What are the likely causes of otosclerosis?
Faulty bone formation
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 re-modelled. In otosclerosis, it seems that the re-modelling process of the stirrup (stapes) - one of the tiny bony ossicles in the middle ear - becomes faulty. New bone is not made properly and abnormal bone forms. However, the reason why this occurs mainly in the stapes (and sometimes the cochlea) is not entirely clear.
Have you inherited it?
Hereditary (genetic) factors seem to be important because a tendency for otosclerosis can be inherited. About 2 out of every 3 people with otosclerosis have other family members who also have this condition. However, some people with otosclerosis have no family history.
Could it be a virus?
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.
Low levels of fluoride
It is also possible that low levels of fluoride may have something to do with the development of otosclerosis. The number of cases of otosclerosis in the UK went down after fluoride was routinely added to drinking water. However, this possible link with low levels of fluoride is controversial.
What is affected in otosclerosis?
Otosclerosis mainly affects the tiny bone (ossicle) called the stirrup (stapes). 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 because 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.
Further reading and references
Niedermeyer HP, Arnold W; Otosclerosis and measles virus - association or causation? ORL J Otorhinolaryngol Relat Spec. 200870(1):63-9
Vicente Ade O, Yamashita HK, Albernaz PL, et al; Computed tomography in the diagnosis of otosclerosis. Otolaryngol Head Neck Surg. 2006 Apr134(4):685-92.
Cruise AS, Singh A, Quiney RE; Sodium fluoride in otosclerosis treatment: review. J Laryngol Otol. 2010 Jun124(6):583-6. Epub 2010 Feb 18.
Otosclerosis; British Tinnitus Association
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