A1 Conductive hearing loss: outer and middle ear

Topic

Sound from the environment enters the auditory system via the external auditory canal, which captures air vibrations and channels them into the inner ear without altering their physical nature. An obstruction in the canal—whether caused by earwax, a foreign object, or a malformation—impedes the sound's progress, causing a portion of the signal to be lost before it reaches the cochlea.

At the end of the canal lies the tympanic membrane, which separates the outer ear from the middle ear. This membrane receives sound pressure and converts it into a mechanical back-and-forth movement that follows the rhythm of the sound wave. The larger the surface area vibrating in unison, the greater the mechanical force transmitted. A perforation in the tympanic membrane reduces this surface area and allows some sound pressure to escape through the opening, thereby diminishing the force that reaches the next stage of the process.

The movement of the tympanic membrane is transmitted to the ossicular chain, which consists of the malleus, incus, and stapes. The stapes—the last of these three small bones—sits directly against the oval window and ultimately transmits the movement to the fluid within the cochlea. The ossicular chain acts as a lever, concentrating the force received across the large surface area of ​​the tympanic membrane onto the much smaller surface area of ​​the oval window. This difference in surface area creates the impedance matching necessary to efficiently transfer vibration from low-impedance air to high-impedance cochlear fluid; without this mechanism, a significant amount of energy would be reflected rather than moving the fluid.

The efficiency of this coupling depends on the mobility of the ossicular chain. In cases of otosclerosis, abnormal bone growth fixes the stapes against the oval window, effectively blocking transmission from that point onward, even though the other components remain intact. In the case of a middle ear effusion, however, the fluid accumulating around the ossicles introduces viscous resistance that dampens their movement—depending on the amount of fluid present—thereby reducing the transmission of vibration proportionally, even without the ossicular chain being physically fixed.

The admittance of the sound-conducting system represents the proportion of the original sound energy that successfully passes through the external auditory canal, the tympanic membrane, and the ossicular chain to reach the oval window. Admittance is highest when the canal is clear, the eardrum vibrates normally, and the ossicles move freely; it decreases in the presence of canal obstruction, a perforated eardrum, fixation caused by otosclerosis, or damping due to an effusion. As admittance drops, an increasing amount of energy is lost along the pathway, and only a small fraction reaches the oval window to displace the cochlear fluid.

In pure conductive hearing loss, this reduction in energy reaching the oval window does not imply damage to the cochlea. The cochlear mechanisms responsible for converting fluid movement into signals interpretable by the brain remain intact when the impairment is located exclusively in the external auditory canal, the tympanic membrane, or the ossicular chain. Consequently, the cochlea retains its normal processing capacity but receives less sound energy due to the reduced efficiency of the conducting system.

Therefore, treatment does not require repairing the cochlea itself, but rather restoring or compensating for the lost admittance. A hearing aid amplifies sound before it enters the external auditory canal; this ensures that, even when passing through a system with reduced admittance, sufficient energy reaches the cochlea for normal processing. Surgery addresses the cause of the loss directly: it can repair a perforated eardrum, mobilize or replace a stapes fixed by otosclerosis, or drain a middle ear effusion. Restoring the continuity and mobility of the conductive system re-establishes the efficient transmission of sound from the environment to the cochlea.

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