|Figure 1. Cross section of ear. Otosclerosis involves the small bones of the middle ear, the malleus (2), the incus (3) and the stapes (4), as well as the bone that surrounds the inner ear, which is called the otic capsule.|
Otosclerosis is a disease of the bones of the middle ear and the bones of the inner ear. The bones of the middle ear are labeled the malleus, incus and stapes (2-4) in figure 1, and are also known in aggregate as the "ossicles". The ossicles become knit together into an immovable mass, and do not transmit sound as well as when they are more flexible. An illustration of one form of this called "stapes fixation" is shown in figure 2. Otosclerosis can also affect the other ossicles (malleus and incus) and the otic capsule -- the bone that surrounds the inner ear.
|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.|
Only about 0.3% of the population has otosclerosis (Donaldson and Snyder, 1993), and the percentage is going downward. Although hearing loss is usually diagnosed in early adult life, the prevalence increases almost 7-fold with age (Niedermeyer et al, 2001). At autopsy, pathologic otosclerosis is very common (2.5 to 10% of temporal bones).
Hearing loss due to otosclerosis usually begins between the ages of 11-30. The hearing loss 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 by surgical procedures called stapedectomy and stapedotomy. VEMPs are usually absent in conductive hearing loss too.
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 usually worth trying, they also may be ineffective.
When otosclerosis involves both the small bones and the cochlea, a "mixed" type hearing loss occurs. This is often found in patients with "far advanced otosclerosis".
Sometime patients with mixed hearing loss are not diagnosed because surgical exploration is needed to be sure in some patients. As hearing is usually easily improved with hearing aids, there may simply be a decision to just leave things alone rather than to pursue a diagnosis of otosclerosis with surgical exploration. This decision, to avoid surgery given that there is little to gain, seems quite reasonable to us.
|Advanced otosclerosis -- mixed hearing loss in both ears. Post stapes surgery R.|
There are also occasional patients with a 2K notch, seen on the bone conduction audiogram. This notch in the bone conduction audiogram is attributed to the "resonance frequency of the middle ear". This is caused the "Carhart notch" (Carhart, 1971). This notch is attributed to decreased mobility of the ossicular chain at 1700 Hz, which changes the resonant frequency of the ossicular chain (Tonndorf, 1971)
Tinnitus is common in otosclerosis.
Vertigo is not so common, but some say that it appears in as many as 30%.
NATURAL COURSE OF OTOSCLEROSIS
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 but rather there may be a "sensory" type hearing loss. The amount of sensory type hearing loss is correlated with decreased bone minearl density of the cochlea (Guneri et al, 1996). The sensory type hearing loss is attributed to leakage of enzymes from bone into the inner ear (Grayeli et al, 2003). It is this process that is thought to possibly be inhibited by medical treatment (sodium fluoride) -- see following.
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. Sensorineural hearing loss typically progresses at a rate of 1 dB/year (Sakihara and Parving, 1999).
Later on, a conductive pattern of hearing loss appears. The conductive type of otosclerosis usually progresses up to a maximum in the 30's. After this, it rarely progresses. Dizziness and 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. Women are affected twice as often as men. People of African-American descent only rarely have otosclerosis -- it is usually a condition found in persons of Caucasian or Asian descent.
Pregnancy often has an adverse effect on otosclerosis. Otosclerosis is often discovered during or just after pregnancy. The effect of hormone supplements post-menopause is unknown -- presumably it is not a good effect however. Usually both ears are affected, although in about 10-15% of patients, hearing loss occurs on one side only. This may be mediated by increased activity of the renin-angiotension system (Imauchi et al, 2008). Renin blockers (tekturna -- Aliskiren) are available, as well as many angiotension convertase inhibitors (ACE), but to our knowledge, no one has attempted their use to block worsening of otosclerosis in pregnancy.
Most feel that otosclerosis is an inherited, an autosomal dominant disease with variable penetrance. This means that you have a 50-50% chance of getting the gene for otosclerosis if one parent has it, but that not everyone with the gene develop symptoms. There are some differences in opinion about this - -as about 100% of monozygotic twins match up with respect to otosclerosis (Fowler, 1966). Many genes have been reported, suggesting that the disease is genetically heterogeneous. This would be consistent with the observation that it is of variable penetrance, as one would expect a single gene to be more consistent in behavior.
An association between the COL1A1 gene, related to osteogenesis imperfecta, and otosclerosis has been demonstrated by McKenna (McKenna et al, 1998). The relevance of this observation is presently unclear.
Variants of otosclerosis exist in which there is a mutation in the NOG gene (Brown et al, 2003), as well as numerous other genes on linkage studies. Linkage studies appear to represent types of otosclerosis that do not represent the usual clinical pattern for the disease, and actually may be "red herrings", -- basically an artifact of the way that genetic research is performed and accepted for publication.
