Otosclerosis primarily causes conductive hearing loss, meaning the problem lies in how sound vibrations travel through the middle ear. However, in advanced cases, it can also cause sensorineural hearing loss or a combination of both, known as mixed hearing loss. The type depends on where the abnormal bone growth occurs and how far the disease has progressed.
Why It Starts as Conductive Hearing Loss
The middle ear contains three tiny bones that form a chain, passing sound vibrations from the eardrum to the inner ear. The last bone in that chain, the stapes, sits in a small opening called the oval window. In otosclerosis, the normal dense bone surrounding the inner ear gets broken down and replaced by softer, spongy bone. This abnormal bone hardens over time and gradually locks the stapes in place, preventing it from vibrating freely.
When the stapes can’t move properly, sound vibrations can’t reach the inner ear efficiently. This is conductive hearing loss: the inner ear itself still works fine, but sound isn’t being delivered to it. In most cases, the abnormal bone growth stays limited to the area around the oval window, which is why conductive hearing loss is by far the most common presentation. The hearing loss tends to be worse for low-frequency sounds, so people often notice they have trouble hearing deep voices or whispered speech before they struggle with higher-pitched sounds.
When It Becomes Sensorineural
In some patients, the abnormal bone growth doesn’t stay confined to the stapes region. It spreads into the bone surrounding the cochlea, the spiral-shaped organ of the inner ear responsible for converting sound vibrations into nerve signals. This is called cochlear otosclerosis, and it causes sensorineural hearing loss, meaning the inner ear itself is damaged.
The damage happens through a specific chain of events. When the otosclerotic bone reaches the inner lining of the cochlea (the endosteum), it disrupts structures that maintain the chemical environment hair cells need to function. The spiral ligament and a tissue called the stria vascularis begin to atrophy, which reduces the electrical potential inside the cochlea. Without that electrical potential, the hair cells that detect sound can’t do their job properly. Studies have also found that otosclerotic bone near the round window causes more direct damage to the hair cells and nerve cells, while bone near the oval window tends to damage the spiral ligament more severely.
On a CT scan, cochlear otosclerosis shows a distinctive “double ring” pattern of demineralized bone around the cochlea, sometimes called the fourth ring of Valvassori. This imaging finding helps confirm the diagnosis when sensorineural hearing loss is suspected.
Mixed Hearing Loss in Advanced Cases
Many patients with cochlear otosclerosis don’t have purely sensorineural hearing loss. Because the disease severe enough to reach the cochlea almost always fixes the stapes as well, these patients typically have mixed hearing loss: a conductive component from the locked stapes plus a sensorineural component from cochlear damage. The conductive portion can potentially be corrected with surgery, but the sensorineural portion generally cannot.
How Otosclerosis Shows Up on a Hearing Test
An audiogram measures two things: air conduction (sound traveling through the ear canal and middle ear) and bone conduction (sound vibrating directly through the skull to the inner ear). In pure conductive hearing loss, bone conduction stays normal while air conduction drops. The gap between these two measurements is called the air-bone gap, and it’s a hallmark of otosclerosis.
Otosclerosis also produces a characteristic dip in the bone conduction reading, most commonly at 2,000 Hz, known as the Carhart notch. This dip isn’t caused by actual inner ear damage but rather by the mechanical effect of the fixed stapes on how the test measures bone conduction. In one large study of surgical cases, about 29% had a Carhart notch at 2 kHz, 19% at 500 Hz, and about 5% at 1 kHz. The notch typically disappears after successful surgery, confirming it was mechanical rather than sensorineural.
If both air and bone conduction scores are reduced and the gap between them narrows, that suggests mixed hearing loss and possible cochlear involvement.
Symptoms Beyond Hearing Loss
Hearing loss from otosclerosis usually begins between ages 15 and 45, most often in the early 20s, and progresses slowly. Unlike noise-induced hearing loss, which hits high frequencies first, otosclerosis tends to affect low-pitched sounds earlier. People often notice a few distinctive patterns. Many speak quietly because their own voice sounds unusually loud to them. Some find they can actually follow conversations better in noisy environments, likely because others raise their voices and speak at higher pitches that are easier to hear. Tinnitus, dizziness, and balance problems can also occur.
How Treatment Differs by Type
The type of hearing loss determines what treatment can accomplish. For conductive hearing loss caused by a fixed stapes, surgery (stapedectomy or stapedotomy) replaces the stapes with a tiny prosthesis, restoring the chain of vibration to the inner ear. Results are generally strong: in a large single-institution study of 468 operations, about 87% of patients achieved closure of the air-bone gap to within 20 dB, and air conduction scores improved by an average of 25 dB.
For patients with mixed hearing loss, surgery can correct the conductive portion but won’t reverse the sensorineural component. If the cochlear damage is extensive enough, a stapedectomy alone may not restore useful hearing, and hearing aids or cochlear implants become the more practical options. This is one reason early diagnosis matters: the conductive component is highly treatable, but once the disease reaches the cochlea, some hearing loss becomes permanent.
Genetics and Risk Factors
Otosclerosis runs in families. It follows an autosomal dominant inheritance pattern, meaning a single copy of the gene from one parent can cause the condition. However, penetrance is only about 80%, so not everyone who carries the gene develops symptoms. Researchers have mapped several genetic loci linked to hereditary otosclerosis, including one on chromosome 16 (designated OTSC4), where all 12 affected members of a studied family inherited the same genetic region. Some family members carried the mutation without ever developing hearing loss, illustrating why the condition can seem to skip generations. Genes in the cadherin protein family, which help cells stick together, have been identified as candidates in the affected regions.