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What is Otosclerosis?

What is Otosclerosis?

Otosclerosis is a disease of the bones of the middle and inner ear (see Figure 1). The ossicles (bones) become knit together into an immovable mass, and do not transmit sound as well as when they are more flexible. Figure 2 illustrates one form of this called stapes fixation. Otosclerosis can also affect the other ossicles (malleus and incus) and the otic capsule.

stapes-fixation-75d.jpeg  

Figure 2. Stapes fixation in otosclerosis. A bony ankylosis (knee) knits the bone of the middle ear to the stapes, preventing normal transmission of sound from the eardrum into the inner ear.

Otosclerosis is usually inherited in an autosomal dominant pattern with variable penetrance. This means that you have a 50% chance of getting the gene for otosclerosis if one parent has it, but that not everyone with the gene develops symptoms. Hearing loss usually begins between the ages of 10 and 30 and can be of two types. When otosclerosis involves the small bones of the middle ear, a conductive type loss is found. This type of hearing loss can be corrected both by a hearing aid as well as a surgical procedure called stapedectomy.

When otosclerosis significantly involves the bone which surrounds the inner ear, called the otic capsule, a sensory type hearing loss occurs. This type of hearing loss is not correctable by stapedectomy. While hearing aids are worth trying, they also may be ineffective. When otosclerosis involves both the small bones and the cochlea, a mixed type hearing loss occurs. More details about hearing loss can be found here.

What is the Natural Course of the Disease?

Hearing loss generally begins between the ages of 10 and 30. Early on, the disease is called otospongiosis. During this time there is active remodeling of bone of the otic capsule. There may be no conductive hearing loss at this point; however, there may be sensory hearing loss. The sensory hearing loss is attributed to leakage of enzymes from bone into the inner ear. Sensory hearing loss is also attributed to atrophy of the spiral ligament caused by involvement of lamellar bone at the inner surface of the cochlear capsule. A third cause is vascular, due to degeneration of the stria vascularis. In early stages, treatment with fluoride may be helpful. The sensory component of the disease may eventually lead to complete deafness, but fortunately, this is unusual. In most cases, both ears are affected, although in about 10 to 15% of patients, hearing loss occurs on one side only.

Later on, a conductive pattern of hearing loss appears. The conductive type of otosclerosis usually progresses up to a maximum when patients are in their 30s. After this, it rarely progresses. Dizziness or imbalance is a feature of otosclerosis in roughly 25% of cases. In about half of all patients, there is a family history of similar problems.

People of African-American descent rarely have otosclerosis -- it is usually a condition found in persons of Caucasian or Oriental descent. Women are affected twice as often as men, and pregnancy often has an adverse effect. Otosclerosis is often discovered during or just after pregnancy. The effect of hormone supplements post-menopause is unknown.

What Causes Otosclerosis?

Most feel that otosclerosis is an inherited, an autosomal dominant disease with variable penetrance. As there is also evidence of viral influences in otosclerosis, a recent hypothesis is that otosclerosis requires a combination of a specific gene with exposure to a specific virus (for example measles) for it to be expressed and for hearing loss to occur (McGuirt et al, 1998). Some feel that chronic measles infection in bone predisposes patients to otosclerosis. Viral materials can be found in osteoblasts in otosclerotic lesions (Nadol, 1998).

Pathologically, otosclerosis occurs only in human temporal bones, and is considered to be a disorder of new bone formation. Histologic otosclerosis, meaning that it is found only on section but has no symptoms, is found in one of every 10 Caucasians in the United States. Clinical otosclerosis, with involvement of hearing, occurs in one of every 10 patients with histologic otosclerosis (Nadol, 1998). This results in the rather amazing conclusion that 1% of the population should exhibit clinical otosclerosis.

How is Otosclerosis Diagnosed?

Diagnosis is usually made by a combination of family history, progressive conductive hearing loss pattern, and exclusion of alternatives. Hearing tests may initially show a sensory pattern and later show the typical conductive loss pattern. Acoustic reflexes may eventually be absent, but early on may show the "on-off" effect. Tympanometry often shows stiffening of the ossicular chain. A CT scan of the temporal bone is specific but insensitive. It may be the only way to document otosclerosis early in the disease.

