How does fluid backup cause hearing impairment?

An ear infection may occur because the closure of the eustachian tube causes a vacuum to form in the middle ear that prevents the normal vibration of the ear drum. If this closure persists, the body tries to fill this vacuum. The normal air-containing cells of the mastoid bone change to mucus-making cells, causing a condition called serous otitis media, or fluid filling the middle ear.

The buildup of the pressurized pus in the middle ear causes earache, swelling, and redness. Since the eardrum cannot vibrate properly, you may notice hearing problems.  Sometimes the eardrum ruptures, and the pus drains out of the ear. But more commonly, the pus and mucus remain in the middle ear because of the swollen and inflamed eustachian tube, a condition called middle ear effusion or serous otitis media. After the acute infection has passed, the effusion remains and becomes chronic, lasting for weeks, months, or even years. This condition makes you subject to frequent recurrences of the acute infection and may cause difficulty in hearing.