Conductive hearing loss (CHL) is a specific type of impairment where the sound energy is physically blocked or impeded before it reaches the inner ear. Unlike nerve-related hearing loss, this condition stems from a mechanical problem in the outer or middle ear structures. This obstruction prevents sound vibrations from being efficiently transmitted to the cochlea, which is the sensory organ that converts these vibrations into electrical signals for the brain.
How Sound Transmission Fails
Sound waves first travel through the external auditory canal, which is the passage leading to the middle ear. These waves strike the eardrum, or tympanic membrane, causing it to vibrate like a drumhead. This vibration is then amplified and passed along a chain of three tiny bones, the ossicles—the malleus, incus, and stapes—located within the air-filled middle ear cavity.
The last bone in this chain, the stapes, presses against a membrane-covered opening called the oval window, which leads into the fluid-filled inner ear. Conductive hearing loss occurs when a physical barrier or structural problem interferes with this precise mechanical process. Any obstruction in the external canal, stiffness of the eardrum, or difficulty in the movement of the ossicles can reduce the amount of sound energy reaching the oval window. This failure of energy transfer means that even with a healthy inner ear, the auditory signal is weak or distorted.
Causes Originating in the Outer Ear
The most common cause of conductive hearing loss originating in the outer ear is impaction of cerumen, or ear wax. Cerumen, or ear wax, is a natural substance that helps clean and protect the ear canal, but excessive accumulation can completely block the passage, preventing sound waves from reaching the eardrum. This blockage results in an immediate and often reversible muffling of sound.
Another frequent cause is the presence of a foreign body lodged in the external auditory canal, which is especially common in children. Infections of the ear canal, known as otitis externa or “swimmer’s ear,” can also cause temporary hearing loss. Severe cases of otitis externa lead to swelling of the canal walls, physically narrowing the passage and thereby impeding sound transmission. These outer ear issues typically resolve once the obstruction is removed or the infection is treated with medication.
Causes Originating in the Middle Ear
The middle ear is an air-filled space behind the eardrum, and issues here often involve the accumulation of fluid. Otitis media, or a middle ear infection, causes inflammation and the buildup of pus or fluid, which is a major cause of conductive hearing loss, particularly in children. This fluid dampens the vibrations of the eardrum and the ossicles, preventing the efficient transfer of sound energy.
When fluid persists in the middle ear without an active infection, the condition is called Otitis Media with Effusion (OME), often referred to as “glue ear.” OME is the most frequent cause of acquired hearing loss in children and creates a sound barrier that the ear’s mechanical components struggle to overcome. Eustachian tube dysfunction often precedes fluid accumulation, as this tube regulates pressure and allows for drainage from the middle ear to the back of the throat. When the Eustachian tube is blocked, negative pressure can build up, leading to the suction of fluid into the middle ear space.
Chronic and Structural Middle Ear Issues
Some causes of conductive hearing loss involve chronic disease or permanent structural changes within the middle ear. A perforation, or hole, in the tympanic membrane disrupts the pressure dynamics necessary for the eardrum to vibrate effectively. Perforations can result from trauma, severe infection, or chronic pressure imbalances, significantly reducing hearing acuity.
Structural damage to the ossicular chain, such as a dislocation or erosion of the tiny bones, prevents the sound signal from being properly transferred to the inner ear. This ossicular discontinuity can occur due to head trauma, chronic middle ear disease, or a growth like a cholesteatoma. Otosclerosis is an inherited disorder where abnormal bone growth fixes the stapes bone in place at the oval window. This fixation prevents the stapes from moving and transmitting vibrations to the inner ear fluid, creating a mechanical barrier that often requires surgical intervention for correction.