Hearing & Ear Anatomy
The Ear is divided into 3 parts: Outer Ear, Middle Ear, Inner Ear.
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 sounds travels 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 a child with microtia can 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.
There are 3 ways to manage the hearing loss in a child with microtia:
1) “Do nothing”
This is an option for a child born with unilateral (one-sided) microtia and atresia. Many years ago, this was a common option for several reasons. The first was the worry of risking damage to the facial nerve (the nerve that controls the movements of the face). With much advancement of CT scan technology and facial nerve monitoring during surgery, now it is much easier to evaluate the anatomy of the facial nerve, middle and inner ear. With this important knowledge, now surgeons can discuss whether a child is a good candidate, medium candidate or poor candidate for ear canal surgery. If a parent decides to give a child with unilateral microtia the option of waiting for hearing correction later, it is hard to argue that choice. There MUST be excellent communication between the surgeon and the parents.
“Doing nothing” is NOT a choice on a child with bilateral (both sides) microtia and atresia. These children have severe hearing loss on both sides and must be aided with a bone conduction hearing aid.
2) Atresioplasty (opening of the ear canal)
Atresioplasty is a very complex surgical procedure which entails using a specialized drill to burr through the bone to form an ear canal. Once the ear canal is drilled, a skin graft is then placed to line the ear canal.
The most common risk of the atresiaplasty is re-narrowing of the new ear canal. The most dangerous risk is actual damage to the facial nerve (although the risk should be low in experienced hands). Therefore, this procedure should only be performed by an experienced otologist.
This can be done in conjunction with the outer ear surgeries, depending on the technique used by the microtia surgeon. Questions that should be asked when seeking an atresioplasty surgeon:
1) How many ear canal surgeries have you performed?
2) What is your complication rate?
3) What kind of hearing results do your patients obtain?
3) Bone-Anchored Hearing Aid (BAHA)?
Another term for the BAHA is a Bone-Anchored Auditory Processor. This is a system that consists of a high-performance sound processor, which snaps on to a titanium implant located behind the microtia reconstructed ear. This implant completely integrates with the skull bone thanks to a process known as osseointegration.
The hearing processor captures the sound vibrations and directs them through the implant, where the body’s natural ability to conduct sound via bone carries the sound to the inner ear.
Osseointegration has been used for reconstructive surgery, dental implants and other hearing systems with great success.
Children with microtia who are less than 5 years of age and are not yet ready for an implant, can use the hearing processor through a Softband. This consists of a soft elastic headband with a plastic snap connector that holds the hearing processor in place.
The band is fit very easily with velcro and can be adjusted so the hearing processor is not always in the same position. This helps avoid discomfort and can be very useful when your child is in a car sear or stroller.
The Bone-Anchored Hearing Processor is worn by thousands of children with microtia and other ear deformities worldwide and is a well-proven, clinically tested and documented hearing care system
To see what a BAHA looks like and to see see how it works, please visit the “BAHA Section”.