ANATOMY OF THE EAR - MICROTIA AND ATRESIA
The Outer Ear consists of the Pinna or external ear and the ear canal all the way up to the Tympanic membrane or eardrum. The most important function of the pinna in hearing is the capture and localization of the sound.
The middle ear is an air filled space or cavity about 1.3cm (one-half inch) across. The middle ear space houses three little bones called the malleus, incus and stapes. These bones conduct sound from the eardrum to the inner ear. You can see to the left how small our middle ear bones are. The bone pictured to the left is called the “stapes” and is actually one of the smallest bones in our body.
The inner ear consists of two structures: 1) The cochlea or hearing organ is shaped like a snail. The cochlea is fluid-filled and is lined by very tiny hair-like structures. When sound causes the eardrum and middle ear bones to vibrate, the last bone (stapes) moves in and out like the piston of a car. This causes a wave of fluid to move across the inner ear. This wave of fluid stimulates the hair-like structures. Through electrical impulses, the hair-like structures communicate with the hearing nerve that in turn communicates with our brain telling us that there is sound. 2) The semicircular canals help us sense our orientation and balance.
How does sound normally travel to the inner ear?
Sound is captured and localized by the pinna or external ear. This sound is then concentrated and directed into the ear canal until it strikes the eardrum. The eardrum then vibrates, causing vibration of the three middle ear bones. The last bone, called the stapes, then moves in and out into the inner ear. This causes a wave of fluid within the inner ear to stimulate the inner ear hair cells. These hair cells communicate with the hearing nerve via electrical impulses. The impulses from the hearing nerve tell our brain that we are hearing sound.
How can a microtia patient hear without an ear canal?
Luckily the inner ear and the outer ear form at a different time in-utero. As a result, even though in most cases when there is an absence of both the outer ear the ear canal, the inner ear is usually perfectly normal. This, of course, must be confirmed with a specialized hearing test. It is very frequently asked how sound can get to the inner ear on the microtic side. It is also commonly asked if the children are deaf without an outer ear and ear canal.
The answer is relatively simple. As long as the inner ear is normal, then sound does not need an outer ear nor an ear canal to reach the inner ear. Sound strikes the skull in any area such as the nose, teeth, jaw, etc… This in turn causes a very subtle vibration that reaches the inner ear. The inner ear than transmits a signal to the brain telling it that there is sound.
An easy way to reproduce what a child with microtia and atresia feels is by placing a finger in the ear canal. Sound cannot make it through the ear canal because it is blocked, but it can make it to the inner ear by vibrations of sound on the bone.
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