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Otosclerosis
ALTERNATIVE NAMES: Otospongiosis.
DEFINITION: New bone formation involving the bony cochlea or labyrinth.
WHAT IS GOING ON IN MY BODY? The middle ear consists of the tympanic membrane and the ossicles. The ossicles are the three bones in the middle ear space that transmit the vibration in the eardrum induced by sound into the cochlea. The cochlea is a snail-shell-shaped structure which has two fluid-filled compartments, one inside the other. Vibration of the bones for hearing is transmitted through the stapes, which is a stirrup-shaped bone, into the inner ear, where it generates a fluid wave. This fluid wave stimulates frequency-specific areas in the cochlea, which then generate an electrical signal. This electrical signal is transmitted through the acoustic nerve into the brain, where the sound is interpreted. In otosclerosis a focus of new bone forms on the bone housing the inner ear structures. If it forms on the window in which the foot plate of the stapes sits, the stapes can be fixed and thus not transmit vibration into the inner ear, with resultant conductive hearing loss. Infrequently the bone growth can start to affect the full thickness of the bony housing of the inner ear structures and then start to affect the nerve and balance components of hearing.
WHAT ARE THE SIGNS AND SYMPTOMS? The main complaint that a patient has is a slowly progressive, gradual hearing loss. It initially usually affects just one ear, but both ears can be simultaneously affected. Many individuals go on to develop otosclerosis in both ears. If otosclerosis affects the full thickness of the bony housing of the inner ear, the patient may still have a gradual hearing loss, but it may be mixed in that it affects both the conductive and nerve components or in certain rare situations can affect the nerve component alone. Another rare form of otosclerosis that can cause conductive hearing loss is one in which the new bone plugs up a structure called the round window. The stapes sits in the oval window and not the round window. The round window's function is to allow the vibration-induced inner ear fluid wave to move completely through the inner ear. Whenever the stapes moves in, the round window bulges out and vice versa. If new bone forms in the round window, the round window membrane can no longer move, and the stapes then cannot generate its fluid wave, with a resultant conductive hearing loss.
On physical examination there are usually very few findings. On occasion a Schwartze's sign is present. This is an area of redness just in front of the oval window in which the stapes sits. It is present when the otosclerosis is active when there is increased blood flow to support the new bone growth.
WHAT ARE THE CAUSES AND RISKS? Otosclerosis is seen in about 3% of the population based upon temporal bone studies, but not every individual with otosclerosis ends up with any form of hearing loss. The only time hearing loss will become evident is if the stapes is fixed, the round window is obstructed, or the otosclerosis begins to injure the nerve or balance portions of the inner ear. Many cases have no known cause. In others there is a clear family history. Osteogenesis imperfecta, which is a genetic disease whereby the bones are brittle, has a fairly high frequency of otosclerosis. There is some thought that drinking nonfluoridated water may increase the likelihood that a susceptible individual may develop otosclerosis.
HOW TO PREVENT THE DISEASE: There seems to have been a decline in otosclerosis with fluoridation of municipal water systems. If an individual with a positive family history does not have access to fluoridated water, then fluoride supplementation in the form of Florical® tablets may be useful.
HOW IS IT DIAGNOSED? It is always helpful if a Schwartze's sign is present. The main symptom is the slow progressive nature of conductive hearing loss that ultimately stabilizes. On hearing testing, the nerve function is usually noted to be normal, but there is difficulty getting sound to the nerve through the fixed stapes. On the hearing test the low-frequencies are usually the most severely affected. If there is some concern that there is otosclerosis damaging the nerve function of the inner ear hearing or balance systems, a high-resolution CT scan of the temporal bone, which houses the cochlea, can demonstrate full thickness involvement of the bony housing of the inner ear nerve structures.
WHAT ARE THE LONG-TERM EFFECTS? In the individual with an asymptomatic otosclerotic focus that stops growing, there should be no long-term effects. If the otosclerotic focus only affects the stapes or the round window, there will be a stable conductive hearing loss. Should the otosclerosis, however, continue to progress and affect the bony housing of the inner ear, nerve deafness or imbalance may result.
AM I PUTTING OTHERS AT RISK? Other than the problems that may result from not being able to hear, other individuals will not be at risk.
WHAT ARE THE TREATMENTS? Should the individual develop a conductive hearing loss, the hearing can be improved with hearing aids. There are surgical procedures, however, that can bypass the fixed stapes and bring the hearing up to what the nerve is capable of doing. For those individuals with a round window otosclerosis, most ear surgeons would not recommend removing the otosclerosis from the round window because of the high risk of permanent deafness but would instead recommend hearing aid placement. Should the otosclerosis be suspected of causing injury to the inner ear, Florical at 6 to 8 tablets per day may help in stabilizing the otosclerotic focus and preventing any further neurologic injury.
WHAT ARE THE SIDE EFFECTS TO THE TREATMENTS? The side effects to hearing aids are really quite few and are mostly limited to the mold that houses the hearing aid. The side effects of the surgery mostly relate to the complications that can occur relative to the procedure itself. There is approximately a 0.5 to 1 % incidence of nerve deafness. Taste disturbance, because of injury to the chorda tympani nerve that often has to be moved out of the way to gain access to the stapes, can occur. Temporary vertigo is fairly frequent but usually resolves. Failure to restore the conductive hearing loss can occur if the prosthesis is of insufficient length or becomes dislodged post-operatively.
WHAT HAPPENS AFTER TREATMENT? Successful surgery usually restores the hearing to what the maximum nerve is.
HOW DO I MONITOR THE DISEASE? Any gradual hearing loss, especially if it is unilateral, needs to be investigated. It may be a result of otosclerosis, but other entities affecting nerve function, such as benign intracranial tumors (acoustic neuromas and cerebellar pontine angle tumors) could also occur.
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