The Types of Deafness

Deafness is a term used to describe complete or partial loss or absence of the ability to hear. In this article, we will explore the types of deafness, as well as the diagnosis, treatment, and prevention of this affliction.

We will start by examining the three major types of deafness:

Conductive Deafness

Conductive deafness is the result of a defect in the outer or middle ear, which prevents normal transmission of sound. It may be present at birth as the result of an inherited defect, an abnormality in development, or an infection of the fetus in the womb. It may also be produced by an injury that perforates the eardrum or that breaks up the linkage of the three very tiny bones -- the hammer, the anvil, and the stirrup -- that normally transmit sound from the eardrum through the middle ear to the inner ear.

The process of hearing is complex, involving a multitude of bodily structures.

In normal hearing, sound causes vibrations of the eardrum. These
vibrations are transmitted to the cochlea by the chain of bones called the hammer, anvil,
and stirrup. The cochlea then sends the signals through the auditory nerve to the brain.

Inflammation of the middle ear, a condition known as otitis media, is another important cause of conductive deafness. Infection from an upper respiratory tract ailment, such as strep throat or the flu, can produce a buildup of pus in the middle ear so great that it ruptures the eardrum. Also, a plugged eustachian tube (the tube leading from the back of the throat to the ear) may trap fluid in the middle ear, creating temporary deafness.

Conductive deafness in the middle and later years is most often caused by otosclerosis. In this inherited condition, new spongy bone grows over the stirrup bone, preventing it from vibrating when sound travels to it through the hammer and anvil bones.

Sensorineural Deafness

Sensorineural deafness is a type of hearing loss that occurs because of damage to the structures of the inner ear, to the auditory nerve carrying sound messages to the brain, or to the hearing center of the brain itself. It can be caused by a head injury during birth, the effects of a rubella infection on the developing fetus, a skull fracture affecting the inner ear or the auditory nerve, fever, bacterial or viral infections (such as mumps or meningitis), syphilis, Meniere's disease, tumors, multiple sclerosis, a hemorrhage or blood clot in the inner ear, drug side effects, normal aging, prolonged or repeated exposure to intense noise, or edema (fluid buildup) caused by a thyroid deficiency.

Most sensorineural deafness is not nerve deafness. It is usually sensory deafness, caused by defects in the structure of the inner ear, especially in the fluid-filled cochlea. The cochlea contains sensory cells that convert sound waves into electrical impulses, which can then be transmitted via the auditory nerve to the brain.

Mixed Deafness

Mixed deafness is a relatively common form of deafness. It is a combination of conductive and sensorineural deafness.

People with pure conductive deafness simply need louder volume to hear all sounds. Those with defects in the inner ear usually can hear low-pitched sounds more easily than high-pitched sounds, and some sounds may be distorted. When there is damage to the hearing center in the brain, the person may be able to hear sounds but has trouble recognizing them and understanding words. (This can also occur with the other types of deafness.)

In the next section, we will explain the diagnosis, treatment, and prevention of deafness.