Otosclerosis is a genetic localized bone disease that causes hearing loss. It occurs only in humans and affects the otic capsule (bony labyrinth) of the temporal bone and the ossicles. In this disorder, the normal mature lamellar bone is absorbed and replaced by woven and more porous less dense bone. Otospongiosis (softening) of bone is actually what occurs.
What causes otosclerosis?
Although otosclerosis is present in 10% of the population, only 1% of people will develop hearing loss. The development of hearing loss depends on the location of the osteosclerotic focus. It is mainly inherited in an autosomal dominant way with invariable penetrance, but other forms of inheritance are possible. Half of the patients with clinical otosclerosis have a positive family history. In 30% of cases, it can also arise sporadically due to non-genetic causes. The trigger that sets the development in these patients is unknown. Hormonal, metabolic, viral infections (measles, mumps, and rubella), vascular and autoimmune causes have been implicated.
Who develops otosclerosis?
Otosclerosis occurs more commonly in caucasians where it is believed to have a prevalence of 0-3%. It is the cause in 5% of patients with hearing loss. It is extremely rare in black Africans and Native American Indians. It occurs twice as common in females than in males and some evidence suggests that hormonal changes in pregnancy stimulate the disease. The risk of developing hearing loss is increased by 25% in pregnant females and those taking oral contraceptives. Bilateral (both ears) otosclerosis occurs in 90% of females and only 65% of males. It is rare in children.
Hearing loss in otosclerosis
Hearing loss usually becomes apparent between 15 and 45 years of age. In females, it is mostly seen in the 20 – 30-year age when pregnancy may be responsible for rapid progression. Both ears are affected in 80% of patients. Hearing loss usually stabilizes at 50 years, a sign that the otosclerotic foci have matured.
If it involves the ossicles, especially the stapes a slow progressive conductive hearing loss is seen. The progress is often stepwise with periods of quiescence alternating with periods of deterioration. The hearing loss may be asymmetric in the two ears. The most common focus of otosclerosis (80-90%) is the fistula ante fenestral just in front of the oval window. Obliterative otosclerosis refers to otosclerosis that obliterates the whole oval window. In some cases, otosclerosis involves the whole footplate of the stapes but not the walls of the oval window. This is referred to as a biscuit footplate. Although otosclerosis involves the round window in 30-50%, hearing loss in this scenario is extremely rare and occurs only if the whole round window is involved.
Less commonly it may involve the cochlea and cause sensorineural hearing loss. In the absence of otosclerotic changes in the cochlea, it is speculated that the release of toxic enzymes or metabolites in the cochlear fluid is responsible for the sensorineural component. In these cases, it is typical for patients to present with mixed hearing loss.
Tinnitus and vertigo in otosclerosis
Tinnitus occurs in 75% and dizziness and vertigo in 25% of patients. The exact mechanism for the development of vertigo is unknown.
Differential diagnosis of otosclerosis
Some conditions may mimic the hearing loss in otosclerosis. These include superior canal dehiscence syndrome (SCDS), tympanosclerosis, osteogenesis imperfecta, and other middle ear conditions. It is of vital importance that these conditions are considered before surgery is attempted.
Examination of otosclerosis
The “Schwartz sign” refers to the finding of a Flamingo pink discoloration behind the intact tympanic membrane (ear drum). It occurs in 10% of patients with otosclerosis due to the increased vascularity of the middle ear.
Varying degrees of conductive or sometimes mixed hearing loss may be present. A typical finding is the “Carhart notch” which indicates sensorineural hearing loss at 2000Hz, but which is a” pseudo” sensorineural hearing loss. Maximal conductive hearing loss due to otosclerosis is about 50dB and if worse additional pathology such as ossicular dislocation should be suspected.
Stapedial reflexes are likely to be absent, indicating a fixed stapes. If present other conditions mimicking otosclerosis such as superior canal dehiscence syndrome (SCDS) should be ruled out.
Computed tomography (CT) or nowadays preferably cone beam CT scan of the temporal bone might help to confirm an otosclerotic focus. It may also help to exclude other conditions such as SCDS. The active otosclerotic focus may also be seen in magnetic resonance imaging (MRI) but it must be stressed that CT remains the imaging modality of choice.
Vestibular examination, video head impulse testing (VHIT), and videonystagmography (VNG) are indicated in cases where vertigo, dizziness, and disequilibrium are present.
Management of otosclerosis
Hearing loss can be managed with hearing aids, medication, surgery, or a combination. Every patient should be informed about all these possibilities. Patients befit from hearing aids as the speech discrimination of patients with otosclerosis is usually good.
Medical treatment with oral sodium fluoride aims to stabilize otosclerosis. It is a long-term treatment and otosclerosis may relapse after treatment is discontinued. Side effects of fluoride include arthritis, skin rash, and gastric irritation.
The most common surgical procedure performed nowadays is a stapedotomy. A small hole is created in the footplate of the stapes where after the prosthesis is placed from the incus into the stapedotomy hole. This re-establishes the conduction of sound. Mobilizing the fixed stapes is not effective and refixation always occurs. In some cases of cochlear otosclerosis with mixed or sensorineural hearing loss cochlear implantation (CI) may be the only option.