Causes of hearing loss
The medical profession is keen to know what causes loss of hearing because if we know the causes, we can treat them or prevent them. Sociologists also want to know what causes deafness because of the effect that it has on communities.
Remember, again, interest in the cause is only of great interest if you want to do anything about it. There are many deaf people for whom the cause of their deafness is only of vague, peripheral interest. It certainly isn't high on the list of things you'd know about someone. It's more important to know if the person has been deaf from birth and if they have any deaf relations.
A similar situation occurs with gay people, when science tries to find out why a man is gay. Most gay men aren't in the least bit interested, but straight people do seem so, perhaps because of an underlying idea that if we know why they are gay, we can "cure" it.
So, for born-deaf members of the Deaf community, the cause of deafness isn't necessarily very important, but it is for many hearing people, such as hearing parents of deaf children, or people who become deaf, or people trying to prevent or cure deafness.
So far, in the last lecture we have mentioned why conductive deafness usually occurs. The canal is blocked, the ear drum is perforated, the ossicles can't move or the middle ear is blocked.
The canal can be cleared, often by surgery or syringing. The ear drum may be stitched up, or even patched. The ossicles can be put back together if they are dislocated or can be freed if they are growing together through otosclerosis. Otosclerosis is surprisingly common in young people. It tends to be found in certain families, so it is obviously some sort of genetically caused problem. Its particularly common in families with fair hair and blue eyes, but also common in India. It affects men and women, but it is particularly likely to start during pregnancy when hormone changes seem to trigger it off, so it is more prevalent in women. Oddly, this fusing of bones only happens with the stapes, and not with any other bones in the body.
Freeing the ossicles fused through otosclerosis is often only temporarily useful, though, because once the bones have started fusing, they often can't be persuaded to stop. An alternative treatment is a stapedectomy where the stapes is taken out and replaced with a plastic one. This is a very common operation and is 90% successful. However, the other 10% that arent totally successful often involve side-effects such as total hearing loss, damage to the facial nerves or damage to the ear-drum. Many people consider a stapedectomy a worthwhile risk, though, because for people who have been hearing, a hearing loss is something to be cured, even if there is considerable risk.
When the middle ear is blocked because of an infection, especially a cold, it is called otitis media (literally "inflammation of the middle ear"). This is very common in children and usually doesn't last for long and clears up by itself. If it gets very nasty and sticky, and won't drain away down the eustachian tube by itself, it is called glue ear. Antibiotics can be used to clear it, or grommets can be fitted into the ear drum and it drains out that way. People usually grow out of these middle-ear infections because the shape of the eustachian tube changes as we grow up. In children it is curved, so there is more chance of infection: in adults, it is much straighter, so potential infections can drain away more easily.
In Britain, and other countries with a well-developed health service, this middle ear deafness is not usually a major problem, although adults need to look out for it because it can cause behaviour problems (eg if the child is bored in school because he or she can't hear the lesson), and if it occurs at a young age and lasts a long time, it can interfere with the natural acquisition of speech.
In other countries, it is a major cause of deafness. Among the aborigines living on reservations in Australia, glue ear is pandemic, and there are many people who are left permanently deaf because of it. The combination of poverty, little health care and a lack of natural immunity to "European" colds causes it. This pattern can be seen all over the world.
Middle ear deafness makes everything quieter. The typical audiogram of someone with middle ear deafness is flat (like someone with little loss) but much lower than that of someone with no loss.
Sound can't get through the middle ear easily, but it can travel through the bone of the head. When people test for middle ear deafness, they can use a bone-conductor. If there is a conductive hearing loss, the person should be able to hear much better when the sound is brought to the inner ear via another route. An audiogram taken from sounds played to the ear and played through the bones will show an "air-bone gap", where hearing is better when the middle ear is bypassed.
Perceptive deafness (inner ear) has many causes. It can occur at any time through life.
a) Before birth:
i) There may be a family history of deafness. This may just be simple deafness, or part of a syndrome such as Ushers (which we will discuss again later)
ii) There may be a genetic accident, such as with Downs Syndrome. Most DS people have at least a mild hearing loss, and some have quite a severe loss.
iii) Some ototoxic ("poisonous to the ear") drugs given to the mother while she is pregnant (eg quinine or thalidomide). There are now alternatives to most ototoxic drugs, but they are not always used, particularly in poorer countries which have to use the cheapest drugs they can get.
iv) The mother contracts rubella. The first trimester is the most dangerous because that is when the organs of Corti develop. This can be prevented by inoculating girls. Because rubella often causes multiple handicap, including deaf-blindness and brain damage, some mothers may be offered the option to abort the foetus.
b) At birth:
i) Lack of oxygen. If the lungs are not fully developed, the baby can be short of oxygen. The inner ear is very sensitive to oxygen and, if there is not enough, the hair cells can be damaged, and high tones especially can be affected.
ii) Too much oxygen. The blood in the unborn baby carries more oxygen than a baby breathing real air. When the baby is born, the baby has to stop taking so much oxygen around. If the red blood cells don't react properly, the baby may have too much oxygen, and the delicate (fussy?) hair cells can be damaged.
iii) Birth trauma. For example an accident with forceps, or anything that leads the baby to stop breathing.
c) After birth:
i) Viruses like mumps, measles, scarlet fever or meningitis may cause deafness, or
ii) the drugs used to treat the diseases might. Antibiotics are designed to attack bacterial cells, not animal cells. Unfortunately, the mitochondria (the little power stations in each cell) can sometimes seem more like little bacteria than parts of an animal cell, and the antibiotics attack them.
