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Gradually, the new bone matures and becomes hard, in a process called sclerosis. In otosclerosis, the consistency of the sound-conducting bones of the ear changes from hard, mineralized bone to spongy, immature bone tissue. The troble is that the lack of vibration prevents the sound from being conducted to the inner, leading to a conductive hearing loss.
Normally, the ear conducting system is divided into three parts, the outer ear, the middle ear and the inner ear. The outer ear collects the sound via the ear drum that vibrates. These vibrations are transmitted to the inner ear by three small bones:
- the malleus (hammer),
- the incus (anvil),
- the stapes (stirrup)
These are the three smallest bones in our body. They work by increasing the sound pressure and transmitting the sound to the inner ear. In the inner ear, the vibrations stimulate the hair cells which transform the vibrations into electrical impulses that are carried by the hearing nerve to the brain.
Pathophysiology of otosclerosis
If there are any problems in the outer or middle ear, a conductive hearing impairment occurs. There are different impairments depending on the localization of the damage.
- If the trouble is in the inner ear, it will lead to some nerve hearing impairment
- When there is difficulty in both the middle and inner ear, it leads to the mixed impairment. The mixed impairments are very common in otosclerosis but the conductive component predominates in most cases.
Why is this stapes bone so important?
It is the stapes which connects to the inner ear in a bony gap called the oval window. The biggest problem is that, when we talk about otosclerosis, the formation of new bone around the oval window stops the stapes from moving which normally leads to complete deafness.
Incidence of the condition
When this condition is present, both ears are usually involved. Several studies have proven that the condition occurs in men and women with almost equal frequency and that it usually begins in the teens or early twenties. Less than 1% of the population is affected by this condition. Although otosclerosis tends to run in families, it does so irregularly; parents with otosclerosis do not necessarily transmit it to their offspring. Some studies have shown that about 10 percent to as high as 18 percent of all caucasian women and 7-9 percent of all caucasian men between the ages of 30 and 50, have histological signs of otosclerosis. Less than 10 percent of these people will actually seek help for hearing loss.
Types of hearing impairment from otosclerosis:
Generally, there are two types of otosclerosis and they are:
During this process, otosclerosis can spread to the inner ear and a sensory-neural hearing impairment may result due to interference with the nerve function. This nerve impairment is called cochlear otosclerosis and once it develops it is permanent and there is no cure for it.
In some cases, otosclerosis can spread to the stapes or stirrup bone, the final link in the middle ear transformer chain which leads to the conductive hearing impairment. This type of impairment is called Stapedial otosclerosis and is usually correctable by surgery.
Possible causes of otosclerosis
Many experts believe that only heredity can be the cause. Otosclerosis runs in families in many cases, but the truth is that genetic cause is not clear. It is very possible that there are a number of different genetic types of otosclerosis but there is still no clear distinction between them. The most common theory has been that there is a dominant gene that can cause otosclerosis.
It is difficult to find the underlying cause of otosclerosis, since it may not be the same in everyone.
It some studies, it is proven that women are more likely to develop otosclerosis than men.
It is proven that susceptible women may develop otosclerosis during pregnancy.
Caucasians tend to be the most commonly affected race.
It is a genetic disorder is characterized by abnormally brittle bones
Some evidence suggests that drinking non-fluoridated water may increase the risk of otosclerosis in susceptible people.
Some people develop otosclerosis even though they don't have any of the known risk factors.
Symptoms of otosclerosis
Deafness is the main symptom and is usually present in both ears. Most patients notice the deafness before the age of 50. Noises in the ear (tinnitus) are present in most cases. Several patients have also noticed:
- slight unsteadiness
- discharge from the ears
- hearing may improve in noisy conditions
Diagnosis of otosclerosis
Diagnosis of otosclerosis is based on a combination of the:
- patient's family history
- the patient's symptoms
- the results of hearing tests
Every doctor should first determine whether the patient's hearing loss is conductive or sensory-neural. The two tests that are commonly used to distinguish conductive hearing loss from sensory-neural are:
- Rinne's test
- Weber's test
A computed tomography (CT) scan or x ray study of the head may also be done to determine whether the patient's hearing loss is conductive or sensorineural.