Hearing and hearing loss

In order to understand hearing loss, it helps to have a general understanding of the parts of the ear and how sound is transmitted to the brain. The ear is divided into three parts:
1. The outer ear
2. The middle ear
3. The inner ear

The middle ear is the cavity behind the eardrum (tympanic membrane). It is an air-filled space and holds the three little bones of hearing (the Ossicles). The bones are called the Incus, the Malleus, and the Stapes. They are so small that you can place all three of them on a five cent piece without touching, and they will not hang over the edge. The Malleus attaches to the eardrum on one end and the incus on the other end. The incus then attaches to the Stapies, which attaches to the Oval window. The Oval window is the division between the middle ear and the inner ear.

The inner ear is called the cochlea. This is a snail shaped cavity filled with three little tubes which are separated by membranous walls and contain fluid. The organ of hearing is housed inside the cochlea. One of the membranes is called the Basilar Membrane, which has lots of little hair cells attached to it. When stimulated, these hair cells communicate with the Auditory nerve (the 8th cranial nerve).

How We Hear
Sound is gathered up by the pinna (outer ear) and funnelled into the ear canal. These sound waves bounce off the eardrum and cause it to vibrate. The vibration of the sound causes the little bones to rock back and forth. The stapes rocks into the oval window, setting it into motion. The motion of the oval window causes a pressure wave to move through the fluid of the inner ear. As this wave passes under the basilar membrane, it causes it to rock up and down. As this happens, the hair cells also move. This movement of the cells results in the release of neurotransmitter, a chemical signal that stimulates the hearing nerve. This sends an electrical impulse which travels up the nerve to the brain, where it is perceived as sound.

Hearing Loss
There are three types of hearing loss: Conductive, Sensorineural, and Mixed.

1. Conductive Hearing Loss
This is a hearing loss that occurs when there is a problem in the Outer or the Middle ear. This problem “blocks” the path of the sound, which prevents it from travelling to the inner ear. The middle ear cannot conduct the sound water to the inner ear. This type of hearing loss is often medically treatable. This can be caused by several things including:

An ear infection with effusion (fluid in the middle ear space)
Failure for the ear canal to develop
A hole (perforation) in the ear drum
A congenital abnormality of the little bones in the ear canal
Fixation of the bones, causing them to not move properly

2. Sensorineural Hearing Loss
This type of hearing loss occurs when there is a problem in the cochlea that prevents the sound from getting from the middle ear to the brain. This can occur when there are damaged hair cells or supporting structures. It can also happen (although rarely) by damage to the auditory nerve. This type of hearing loss is permanent. Causes of Sensorineural hearing loss include:

Infections or viruses in pregnancy
Inherited hearing loss. (If it is recessive, then the child may have no other family members who are deaf)
Meningitis and mumps
Ototoxic drugs given for extreme illness
Sudden very loud sounds or prolonged exposure to loud sounds
Birth asphyxia
Hyperbilirubinemia (Severe Jaundice)
Failure for the 8th nerve to develop (very rare)

3. Mixed Hearing Loss
This occurs when there are two problems: one at the level of the outer or middle ear and one at the level of the inner ear or eighth nerve. An example of this would be if there is a congenital sensorineural hearing loss and a middle ear infection. In this case, once the fluid cleared, you would have only a sensorineural remaining hearing loss. Another example is where the ear canal fails to form, and the cochlea is not fully formed. Then you may have a permanent mixed hearing loss.

For more information on hearing see The Power of Hearing, How Do We Hear?, Hearing Age and Welcome to Holland.