But, despite all this, about 50% of all childhood deafness is from unknown causes. Some deaf people find they have felt obliged to make up a reason because they don't know, and a reason seems expected.
d) In adult life
i) Some deafness is hereditary, but only develops in adulthood. The genes are there at birth but deafness doesn't start until later. In "Adjustment to Acquired Hearing Loss", Maggie Woolley gives a good account of her hereditary deafness that began to show itself in her teens.
It is important to be clear about the difference between "congenital" and "hereditary".
Hereditary deafness means that the deafness comes from the parents' genes. It might show itself at birth or later, but it is caused by genes. Congenital deafness means that the deafness is present at birth. It may be caused by genes or by any of the other things we've mentioned.
ii) Illness and accidents. Viruses can cause partial or total deafness (Jack Ashley went deaf after an illness). Motor accidents and other physical trauma can also be causes (I King Jordan, the president of Gallaudet was deafened through a motor-cycle crash).
iii) Life-saving Surgery. Sometimes a person may have a tumour on or near the auditory nerve, and this must be removed to save their life, but the result is deafness.
iv) Medical blunders. There are quite a few cases of people who went into hospital for a minor ear operation and came out totally deaf. Ears are very delicate and any intrusion will disturb them greatly. Even if deafness doesn't result, tinnitus may be caused.
e) Old Age (Presbyacusis)
Hair cells wear out and die off. The auditory nerve can also start to degenerate. This form of deafness starts in early adulthood and progresses. It usually doesn't start to be noticeable until the late 50s, but is quite pronounced by the 70s.
Some of this is because we get old, but some is because the ears have had decades of misuse, living in a noisy environment. A study on some people who live in the mountains of Ethiopia (generally regarded as a nice, quiet place) found that the 70-year olds there had hearing equivalent to British teenagers.
This is by far the most common cause of deafness in Britain. There is very little research into it. Remember that the larger the group, the less study there is on it. It is important to bear old people in mind when considering the overall scheme of deafness because they will often have other problems, such as arthritis and poorer sight as well. But on the other hand, they tend not to be as demanding on society as younger people. Perhaps this is because in our society, we are taught to accept deafness as something that just happens as we get older.
f) Noise-induced deafness.
This is usually started in working life, but manifests itself in later life.
Very loud noises (eg close gunfire or an explosion) can rip the Organ of Corti apart and tear the hair cells. It can also rupture the ear-drum, and tear the stapedial muscles.
Continuous loud noise has serious effects, too, but it takes a while to show. It can damage or kill the hair cells by overworking them. When a noise is loud, the cells have to work hard, and they need extra oxygen and food to do so. They also need to get rid of the waste products of metabolism. Normally, supply of food and oxygen and waste removal is fine, but if the cells are really working hard, the system can't cope, so the cells end up starving, suffocating and drowning in their own waste (not a nice way to go....).
Even if they don't die, they can get very tired if exposed to loud noise for a while (eg at a concert or at a club). This is why when you come away from an evening of loud noise, everything sounds very quiet.
Dead cilia do not naturally regenerate, (like, for example, blood or hair), so once they are dead, that's it. Until recently, there was no medical cure, either. However, it is possible that cilia can be persuaded to regenerate. Research in the last three or four years shows it can happen. The hair cells in the ears of chickens and guinea pigs have been persuaded to regrow, by giving them a large dose of retinoic acid (a first cousin to Vitamin A), which we know can cause ordinary cells to take up specialised jobs, such as becoming part of nerves. So far, this has only been done in test-tubes, and the amount of retinoic acid needed to regenerate the hair cells in humans is toxic. Still, it seems it is possible to restore some sort of hearing. When this will be possible, and at what cost, and how useful it'll be remains to be seen.
Noise-induced deafness is recognised by the EC as an important industrial injury (although some people are worried that younger people can damage their hearing using a walkman). It is reckoned that about 1.6 million UK workers are exposed to over 85 dB regularly, and 650,000 to over 90 dB. European law now says that workers should not be exposed to levels above 90 dB without protection, and that at 85 dB, workers should be provided with protection, although they are not obliged to wear it. This law is very weak, and there is not nearly enough education of workforces. Noise-induced deafness is not noticed at first, so preventive measures are not seen as important, and other factors such as comfort (protectors are hot and uncomfortable) and machismo (they look cissy) are given priority. 85 dB doesn't really sound too loud, but constant exposure (over 8 hours a day) will cause damage over time. British law, since 1989 has required people exposed to over 90 dB of noise in industrial situations to wear ear-protectors. However, this law is not as rigorously enforced as it could be.
France has recently passed a law limiting the intensity of output on personal stereos because the French government is worried that using a Walkman at loud volumes for long periods of time is creating a nation of teenagers and young people with hearing loss.
The high-frequency is usually lost in noise-induced deafness. This is probably because the hair cells are nearest the oval window, and there are fewer of them. Cells for lower sounds are spread along the cochlea.
Another nasty side-effect is tinnitus. This will be discussed more in a later session when we think more about tinnitus.
One good thing about the decline of Britain's heavy industry, like steel, ship-building and power generation, is that there is less noise-induced deafness.
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