Normal bone remodeling occurs at a rate of 10% per year throughout skeletal regions; however, a normal otic capsule has very little bone remodeling—only 0.13% per year.2 In patients with otosclerosis, bone remodeling within the otic capsule is increased, leading to accumulation of bone deposits that damage audiologic structures and worsen normal sound transmission. The extent of aberrant bone remodeling in the otic capsule directly correlates to the abnormal audiologic findings.
Abnormal bone remodeling in otosclerosis occurs in three phases:
- The otospongiosis phase, which represents an increase in both osteoclast activity and microvascularity.4
- The transitional phase, which begins with deposits of spongy bone by osteoblasts in areas of previous bone reabsorption.4
- The otosclerotic phase, characterized by spongy bone deposits developing into dense bone that narrows the microcirculation previously developed in the otospongiosis phase.4
These aberrant lesions can occur in many regions in the following areas: anterior to oval window and stapes footplate (80%), round window (30%), pericochlear region (21%), and anterior segment of the internal auditory canal (19%).5
CAUSES AND RISK FACTORS
Genetic influences can contribute to otosclerosis; 60% of patients report a family history of the disease.4 Most researchers consider otosclerosis to be a condition of autosomal dominant inheritance with an incomplete penetration; although in 40% to 50% of patients, otosclerosis occurred spontaneously or with variable patterns of inheritance.4,6 Clinicians should consider otosclerosis as a cause of hearing loss in patients who report a family history of the disease.
Hormonal conditions such as puberty, pregnancy, and menopause may be associated with exacerbation of hearing loss in patients with preexisting otosclerosis.4 Researchers found estrogen receptors on otosclerotic cells, although the specific regulatory mechanism of these receptors is unknown.7 Lippy and colleagues compared pregnant to nonpregnant patients with otosclerosis and found no direct association between pregnancy and exacerbation of hearing loss.8 Although additional research is needed to identify the specific influence hormones may exhibit on hearing loss, clinicians should suspect preexisting otosclerosis in patients who develop hearing loss during times of increased hormonal production.
Measles exposure is considered a risk factor for developing otosclerosis. Recent studies found viral materials in the nucleic acid of the stapes footplates and antibodies to the measles virus in the inner ears of patients with otosclerosis.9,10 Paradoxically, Komune and colleagues found that a complete mRNA sequence of measles had not been isolated from any otic sample.11 The exact etiologic function of measles in the development or progression of otosclerosis is still unknown.
Inflammation secondary to inflammatory and regulatory cytokines has been implicated in the development of otosclerosis. Inflammatory cytokine and cytotoxic mediators are released from spongy bone deposits during the early stages of the disease.4 Tumor necrosis factor alfa, an inflammatory cytokine, has been found in otosclerotic bone.9 Research on this topic is in the nascent phase and no specific inflammatory or autoimmune condition has been found to directly cause otosclerosis.
HISTORY AND DIAGNOSTIC EXAMINATION
Patients with otosclerosis present with progressive hearing loss that is worse in lower tones and/or frequencies. For example, patients often report difficulty hearing male voices or vowel sounds in words. About 50% of patients also have tinnitus.4 Only 10% of patients report vertigo, which is not present unless otosclerosis has extended to the inner ear, affecting the semicircular canals responsible for balance.2 Otosclerosis is found bilaterally in 80% of patients; however, patients often present with unilateral involvement early in the disease.6
An otoscopic examination typically is normal, with the exception of an increased redness along the promontory of the tympanic membrane (Schwartz sign). The Schwartz sign is inconsistently found in patients with otosclerosis and is not necessary for diagnosis.12
Audiometric screenings are general assessments of hearing loss and can be performed quickly in any quiet clinical setting. Clinicians can perform a number of screenings that may aid in audiometric referral for patients with otosclerosis, including questionnaires, tuning fork tests, whisper-voice test, and audioscope (Table 1). A systematic review examined the accuracy of these commonly used screening tools in identifying hearing loss within a clinical setting. Researchers found the Hearing Handicap Inventory for the Elderly Screening Version, a commonly used questionnaire that quantifies hearing handicap, to accurately correlate to hearing loss verified on audiometric findings.13 Although the meta-analysis was limited in quality studies regarding tuning fork accuracies, researchers concluded tuning fork tests to be inaccurate screening tools in identifying hearing loss of any cause due to Rinne's inability to distinguish sensorineural hearing loss from normal hearing and Weber's inability to identify bilateral hearing loss.13 The whisper test and audioscope were found to have appropriate and similar diagnostic accuracy in identifying hearing loss.13 Recent studies also have evaluated a new screening tool, the uHear iPhone app by Unitron, and have found this app to be a useful screening tool for identifying hearing loss across a variety of age groups.14-16 Hearing screening should not take the place of formal audiometric testing in patients with suspected otosclerosis or other audiologic pathologies.