As there is also 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 (e.g. measles) for it to be expressed and hearing loss to occur (McGuirt et al, 1998; Karosi, Konya et al. 2004). Nuclear inclusions similar to paramyxoviral nucleocapsids have been observed in otosclerosis osteoclasts (McKenna et al, 1986; Chole and McKenna, 2001). Measles virus RNA has been found in the stapes of otosclerotic persons (Potocka-Baklazek et al, 2014).
Blue-grey sclera (i.e. "white" of eye) in individual with both osteogenesis imperfecta and otosclerosis
The gene that predisposes to otosclerosis may be similar to the gene that causes osteogenesis imperfecta -- a generalized bone disease (McKenna et al, 2002), that is generally transmitted in an autosomal dominant fashion. Persons with osteogenesis imperfecta often develop conductive hearing loss, and are treated similarly as are patients with otosclerosis (van der Rijt and Cremers, 2003). Their eyes sometimes have a blue-gray cast, called "blue sclera". This is not a bright blue -- see above. The genes that confer susceptibilty to otosclerosis may also provide some protection against otitis media (Manolidis et al, 2003).
Some feel that chronic measles infection in bone predispose patients to otosclerosis. Viral materials can be found in osteoblasts in otosclerotic lesions (Nadol, 1998) and in the stapes footplates of persons with otosclerosis (Karosi, Konya et al. 2004; Karosi, Konya et al. 2005). On the other hand, about 40% of stapes from otosclerosis patients are negative for measles (Karosi et al, 2007), and it is also unclear why measles should affect the temporal bone, but not affect other bones in the body.
Cases that are virus positive also are positive for TNF-alpha, suggesting a possible link to autoimmune inner ear disease. (Karosi, Konya et al. 2005). To our knowledge, nobody has yet attempted to apply this finding with anti-TNF drugs such as entanercept.
Pathologically, otosclerosis occurs only in human temporal bones, and is considered to be a disorder of new bone formation. The statistics here are rather amazing. Histologic otosclerosis, meaning that it is found only on section but has no symptoms, is found in one of every 2.5 to 10% of whites in the United States (Altman et al, 1967; Declau et al, 2001). Clinical otosclerosis, with involvement of hearing, occurs in one in every 10 patients with histologic otosclerosis (Nadol, 1998). This results in the conclusion that 0.25% to 1% of the population should exhibit clinical otosclerosis. The author's clinical experience suggests that this estimate from papers about 20 years ago may be an order of magnitude too high, as practically, otosclerosis is a rare diagnosis and getting rarer. The prevalence of otosclerosis may be dropping due to measles vaccinations (see above).
In otosclerosis, the most commonly affected portion of the bone around the inner ear (otic capsule) is the anterior oval window (about 96%). It can also involve the round window niche (30%), the internal auditory canal, and occasionally ossicles other than the stapes (Schuknecht and Kirchner 1974; Schucknecht, 1993). Otosclerosis is thought to begin with otospongiosis, which is a localized softening of the normally very hard bone of the otic capsule (Shambough 1971). There appear to be three stages of otosclerosis -resorptive osteoclastic stages with signs of inflammation, followed by an osteoblastic stage involving immature bone, followed by mature bone formation. Recent reports (e.g. Brown et al, 2019) suggest that lucency (i.e. thinning) of the "fissula ante fenestram" is the most specific sign on Xray of otosclerosis. So we have this odd situation where otosclerosis is thought to be too much bone, but the diagnosis is made by too little bone.
Bone in the otic capsule is different from bone elsewhere as it undergoes very little remodeling -- less than 2% per year, compared with 10%/year in long bones (Sorensen, 1994; Frisch et al, 2015). Bone turnover varies between the bone in the periphery (about 10%/year) compared to the labyrinth (about 0.1%/year) (Frisch et al, 1998). The difference in the rate of turnover is thought to be mediated by osteoprotogerin (OPG). OPH is found in high concentration in the perilymph (Zehnder et al, 2005), and thus lack of OPG might be thought of as a possible explanation for the the otospongiosis part of otosclerosis.
It is generally accepted that otosclerosis is "dying out" -- there are less and less patients operated every year. It is also conjectured that this is due to a combination of good "case finding", and another factor -- perhaps measles vaccinations, that has reduced the "input" side.
DIAGNOSIS OF OTOSCLEROSIS
Disorders that can be mistaken for otosclerosis (a disease) include disorders of the inner ear bones (ossicles), and fluid within the middle ear.