Dizziness can occur in otosclerosis and was reported in one study to occur in 15% of patients. Pathologically, there is degeneration of the vestibular ganglion (Scarpa's ganglion). The mechanism for dizziness is unknown, although there is speculation that it derives from release of enzymes from metabolically active bone into the inner ear. Other possibilities might be a "halo" effect -- persons visiting ear doctors may be more likely to attribute dizziness to their ear than others, occlusion of fluid pathways within the inner ear from bony overgrowth, or another effect on the ear caused by the same underlying (unknown) cause as otosclerosis.

How is Otosclerosis Treated?

There are four treatment options:

Do nothing (conservative approach)

Otosclerosis does not have to be treated. It is usually advisable to have a hearing test repeated once a year (or earlier if hearing drops).

Amplification

Hearing aids are usually effective for conductive hearing loss.

Medical treatment

To date, the only proposed medical treatment has been sodium fluoride, which is a dietary supplement (not a drug). This treatment is not widely accepted, and has not been proven to be effective. A large uncontrolled study of about 1500 patients by one of the founders of the AHRF, Dr. Shambaugh, suggested that it was effective (Shambaugh, 1990). The idea of using fluoride is not unlike that of using it for teeth -- fluoride speeds up hardening of bone. Fluoride is the only medical treatment available.

Florical and Monocal are the two fluoride preparations available over the counter. Side effects include occasional stomach upset, allergic itching, and increased joint pains. If aggravation of arthritis occurs, the fluoride is stopped and the joints return to their previous state in a few weeks. In such a situation, patients can pace themselves, taking as much of the medication as can be tolerated. Typical doses are one tablet three times a day (Florical) and one or two tablets three times a day (Monocal). After two years of fluoride treatment, the dose is reduced from three times a day to once a day. Once the otospongiosis phase of otosclerosis is over and there is a clear cut otosclerosis documented by conductive hearing loss, fluoride may be stopped.

In theory, avoidance of estrogens or use of estrogen blockers might be helpful in individual with otosclerosis, since otosclerosis frequently worsens during pregnancy, suggesting hormonal modulation. To our knowledge, this hypothesis has not been studied.

Surgical treatment

For conductive hearing loss, in 1957, Dr. John Shea invented the procedure of stapedectomy, which produced excellent hearing results, which remain good for many years after the surgery. This procedure may allow avoidance of hearing aids. However, it does not help the sensory component of the hearing loss and, at best, may close the "air-bone" gap. It also does not affect the vertigo that is sometimes associated with otosclerosis. According to Nadol, stapedectomy is indicated in patients with good bilateral inner-ear function, and conductive hearing loss ranging from 25 to 30 dB in elected frequencies. Stapedectomy is unreasonable if discrimination scores are lower than 65%, as this indicates that there is a substantial sensory component. Patients with stapedectomy may attain better results with hearing aids because of the need for less amplification.

Stapedectomy may fail for a number of reasons. It is a somewhat difficult and delicate procedure. There may be displacement of the prosthesis, reclosure of the fenestra (window), or erosion of the incus. Disease may progress so that correction of the conductive component is inadequate.

 

Research on otosclerosis

Considerable research has been done on otosclerosis in the last 40 years, and a search on pubmed for "otosclerosis" yielded 2746 articles published since 1966. In spite of this large outpouring of effort, we are presently only beginning to understand the disease. The AHRF has supported research on otosclerosis in the past, and expects to do so in the future. In our opinion, particularly helpful would be a method of preventing progression of the disease, and also and a method of determining who is susceptible to otosclerosis. If you would like to contribute to the AHRF's research efforts on otosclerosis or other disorders, please click here.

Acknowledgments

Images marked as copyright Northwestern University were developed with the support of an NIH grant to Northwestern University Dept. of Otolaryngology, and are used with permission.

References

 

  • McGuirt Wt and others. Linkage of a gene for otosclerosis to chromosome 15q25-q26 and identification of a candidate gene: Aggrecan. ARO abstracts, #428, 1998.
  • Nadol JB. Otosclerosis: clinical aspects and management. Audio-Digest Otolaryngology 31,#2, Jan 15, 1998
  • Shambaugh GE Jr. Fluoride therapy for otosclerosis.Arch Otolaryngol Head Neck Surg 1990 Oct;116(10):1217

 

 

 

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