Audiograms, in addition to medical history and physical examination, have traditionally been used for diagnosis of otosclerosis.17 An audiogram measures air and bone conductions and interactions throughout various frequencies (Hz) at various loudness levels (dB). An audiogram that results in hearing thresholds greater than 25 dB is abnormal. Otosclerosis typically presents with low frequency conductive hearing loss (Figure 1).18 A loss of bone conduction at the frequency regions around 2,000 Hz (Carhart notch) historically has been considered an indicator of otosclerosis; however, recent research has found the Carhart notch cannot be used to confirm diagnosis.19
Otosclerosis progression can be monitored by an audiogram because the progression of the disease directly correlates to hearing loss. When the ossicles stiffen and the connection between the stapes and oval window begins to change, a low-frequency mild conductive loss (small air-bone gap) will occur (Figure 1).18 The air-bone gap is the difference between air and bone conduction; a value greater than 10 dB is considered abnormal. As the stapes footplate becomes fixed to the oval window, the conductive loss worsens (indicated by a widening air-bone gap) and begins to involve all frequencies.18 If cochlear lesions develop, as is the case in 10% of patients, high-frequency sensory loss results in a mixed sensorineural and conductive hearing loss pattern on the audiogram.2,18 Extensive cochlear progression will result in mixed hearing loss in all frequencies.
Tympanometry is the measure of acoustic energy transmission. Tympanograms often are normal in patients with otosclerosis. Only in extensive cases of otosclerosis may the patient's tympanogram demonstrate some flattening secondary to severe ossicular chain fixation.18
High-resolution CT is beginning to be used in diagnosis and surgical planning of otosclerosis due to improvements in technology allowing for identification of smaller bony lesions.17 High-resolution CT has high diagnostic sensitivity and specificity, and reveals variants in patient anatomy and severity of disease.17 Common findings of otosclerosis on a high-resolution CT include areas of increased bony radiolucency in the otic capsule around the anterior footplate, thickening of the stapes, and widening of the oval window.17 High-resolution CT also can reveal cochlear involvement by demonstrating a demineralized area outlining the cochlea (double-ring sign).17 The main disadvantage to the use of this test is its high cost.
Stapes surgery restores the mechanical transmission of sound through the middle ear, correcting conductive hearing loss. It does not correct sensorineural hearing loss secondary to otosclerotic extension into the cochlea. Stapes surgery is a minimally invasive one-day procedure performed under general anesthesia; more recently, some surgeons have begun to perform stapes surgery under local anesthesia.6 The two variations of the surgery are:
- Stapedectomy, in which the stapes footplate and the crura are removed and replaced with a prosthesis.
- Stapedotomy, in which a small hole is made in the central aspect of the stapes footplate for the prosthesis without the removal of the structure.
Indications for stapes surgery include conductive hearing loss, air-bone gap of at least 20 dB, speech discrimination score of 60% or greater, and good patient health.12 Contraindications include poor patient physical condition, fluctuating hearing loss with vertigo, tympanic membrane perforation, infection, and hearing loss of 70 dB or worse unless the patient has a speech discrimination score of 80% or better.12
Vincent and colleagues reviewed 3,050 stapedotomies and found the surgical procedure to be safe and successful in treating conductive hearing loss in 94.2% of patients.20 Surgical complications are rare but can include deafness, necrosis of the incus, tympanic membrane perforation, facial nerve injury, disturbance of taste, perilymph gusher, floating or subluxed stapes footplate, and vertigo. The surgical failure rate commonly results from prosthesis malposition or inappropriate prosthesis length.18
Due to the progressive nature of the disease, 10% to 20% of patients will require surgical revision.21 Who will develop disease progression or cochlear involvement cannot be predicted. Following stapes surgery, hearing loss can progress at variable and unpredictable rates.22 Redfors and colleagues looked at 30 years poststapedectomy data and found that 88% of patients had bilateral involvement and 66% of patients showed moderate to profound loss secondary to progressive development of sensorineural involvement.23
Hearing aids are an alternative for patients who are not candidates for stapes surgery or are in need of sensorineural hearing loss correction. Hearing aids amplify sound, transmitting greater energy through the stiffened ossicles and improving sound transmission into the inner ear. Patients with a hearing loss greater than 25 dB are candidates for hearing aids.24 Hearing aids can be customized to amplify only the frequencies that are needed based on findings from the patient's audiometry. As otosclerosis progresses, additional adjustments in amplification may be required. Hearing aid technology has improved greatly over the last few years—they can be used more easily with telephones, and some interact directly with smartphones and tablets. Federal Communications Commission rules require cell phone companies to make phones that are compatible with hearing aids and cochlear implants.25 Hearing aids can be very expensive and may require multiple visits to an audiologist for sizing and adjustment. Patients also may have increased irritation and infection of the ear canal.
