Otosclerosis is a bone disorder of the middle ear that leads to gradual hearing loss. It occurs when abnormal bone growth around the stapes bone prevents it from vibrating properly, disrupting the transmission of sound to the inner ear.
The condition typically affects adults between the ages of 15 and 45, with a higher prevalence in women and individuals of Caucasian descent. It is responsible for approximately 5 -10% of all cases of adult hearing loss. In some cases, otosclerosis may also affect the inner ear, leading to balance disturbances.
How the Ear Works
Image credit: National Institute on Deafness and Other Communication Disorders (NIDCD), NIH.
To understand how otosclerosis affects hearing, it’s important to understand how the ear functions. The ear is divided into three main parts: the outer ear, middle ear, and inner ear. Each part plays a specific role in the hearing process.
Sound waves enter through the outer ear and travel down the ear canal to reach the eardrum. Vibrations from the eardrum are transferred to the middle ear, which contains three small bones known as the ossicles: the malleus, incus, and stapes. These bones amplify and transmit sound vibrations to the inner ear.
The stapes, the smallest bone in the human body, is the final link in this chain. It pushes against the oval window, a membrane that leads into the fluid-filled cochlea of the inner ear. Movement of this fluid stimulates tiny sensory cells, which convert the mechanical vibrations into electrical signals. These signals are then transmitted to the brain via the auditory nerve, allowing us to perceive sound.
This process can be compared to a piano key mechanism. When functioning normally, the key moves freely and produces a distinct tone. However, if the key becomes stuck, pressing it yields no sound, regardless of the force applied.
Disruption at any point in this process can lead to hearing loss. In otosclerosis, the stapes become fixed due to abnormal bone growth, preventing it from moving freely. This interrupts the transmission of sound and leads to a type of hearing loss known as conductive hearing loss.
What is Otosclerosis?
Otosclerosis is a condition that affects the bones in the middle ear, most commonly the stapes. In a healthy ear, the stapes moves freely to pass sound vibrations into the inner ear. In otosclerosis, abnormal bone growth forms around the stapes, causing it to become stiff or fixed. This limits its movement and blocks sound from reaching the inner ear, leading to conductive hearing loss.
In some cases, the abnormal bone growth extends to the cochlea, the part of the inner ear that processes sound signals. This can result in sensorineural hearing loss or a combination of both types, known as mixed hearing loss.
As noted above, otosclerosis tends to affect adults between the ages of 15 and 45 and is more common in women, particularly during hormonal changes such as pregnancy. Genetic factors may play a role, and certain populations appear to be more predisposed to developing the condition. A possible link to viral infections such as measles has also been suggested, but evidence remains inconclusive.
The condition usually develops slowly. In rare cases where the inner ear is involved, people may also experience dizziness or balance issues in addition to hearing loss.
Characteristics of Otosclerosis
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Begins in the middle ear, often near the boundary between the stapes and the oval window
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Progresses gradually, typically over several years
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Often affects both ears, though one side may be more advanced
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Early stages may not cause any noticeable symptoms.
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Occasionally involves the inner ear structures, not just the middle ear
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Bone changes are not visible externally and do not cause pain or swelling
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Detected through audiometric tests and imaging, not physical examination
Symptoms of Otosclerosis
The most common symptom of otosclerosis is gradual hearing loss. It often starts in one ear and later affects both. People may notice that they have trouble hearing low-pitched sounds or soft voices, especially in quiet environments.
Other symptoms may include:
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Tinnitus – a ringing, buzzing, or roaring sound in the ears
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Difficulty hearing in noisy settings, even when others can
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Better hearing in loud environments than in quiet ones (a phenomenon called Paracusis of Willis)
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Dizziness or balance issues – rare, but possible if the inner ear is involved
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No pain or visible changes in or around the ear
Because symptoms develop slowly, they may be mistaken for other causes of hearing loss, such as aging or earwax buildup. A hearing test is usually needed to confirm the diagnosis.
Understanding the Audiogram
An audiogram is a chart used to measure how well someone hears various frequencies. It’s an essential tool for diagnosing hearing loss and can help distinguish between different types, such as conductive, sensorineural, or mixed hearing loss.
In otosclerosis, a classic finding is a gap between air and bone conduction thresholds, known as an air-bone gap. This happens because the stiffening of the stapes bone prevents normal movement, making it harder for sound to travel through the middle ear. As a result, air conduction scores (marked by X’s for the left ear and circles for the right ear) are poorer than bone conduction scores (shown with bracket symbols).
Another key indicator often in otosclerosis is Carhart’s notch, a dip in bone conduction around 2000 Hz. This notch does not reflect true inner ear damage but instead represents the mechanical effect of the immobilized stapes.
In the audiogram shown below:
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The right ear (circles) shows mild sensorineural hearing loss.
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The left ear (X’s) shows moderate mixed hearing loss, with a clear Carhart’s notch at 2 kHz.
This combination of findings is consistent with a diagnosis of otosclerosis.
Diagnosis and Treatment
An audiogram is often the first step in diagnosing otosclerosis. It typically shows a pattern of conductive hearing loss, with a noticeable dip in bone conduction around 2000 Hz. This dip is known as Carhart’s notch, a classic audiometric finding associated with stapes fixation. While it may look like sensorineural loss, it reflects a mechanical issue in the middle ear and often improves after successful stapes surgery.
Additional diagnostic tools may include:
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Tympanometry – to assess movement of the eardrum and middle ear pressure
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CT scan of the temporal bone – used when imaging is necessary to evaluate the extent of bone changes, especially before surgery
Once diagnosed, treatment depends on how much the hearing loss affects daily life. Options include:
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Hearing aids – commonly used to amplify sound and compensate for reduced hearing
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Stapedectomy or stapedotomy – surgical procedures that replace or bypass the fixed stapes bone with a prosthetic device to restore sound transmission
There are currently no known ways to prevent otosclerosis. However, with proper diagnosis and management, most individuals can maintain good communication and quality of life.
Otosclerosis is a progressive ear condition that leads to hearing loss and, in some cases, balance issues. Though it cannot be prevented, early diagnosis and treatment, through hearing aids, surgery, or cochlear implants, can significantly improve outcomes.
If you’re experiencing changes in your hearing, take the first step today. You can complete a free online hearing test in under 4 minutes and have your audiogram results emailed directly to you. It's quick, easy, and could help you catch important signs early.
