Otosclerosis

What is Otosclerosis?

Otosclerosis is a complex condition affecting the temporal bone - the bone in the skull that contains ear. The term comes from "oto" (ear) and "sclerosis" (abnormal hardening). In this condition, the normal bone in the ear is replaced by spongy bone that later hardens. This process primarily affects the stapes, the smallest bone in your body, which is shaped like a stirrup and plays a crucial role in transmitting sound vibrations. The condition typically starts in an area just in front of the oval window of the inner ear, known as the fissula ante fenestram (the oval window is one of two openings - called windows - in the cochlea). From there, it can spread to other parts of the ear. This abnormal bone growth will eventually interfere with the movement of the stapes, leading to hearing problems.

How Common is Otosclerosis and Who Gets It? (Epidemiology of otosclerosis)

Otosclerosis is relatively common, affecting about 0.04% to 1% of white individuals (1). However, it's important to note that in reality this number could be even higher if we rely on histological markers (changes that are only visible under a microscope in a laboratory, not with a clinical exam) of otosclerosis, as they may be seen in up to 10% of this population group, even if they don't have symptoms. The condition is less common in other racial groups. For example, histological otosclerosis is found in about 1% of Black individuals and 5% of Asian individuals (1).Otosclerosis usually affects young adults, usually starting in their 30s or 40s. Women are more likely to develop otosclerosis than men, with a ratio of about 2:1 (1). This gender difference, along with the fact that symptoms can worsen during pregnancy, suggests that hormones may play a role in the development of the condition.

What Causes Otosclerosis?

The exact cause of otosclerosis isn't fully understood, but research suggests it's a complex mix of genetic and environmental factors (1):

1. Genetics: Many patients have a family history of otosclerosis. It appears to be inherited in an autosomal dominant pattern, meaning if one parent has the gene, there's a 50% chance of passing it to their child. However, not everyone who inherits the gene will develop symptoms, a phenomenon known as reduced penetrance.
   
   
2. Hormones: The higher prevalence in women and potential worsening during pregnancy suggest hormonal influences.
   
   
3. Viral Infections: Some research has found evidence of measles virus RNA in the stapes of otosclerosis patients, suggesting a possible viral trigger. This needs to be studies further.
   
   
4. Autoimmune Factors: Some researchers believe autoimmune reactions might contribute to otosclerosis. This also needs to be studies further.
   
   
5. Bone Metabolism: Genes associated with bone remodeling, including those for type I collagen, TGF-beta 1, and parathyroid hormone, have been implicated in otosclerosis.

How Does Otosclerosis Affect the Ear? (Pathophysiology)

Otosclerosis affects the ear through a process of abnormal bone remodeling. Here's a more detailed look at what happens:

1. Initial Stage: The process typically begins in the fissula ante fenestram, an area just in front of the oval window of the inner ear. Here, osteoclasts (cells that break down bone) become overactive, replacing normal bone with connective (fibrous) tissue.
   
   
2. Progression: As the condition progresses, this abnormal bone growth can spread to other areas, including the stapes (particularly its footplate, the bottom part), the cochlea, and other parts of the temporal bone.
   
   
3. Stapes Fixation: The most common and significant effect is on the stapes. The abnormal bone growth will cause the stapes to become fixed in place, unable to vibrate freely. This is what leads to conductive hearing loss. This fixation can be corrected by surgery by replacing the stapes bone with an implant (see below).
   
   
4. Cochlear Involvement: In some cases, the abnormal bone growth can spread to the cochlea, leading to a condition called cochlear otosclerosis. This will cause damage to the cochlea and lead to sensorineural hearing loss. This is not reversible by surgery. We don't really understand why some people develop cochlear otosclerosis and others don't.
   
   
5. Bone Changes: At a microscopic level, the affected areas show disorganized bone with increased numbers of osteocytes (bone cells) and widened marrow spaces filled with vascular and connective tissue. Over time, these spaces are replaced by dense, disorganized, sclerotic bone.

Understanding this process helps explain why otosclerosis can cause both conductive and sensorineural hearing loss, and why its effects can progress over time.

What Are the Symptoms of Otosclerosis?

The symptoms of otosclerosis typically develop gradually over time. The progression is usually slow, over years. Here's a more detailed look at what patients might experience:

1. Early Symptoms:
   • Gradually worsening hearing loss, often starting in one ear
   • Difficulty hearing low-pitched sounds or whispers
   • Tinnitus (ringing in the ears)
     
     
2. Progressive Symptoms:
   • Hearing loss in both ears (bilateral hearing loss occurs in about 25-30% of cases)
   • Speaking quietly because your own voice sounds loud to you (autophony)
     
     
3. Advanced Symptoms:

• • Severe hearing loss
  • Dizziness or balance problems in some cases, especially if there's cochlear involvement

It's important to note that the progression and severity of symptoms can vary greatly between individuals. Some people may have very slow progression over decades, while others may experience more rapid hearing loss,

What Does the Physical Exam Show?

When I examine patients with otosclerosis, here's what I typically find:

1. Normal Eardrum and External Ear: In most cases, the eardrum (tympanic membrane) looks completely normal. This can be confusing for patients who are experiencing significant hearing loss.
   
