Otosclerosis is a condition of the middle ear and mainly affects the tiny stirrup (stapes) bone. It causes gradual hearing loss. Treatments include hearing aids and surgery.
What is the ear like and how do we hear?
The ear is divided into three parts - the outer (external) ear, the middle ear, and the inner ear. The middle ear behind the eardrum is filled with air. Air comes from the back of the nose up a thin channel called the Eustachian tube. In the middle ear there are three tiny bones (ossicles) - the hammer (malleus), anvil (incus) and stirrup (stapes). The inner ear includes the cochlea and semicircular canals.
Sound waves come into the outer ear and hit the eardrum. The sound waves cause the eardrum to vibrate. The sound vibrations pass from the eardrum to the middle ear bones. The bones then transmit the vibrations to the cochlea in the inner ear. The cochlea converts the vibrations to sound signals which are sent along a nerve from the ear to the brain, allowing us to hear.
The semicircular canals in the inner ear contain a fluid that moves around as we move into different positions. The movement of the fluid is sensed by tiny hairs in the semicircular canals. These send messages to the brain along the ear (auditory) nerve to the brain to help maintain balance and posture.
What happens in otosclerosis?
Otosclerosis is a condition that mainly affects the stirrup (stapes), one of the tiny bony ossicles in the middle ear. To have normal hearing, the ossicles need to be able to move freely in response to sound waves. In otosclerosis, abnormal bone material grows around the stapes. The foot of the stapes, where it attaches to the cochlea, is usually where the condition starts. The abnormal bone reduces the movement of the stapes, which reduces the amount of sound that is transferred to the cochlea. The growth of the abnormal bone is very gradual. However, eventually the stapes can become fixed, or fused, with the bone of the cochlea. This can cause severe hearing loss. The hearing loss is known as conductive hearing loss (sound vibrations cannot be conducted (transmitted) from the stapes to the cochlea).
In most cases, it is just the stapes which is affected. However, sometimes, over time, otosclerosis can also affect the bony shell of the cochlea and the nerve cells within it. If this is the case, the damage to the nerve cells means that the transmission of nerve impulses to the brain can be affected. A different type of hearing loss, called sensorineural hearing loss, can then occur.
Both ears are usually affected in otosclerosis but sometimes only one ear is affected.
What causes otosclerosis?
The exact cause is not known. Bone is a living tissue and contains cells that make, mould and take back up (resorb) bone. Normally, bone is continually being broken down and remodelled. In otosclerosis, it seems that the remodelling process of the stirrup (stapes) becomes faulty. New bone is not made properly and abnormal bone forms. However, the reason why this occurs just in the stapes (and sometimes the cochlea) is not entirely clear.
Hereditary (genetic) factors seem to be important because the tendency for otosclerosis can be inherited. About 2 in 3 people with otosclerosis have other family members who also have this condition. However, some people with otosclerosis have no family history. It is also thought that a virus may play a part and the measles virus has been suggested. Indeed, the number of people diagnosed with otosclerosis seems to have decreased since the measles virus vaccination has been given. It may be that a genetic tendency to develop otosclerosis is inherited by some people. Then a trigger, such as a viral infection, actually causes the condition to develop.
It is also possible that fluoride may have something to do with the development of otosclerosis. The number of cases in the UK went down after fluoride was routinely added to drinking water. However, this possible link with low levels of fluoride is controversial.
Who gets otosclerosis?
Otosclerosis affects about 1 or 2 in 100 people in the UK. It usually first develops between the ages of 15 and 35 but sometimes develops in younger children. Women are affected twice as often as men. Pregnancy is not a cause but may make the condition worse, so symptoms are commonly first noticed during pregnancy.
What are the symptoms of otosclerosis?
Hearing loss is the main symptom
The hearing loss may remain mild but commonly it gradually becomes worse. It usually affects both ears, but not always. In some people the hearing loss stays mild for a number of years before getting worse. In others the hearing loss quickly becomes worse. Without treatment, in time, the affected ears often become totally deaf.
In addition to hearing loss:
- Quiet speech is common in someone with otosclerosis.
- Paracusis is also common. If you have this, you can hear better when there is a lot of background noise. For example, when talking to someone in a pub or cafe that is full of other people.
- Tinnitus occurs in about 4 in 5 people with otosclerosis. Tinnitus is an abnormal noise which you hear but it does not come from outside the ear. Noises heard include ringing, whistles, roaring, machine-type noises, etc. Tinnitus tends to worsen as hearing loss worsens.
- Vertigo (dizziness and balance problems) develops in some people with otosclerosis, although this is less common. This occurs when the balance mechanism in the inner ear (the semicircular canals) is affected.
How is otosclerosis diagnosed?
In otosclerosis, your eardrum usually looks normal when your doctor looks inside your ear, using an auriscope. This is the common instrument used to look inside your ears if you have earache. Generally, you will need to be seen by an ear, nose and throat specialist to have a diagnosis of otosclerosis. Hearing tests will show a specific pattern of hearing loss in otosclerosis. The specialist may also use a small device that is placed in your ear, called a tympanometer. This can help them look at the movement of the bones within your ear. In otosclerosis, the stirrup (stapes) will move less. This test is very quick and does not cause any pain. CT scanning may also sometimes be used.
What is the treatment for otosclerosis?
At first, when the hearing loss is mild, you may not need any treatment. As the disease progresses and hearing loss becomes worse, hearing aids can make a big difference. However, when the hearing loss becomes severe, hearing aids may not be of much help.
The most common operation that is done is to replace the stapes with an artificial bone made of plastic or metal. The operation is called a stapedectomy (or sometimes a stapedotomy). In most cases, this operation is successful and restores hearing. It may also reduce the chance of otosclerosis progressing to affect your inner ear. However, it is a very delicate operation. There is a small risk that the operation will fail and cause total deafness in the operated ear. Also, there is a small risk of damaging other nerves during the operation and of causing disturbance to balance or taste. The operation may not cure tinnitus, and will not improve hearing in the small number of cases which affect the cochlea. You should ask your surgeon about their success rates for this type of surgery.
So, although the operation is usually successful, it may be a difficult decision about if, or when, to opt for an operation. Because of the small chance of serious complications, some people decide to stick with hearing aids. They do so until their hearing becomes so bad that hearing aids are not helping very much. Other people opt for surgery earlier so as not to need hearing aids. When surgery is decided upon, the ear that is most badly affected is operated on first. This is the ear with most to gain if the operation is successful. It means that the best ear is still preserved in the small number of cases where the operation does not work.
There is debate about whether the second ear should be operated on in the future. Some surgeons feel not, because if anything were to go wrong with the ear already operated on, you would still have the possibility of wearing a hearing aid and hearing something with your second ear. You should discuss this with your surgeon.
There is some limited evidence that fluoride tablets may possibly slow the progression of the otosclerosis in some cases. They may help to preserve hearing and also help to reduce the symptoms of dizziness and balance problems. However, such treatment is not widely used in the UK.
Hormone tablets and the contraceptive pill
Some doctors feel that taking the contraceptive pill or hormone replacement therapy may make otosclerosis worse. If you have otosclerosis and are considering taking hormone treatment such as this, you should discuss the pros and cons fully with your doctor.
Dr Tim Kenny
Dr Jan Sambrook
Dr Helen Huins