different points; the patch of hair cells being stimulated is interpreted by the brain as pitch.

90-95% of the auditory nerve fibers connect to inner hair cells, but there are also outer hair cells, which have one longer cilia (AKA a kinocilium), the end of which is embedded in a membrane (the tectorial membrane) that moves when sound waves pass through the cochlea. This long cilia will contract when the shorter outer hair cell cilia are stimulated, causing the tectorial membrane to move more than it would otherwise, thus amplifying the sound waves, particularly in the lower frequencies.

Hair cells are technically not nerve cells -- they are 'non-neural receptor cells', releasing neurotransmitters which stimulate the sensory neurons. The term 'sensorineural hearing loss' includes the destruction of the hair cells in part because damage to them tends to lead to retrograde degeneration of the underlying afferent nerve cells; in other words, when the nerve cells stop receiving valid input, they atrophy.


2. Noise induced hearing loss, assuming that the noise is spread reasonably evenly across common frequencies, will cause hearing loss that is most severe in frequencies near 4000 Hz. This 4K notch is so predictable that it is treated as a reliable diagnostic indicator. There are two notes to make here: first, we have no idea what causes this, although there are various theories. And second, this notch may actually be centered anywhere in the frequencies between 3000 Hz and 6000 Hz, depending on the individual; we call it the 4k notch because the standard audiometric test only hits 500, 1000, 2000, 4000, and 8000 Hz, and hence we only see the 4k portion of the notch unless we go looking for more information. Note that hearing loss may occur at all frequencies; the 4k notch only refers to the fact that it is most severe in the 4k range.


References:
Audiology: The Fundamentals, 4th ed. by Fred H. Bess and Larry E. Humes
Human Physiology: An Integrated Approach, 4th ed. by Dee Unglaub Silverthorn
Cranial Nerves: In Health and Disease 2nd ed. by Wilson-Paulwels, Akesson, Stewart, Spacey.

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