Examples of disorders of the inner ear bones other than otosclersosis include ossicular chain disruption and ossicular fixation. These are not diseases but mechanical disorders of the small bones in the middle ear.
In congenital ossicular fixation, the normal articulation of the small ossicular bones does not occur. The stapes can be fused to the otic capsule. The malleus can be fixed at the anteior nuchal ligament, or the head can be adherent to the medial-most portion of the ear canal.
Tympanosclerosis -- scar tissue can case the inner ear bones to knit together. Tympanosclerosis is very common.
Temporal bone fracture can result in scarring and eventual fixation.
Diagnosis of otosclerosis is usually made by a combination of family history and a progressive conductive hearing loss. Hearing tests may initially show a sensory pattern and later show the typical conductive loss pattern. Acoustic reflexes may eventually be inverted or absent.
There is an odd variant of otosclerosis called "cochlear otosclerosis". Some clinical papers (e.g. Lippy and Berenholz) suggest that this condition can be diagnosed with a flat sensorineural hearing loss, good speech discrimination, absent acoustic reflexes, and a family history. To us, this clinical picture would seem rather difficult to distinguish from any other type of sensorineural hearing loss. As the treatments for cochlear otosclerosis vary from doing nothing to an unproven medical treatment (sodium fluoride), we would favor simply diagnosing such patients as having sensorineural hearing loss.
The otologic clinician should confirm audiological testing suggesting a conductive hearing loss with bedside examination. The reason for this is that mistakes can be made, and when there is a significant hearing loss, inexperienced audiologists may omit or use masking incorrectly. It is best to confirm the conductive hearing loss both with the tuning fork test (512 hz) -- both Weber and Rinne, and with the VEMP test. VEMP's should be absent in a conductive hearing loss but present in sensorineural hearing loss. Rarely (about 10%) there is a redness of the promentory of the tympanic membrane. This is called Schwartz sign -- it is not a reliable sign and should not be relied upon (Lippy and Berenholz, 2008). In our practice, we see redness to the promentory in all sorts of random people -- usually those who have recently q-tipped their ears, or those who flew in from out of state, and we think that Schwartz's sign is a useless physical sign.
|Absent VEMP responses in patient above (on left side) with advanced otosclerosis. VEMPS are almost always absent in conductive hearing loss. The other patient also had no VEMPs on either side.|
Tympanometry can show stiffening of the ossicular chain. We have not found this to be useful and in fact we find tymps to be almost universally normal (see tymps in figure towards top of this page).
|Inverted reflexes (i.e. upward) in left ear, and absent reflexes in right ear. This is the same patient as above (right) where the Right ear was operated, and the left ear unoperated, but both clearly had stapedial otosclerosis.|
Acoustic reflexes are very useful in otosclerosis, as they show a characteristic "inversion" pattern. They are far more useful than tymps (which are nearly normal)
The temporal bone CT scan is both nonspecific and insensitive (in our opinion of course). We have noticed extremely discordant differences in readings offered by certain otologists and radiologists, in which otologists routinely see otosclerosis and radiologists never see otosclerosis. This experience, however, differs from that reported by others (Wycherly et al, 2010). Whether or not temporal bone CT is sensitive to otosclerosis, a reason for doing temporal bone CT anyway, at least in persons with a conductive hearing loss, is to detect patients with superior canal dehiscence, as they can also show a conductive hearing loss (Mikulec et al, 2004). The better way to detect SCD though is to screen with a VEMP first as VEMPs are present in SCD but absent in true conductive hearing loss. Note that temporal bone CT's require considerable radiation exposure. Note also that up to 10% of the normal population has histologic otosclerosis, and thus reading otosclerosis on a temporal bone CT would be statistically likely to generate an immense # of false positives. Cone-beam tomography may provide a lower-radiation route to diagnosis (Liktor, 2013).
VEMP testing can also be used to diagnose superior canal dehiscence and distinguish otosclerosis from SCD. As mentioned above, VEMP's should be absent in conductive hearing loss, but present in sensorineural hearing loss.
The symptom of paracusis Willisii, where the patient perceives speech better in a noisy background, is said to be frequently present in otosclerosis as well as other causes of conductive hearing loss. It is said to be due to the tendency of people to speak louder in noisy environments. This being said, it is still difficult to see the logic and we ourselves have never seen a patient who has volunteered this observation. Another possible explanation is that persons with sensorineural hearing loss have great problems hearing in noise, perhaps because of loss of outer hair cells, but this is simply not the case in conductive hearing loss.
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 (Causse et al, 1982). There are also many papers reporting hydrops in otosclerosis, and thus an overlap with Meniere's is a feasible explanation. 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 (? measles ?) cause as otosclerosis.
This subject is covered in depth here. There are four treatment options: