There are many causes of hearing impairment, including age, disease and injury, as well as heredity, birth defect, environmental factors and even physical blockage. Each of these conditions interrupts the proper function of a portion of the ear, either the outer, middle or inner ear. Understanding the process of hearing makes understanding the causes of hearing impairment possible.
Sound is received by the outer ear, passing through the ear canal to the eardrum. Vibrations pass through the eardrum and are amplified by three bones called the ossicles, then pass to the cochlea, a shell-shaped organ filled with fluid and lined with tiny hairs. These vibrations travel through the fluid, moving the hairs, which transmit signals to the brain.
The primary causes of hearing impairment relating to the outer ear involve some sort of blockage. Earwax might block off the ear canal, impairing hearing. A foreign object might also cause a temporary loss of hearing. In fact, ear plugs work by deliberately blocking sounds.
When fluid collects behind the eardrum, the ossicles are unable to move properly. Most causes of hearing impairment affecting the middle ear cause this portion of the ear to fill. Infections, for instance, cause pus to collect, and otitis media causes similar conditions without infection.
Ostosclerosis can impair hearing by hampering the movement of the ossicles. When this hereditary disease is present, bone tissue is deposited on the ossicles. This deposit prevents the ossicles from properly conducting the vibrations to the inner ear. Other growths in the middle ear, such as tumors, will have a similar effect.
Many causes of hearing impairment affect the inner ear, the most delicate portion of the hearing process. The number of hairs in the cochlea diminishes with age, meaning that fewer impulses are sent to the brain, and the perception of sound is impaired. Hairs damaged by noisy environments or injury might also be responsible for hearing loss.
Fistula is a condition where fluid from the cochlea leaks out. Without a fluid medium to carry vibrations, sounds cannot be heard. Injury is usually responsible for fistula, but rapid air pressure change might also cause leaking.
Too much fluid in the cochlea can also impair hearing. When pressure in the inner ear is too high, a fluctuation in hearing loss might be experienced. Hearing impairment is often most severe in the lower register, such as the human voice. Tinnitus might also be caused by pressure on the inner ear.
Some conditions, such as infection, might interrupt more than one stage of the hearing process. Neural dysfunction, as opposed to any problem within the ear itself, might also prevent the brain from receiving signals from the inner ear. Causes of hearing impairment relating to neural problems include multiple sclerosis, acoustic neuroma and ototoxicity.