Function of the Middle Ear:
The middle ear works as an energy transducer and must be present to address the impedance mismatch that exists between air and water. Essentially, the middle ear acts as a middle man in the hearing process. The action of the middle ear is to convert acoustical energy from the outer ear into mechanical energy, which stimulates the structures of the inner ear. If no middle ear was present only, 0.01% of the acoustic wave energy traveling through air would enter the fluid filled cochlea and 99.99% would be reflected. In addition, the middle ear provides protection from loud sounds and is a pressure equalizer.
Anatomy of the Middle Ear:
The malleus (hammer), incus (anvil), and stapes (stir up) are three tiny bones found in the middle ear, known as the ossicles. The area ratio, lever action, membrane buckling of these bones contribute to the impedance matching mechanisms, and serve to increase sound pressure level by about 33 dB from the tympanic membrane to the oval window.
- Malleus: Shaped like a hammer and transmits the vibration from the eardrum and to the incus.
- Incus: Shaped like a anvil and transmits sound vibration from the malleus to the stapes.
- Stapes: Shaped like a stirrup and transmits sound vibration from the incus to the membrane of inner ear inside the oval window. The stapes is the smallest and lightest bone in the human body.
Middle ear muscles: stapedius and tensor tympani muscles. They are normally activated by loud sounds (>70 dB SPL) to protect the ear from damage.
The Eustachain Tube provides air and drainage for the middle ear. At rest, the tube is normally closed. If tube fails to open properly, it can lead to a build up of negative pressure in the middle ear functions as a pressure equalizer.
Disorder of the Middle Ear: Otitis Media (OM):
Otitis media is an inflammation of the middle ear, or a middle ear infection. It develops when viruses and bacteria get into the ear and multiply. Otitis Media occurs between the tympanic membrane and the inner ear, including the Eustachian tube.
Signs and Symptoms:
- Intense pain, caused by the build up of pressure behind the ear drum
- Drainage from the ear (May appear as blood, colorless fluid, pus, or as a dry crust on the outer portion of the ear after sleeping.)
- Hearing loss
- Disturbed sleep
- Some children show no symptoms
- 75% of young children will get an ear infection before their third birthday
- Age: Children between 6 months and 6 years old are prone to ear infections because of their shorter and straighter Eustachian tubes
- Upper respiratory infection
- Exposure to groups of children
- Exposure to smoke
- Allergies and asthma
The goals of treatment are to eliminate the infection and prevent hearing loss and other complications.
- Medication: Antibiotics to clear infection, if present. About 80% usually show improvement in 48-72 hours
- Myringotomy (“Tubes”): A small incision is made in the tympanic membrane and tube is inserted to drain fluid. This step is taken if the child is non-responsive to medication.
Impact on Hearing:
All children with Otitis Media will have a slight degree of hearing loss. The equivalent to a child wearing ear plugs. A child experiencing hearing loss due to an infection will hear muffled sounds and speak loudly. In noisy environments, such as a classroom, the child may misinterpret speech leading to a delay in learning important language skills. Conductive hearing loss due to an infection is temporary and once treated hearing will return to normal.
If otitis media is treated in a timely manor a child’s hearing will not be negatively impacted, and the infection and symptoms will go away quickly. Opting to not treat an infection can have adverse effects on a child’s learning development. The long lasting accumulation of fluid within the middle ear is a risk for both hearing and speech disorders and for the repeated development of infections.
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