   
2. Schwartze Sign: In some cases, particularly in early or active otosclerosis, I might see a reddish tint behind the eardrum. This is called Schwartze's sign and is due to increased blood vessels in the promontory (base) of the cochlea. However, this sign is only present in about 10% of cases (1).
   
   
3. Tuning Fork Tests: These are simple tests we use in the office to distinguish between types of hearing loss. In otosclerosis:
   • The Weber test (placing a vibrating tuning fork on your forehead) typically lateralizes to the more affected ear (you will perceive the vibrations more in your affected ear, which might seem counterintuitive).
   • The Rinne test (placing the vibrating tuning fork on the bone behind your ear than placing it in front of your ear) is often negative, meaning bone conduction is better than air conduction.

Remember, the lack of visible abnormalities in most cases of otosclerosis is why it's often described as a condition where "the doctor sees nothing, and the patient hears nothing".

What Does the Hearing Test Show?

The audiogram (hearing test) for otosclerosis typically shows a characteristic pattern:

1. Conductive Hearing Loss: In early stages, there's usually a conductive hearing loss, meaning sound isn't being transmitted efficiently through the middle ear.
   
   
2. Low-Frequency Loss: The mechanical hearing loss often affects low frequencies first, then moves on to affect the other frequencies.
   
   
3. Air-Bone Gap: There's typically a gap between air conduction and bone conduction thresholds, especially at lower frequencies. You can read more about the air-bone gap here.
   
   
4. Carhart's Notch: This is a dip (loss) in bone conduction thresholds around 2000 Hz, which is characteristic of otosclerosis.
   
   
5. Mixed Hearing Loss: In advanced cases or with cochlear involvement, there may be a mixed hearing loss (both conductive and sensorineural).

To understand more about how to interpret an audiogram (hearing test), read this article.

How Do We Diagnose Otosclerosis?

Diagnosing otosclerosis involves several steps:

1. Patient History: We start by discussing the patient's symptoms, their progression, and any family history of hearing loss.
   
   
2. Physical Exam: As mentioned earlier, we examine the ear, looking for any visible signs (though these are often absent in otosclerosis).
   
   
3. Audiometry: This is crucial for diagnosing otosclerosis. We look for the characteristic patterns described above.
   
   
4. Tympanometry: This test measures the movement of the eardrum (read more here). In otosclerosis, it typically shows normal movement or a reduced compliance (stiff eardrum), which we call a type "As" curve.
   
   
5. Acoustic Reflexes: These are often absent or diminished in otosclerosis (this is a special audiometry test, find out more here).
   
   
6. CT Scan: While not always necessary, a high-resolution CT scan of the temporal bones can confirm the diagnosis and show the extent of the abnormal bone growth. It's particularly useful for surgical planning. In 25% of individuals with otosclerosis, the CT scan will be normal (3). This does not mean that patient does not have otosclerosis, it just means the scan was unable to detect it.

How Do We Treat Otosclerosis?

We have several options for treating otosclerosis, and the choice depends on the severity of the condition and the patient's preferences:

1. Hearing Aids: For mild to moderate cases, for patients who aren't candidates for surgery or for patients that do not want surgery, hearing aids can be very effective. Modern hearing aids are small, discreet, and can be precisely tuned to the patient's hearing loss.
   
   
2. Medical Treatment: Some doctors use sodium fluoride or bisphosphonates to try to slow the progression of otosclerosis. The idea is to stabilize the abnormal bone growth. However, there's some controversy about how effective these treatments are, and they're not universally used (1).
   
   
3. Surgery: The main surgical treatment for otosclerosis is called a stapedectomy. In this procedure, we remove all or part of the stapes bone and replace it with a prosthesis. This can dramatically improve hearing in over 90% of cases.
   
   
4. Cochlear Implants: In rare cases of very advanced otosclerosis with severe ear damage and hearing loss ("mixed" or "sensorineural" hearing loss, the kind that is not amenable to stapedectomy), only cochlear implants can restore hearing (even hearing aids won't work).
   

What's Involved in Stapedectomy Surgery?

I explain the procedure in more details in this article (coming soon).

Stapedectomy (or stapedotomy) is a microsurgical procedure that I perform regularly for patients with otosclerosis. The surgery is typically done under local anesthesia with sedation, although some centres do them under general anesthesia (not me). I perform them under local anesthesia so that at the end of the procedure, I can whisper words to my patient and confirm 1) that they hear correctly, and 2) that they do not experience dizziness. I use a microscope and delicate instruments to remove the stapes bone and insert a titanium prosthesis to connect the incus (the 2nd bone of the middle ear) to the oval window (the inner ear opening). The total duration of the procedure is about 45 minutes.

The success rate for this surgery is quite high:

• 90% of patients experiencing significant (complete or near-complete) hearing improvement.
• 9% of individuals will obtain a partial hearing recovery (meaning hearing gets better, but not it's full potential).
• 1% might experience hearing loss. This hearing loss can be mild, moderate or severe (to a point that even hearing aids would not work anymore).

Some of the other risks including taste disturbances, dizziness and infections.

What Happens After Surgery?

After stapedectomy, patients typically go home the same day (day surgery). I usually see patients back in the office about a week after surgery to check the ear and remove any packing. We'll do a hearing test about 10-12 weeks after surgery to assess the improvement.

It's important for patients to avoid heavy lifting, straining, or air travel for a few weeks after surgery (4-8 weeks depending on the surgeon) to allow proper healing.