Implantable hearing aids, such as middle ear implants and bone conduction implants, are now being used in patients with otosclerosis who do not tolerate traditional hearing aids.26 These implantable hearing aids, like traditional hearing aids, enhance the acoustic signal transmitted to the cochlea; however, the devices are technically very different (Table 2).
Middle ear implants amplify sound by mechanically vibrating the ossicles in which they are surgically affixed. These devices require ossicular chain motion, which is often limited in patients with otosclerosis due to bony deposits; therefore, middle ear implants should only be implanted at the time of stapes surgery or after stapes surgery.27 Research found similar improvements in hearing regardless of whether implantation occurred at the time of stapes surgery or after stapes surgery.27 Middle ear implants are indicated for sensorineural hearing loss and provide hearing improvement similar to traditional hearing aids.28
Bone conduction implants are indicated for patients with conductive losses or mixed hearing loss with minor sensorineural involvement. These devices bypass the outer and middle ear, are attached to the temporal bone, and transmit vibrational energy directly to the cochlea. Bone conduction implants can be implanted bilaterally but are typically implanted unilaterally because the vibration is often strong enough to stimulate the contralateral cochlea.29 Research conflicts on whether bone conduction implants are better than traditional hearing aids in correcting conductive losses.29 Bone conduction implants are expensive and patients should try a traditional hearing aid first.29
Cochlear implants do not amplify acoustic signals like hearing aids. These devices convert acoustic signals to electric signals that are transmitted via electrodes to the auditory nerve (Table 2). Bypassing the natural transmission of acoustic energy provides greater amplification in patients with sensorineural hearing loss.26 Cochlear implants pose some challenges in patients with otosclerosis. They may be more difficult to position surgically and patients may have an increased risk postoperatively of cochlear ossification and facial nerve stimulation.30,31 These factors may result in reduced functioning of the implant itself or require more frequent implant revisions or reimplantations.32 Lenarz and colleagues found that patients with otosclerosis and moderate-to-severe mixed hearing loss benefitted from cochlear implants; improved hearing was measured by audiometric testing.26
Although cochlear implants are beneficial for some patients, other research suggests that stapedotomy combined with hearing aids results in good outcomes in patients with severe mixed hearing loss.30 This approach is recommended as first-line treatment, before considering a cochlear implant because of the permanent nature of the implant surgery.30 In 2014, the FDA approved the first hybrid cochlear implant/hearing aid system for patients age 18 years and older.33 The hybrid system reduces the risk of intracochlear trauma due to implantation and increases the chances of preserving some residual hearing. Because of the built-in hearing aid, the hybrid system also can amplify low-frequency hearing. More research is needed to identify whether hybrid systems should be used as treatment before traditional cochlear implant surgery.
Pharmacological options are not considered mainstream treatment for otosclerosis; the efficacy of various treatments is still in question.34 Although sodium fluoride is the most commonly prescribed medication, evidence to support its use is limited and conflicting.34 Sodium fluoride acts as an antagonist to bone remodeling and osteoclast activation throughout the skeletal system.34 The adequate dosage of sodium fluoride required to arrest bone remodeling in the otic capsule has yet to be determined.34 Bisphosphonates and vitamin D also are being considered as possible future treatments for patients with otosclerosis; however, research is in an early phase.34
Otosclerosis is a progressive yet treatable form of hearing loss. Improvements in technology and research have paved the way for additional diagnostic techniques and advancement in treatments. Understanding of this complex disease leads to earlier diagnosis, referral, treatment, and improved patient education for those with otosclerosis.
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Keywords:Copyright © 2017 American Academy of Physician Assistants
otosclerosis; hearing loss; bone remodeling; audiometry; cochlear implant; otic capsule