The Middle Ear

The Tympanic Membrane is the first important structure in the Middle Ear. • • • • When children stick things in their ears the condition can last for a long time. The impact of a sustained conductive hearing loss on speech and language development in a young child could be considerable. Some children are born without a Pinna or an External Auditory Meatus. In the case of the Pinna, in the old days, long hair simply covered the loss. Today, in both cases, plastic surgery can do wonders for both appearances and hearing. The Middle Ear: The beginning of the Middle Ear is the Ear Drum (if this was CD 261), but seeng that this is a 300 level course we will call it the Tympanic Membrane. Some texts include this as part of the outer ear. But God confided in me that this is not the case. She said that it is really part of the middle ear system that actually increases the loudness of sound as it is conducted into the Inner Ear. The three Ossicles are the second important structure in the Middle Ear. • • Why should sound be amplified? It is because the Inner Ear, as we shall see, is filled with fluid. When sound waves travel from one medium (like air) to another (liquid), energy is lost. This is a phenomenon called impedance. We actually lose around 50 Db, which is the sound level of a good conversation. Without some means of recovering the loss, we would have to shout at each other to hear. This is really the major contribution of the middle ear--to amplify sound. • • The mechanism that does this is the Tympanic membrane and the three tiny bones (Malleus, Incus & Stapes). Naturally, because this is a 300 level course, we don't refer to them as bones but as ossicles. As a point of interest, these are the smallest bones in your body! All three would fit on your fingernail. They originated, phylogenetically, from jaw bones. Hence, we have fewer bones in our jaw than species in lower phylums that have jaw bones. Using principles of leverage the Middle ear amplifies the sound signal. • • • The amplification mechanism works on the same principle as my car jack. Leverage is the name of the game. First of all, the Tympanic membrane, which initially receives the sound has a larger surface area than the third ossicle (Stapes) which imparts the signal to the Inner Ear. This acts to concentrate the energy at this point. Also the special arrangement of the three ossicles, which form a bridge between the tympanic membrane and the middle ear, also provides leverage. The Tympanic membrane itself has a convex shape toward the outside, which acts to gather and concentrate the energy of the sound as it is transmitted to the first ossicle (Malleus) which is attached to the membrane. • • In addition to the Tympanic membrane and the three ossicles, the Middle Ear includes some other important structures. One is the Middle Ear Cavity which houses the three ossicles. This is a cavity filled with air. Below is a Diagram from the text of the Middle Ear. • You can see three ossicles of the middle ear. The major contribution of the Middle Ear is the amplification of sound. • • • • • The Middle Ear Cavity sits opposite the external auditory meatus separated by the Tympanic membrane. Incidentally, the bone surrounding this cavity is very porous like a sponge. God didn't tell me why but I suspect is helps to make the skull less heavy. There are some serious draw-backs to this as you will shortly see. One problem with the Middle Ear is that the air pressures on both sides of the tympanic membrane are continually changing. On the outside, changes in weather and/or changes in altitude cause the Tympanic membrane to be either sucked out or pushed in. Air on the inside (middle ear cavity) also is continually being absorbed into the body causing a partial vacuum which sucks the tympanic membrane in. This can be very painful as anyone with an earache can attest. Nature has provided a mechanism (the Eustachian Tube) to equalize the pressure in the middle ear cavity. The Eustachian Tube equalizes the air pressure in the Middle Ear Cavity. • The Eustachian Tube extends from the middle ear cavity to the back of the naso-pharyngeal port (where the adenoids are). • • • • One of the problems with adenoids, if they become swollen, is that they can block the Eustachian tube and hinder the air equalization process. The tubes are usually collapsed. But the muscles that close the soft palate during eating, also open the tubes as part of the same process. That is why when you chew gum while taking off in an airplane,the tubes are open and the air is continually equalized in the middle ear cavity. In the dark ages (60 years ago) it was a popular trick, in order to get attention at a party, to force cigarette smoke from the mouth up through the Eustachian Tubes and out the ears to make smoke rings. Today, that would be considered pathetic, I'm happy to say. The Eustachian Tube is also a passage way for germs from the nose to the Middle Ear cavity. • • • There is a down side to the Eustachian Tube. In adults, it is sloped downward (no pun intended) from the ear to the mouth. But in very young children, whose faces have not yet developed downward, it is almost parallel. Children in addition, have immune systems that are not as well developed as adults, but like many professors, they are continually putting their hands in their mouths (in my case it's usually my foot). This is a perfect formula for the development of a nose cold. If you want to see a dozen or more, go to any kindergarten class. The problem is that the germs causing the cold have direct access to the Middle Ear through the Eustachian tubes. • Aside from the serious pain, the ears may fill up with fluid which hampers the movement of the tympanic membrane and ossicles. A Middle Ear infection can cause a Conductive hearing called Otitis Media. • • • • • The constriction of the movement of the Tympanic Membrand and/or ossicles frequently causes a socially significant conductive hearing loss. By socially significant, I mean it interferes with social interaction, and for children it retards the development of language. An infection in the middle ear is called Otitis Media. If it persists, the fluid in the middle ear cavity can become laden with dead white corpuscles. It then becomes viscous, and interferes with hearing even more severely. It can actually erode the ossicles and break into the inner ear which would be potententally a major disaster. To make matters worse, the germs become lodged in the porous bone surrounding the middle ear cavity. Here, conditions for germ survival are paramount. A Middle Ear infection can last for months and retard language development in very young children. • • • • • • Long after the nose cold has abated the middle ear infection will continue, fueled by the germs multiplying and dropping down from the surrounding porous bone. It can last for a week or for months. For young children the impact on language and speech development can be considerable. Swift diagnosis by an audiologist and/or an otolaryngologist and medical treatment is warranted. Children with a history of colds and ear infections should have their hearing monitored frequently to insure that the infection has not returned and lodged in the middle ear. As with most conductive losses, medical remediation will return the hearing to where it was before the infection. Incidentally, middle ear infection accounts for the highest incidence of socially significant hearing loss in the United States today. Otosclerosis is a conductive hearing loss caused by plague on the ossicles. • • • • Another type of conductive hearing loss that occurs more in girls than boys around the early twenties is otosclerosis. This is a condition in which a sort of plaque covers the ossicles to the point where they become immobilized. This can create a severe conductive hearing loss. If it is not rectified, a person would probably have to learn speech reading and/or sign language to communicate. Medical intervention can, fortunately, usually restore hearing to its former level. A doctor will simply peel back the eardrum (excuse me, Tympanic membrane) and insert a probe to jostle the ossicles. This will free them and restore hearing. If otosclerosis is not treated, it can cause complications which will make treatment more complex.