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How the hearing system works
This is a drawing of the different parts of the ear. The outer ear comprises the pinna (the outside flap of the ear) and the ear canal (external auditory canal). Sound waves are funneled down the ear canal to the middle ear, which consists of the tympanic membrane (eardrum), tympanic cavity (middle ear space), the Eustachian tube and the ossicles (three tiny bones in the middle ear space). The inner ear consists of the bony, fluid filled spaces of the snail shell-shaped cochlea (the sense organ of hearing) and the semicircular canals (the organs of balance). Sound waves move the eardrum and the ossicles like a chain, and the last ossicle moves back and forth into a small opening in the cochlea, creating a small wave that travels through the fluids of the cochlea. All along the cochlea are tiny sensory cells that respond to different frequencies of sound. Depending on the sound that is heard, the wave sent from the middle ear into the cochlea will reach the corresponding place along the cochlea, stimulating the little sensory cells in that area. When these cells are excited, they send electrical signals to the auditory nerve, which travel along the nerve to the brainstem and the brain, where these impulses are translated into information about sound, speech or the body’s position in space.
The mechanics of hearing are fascinating. Click here for a great animated YouTube video showing how the ear works.
The types of hearing loss
There are many types of hearing loss. The type of hearing loss your child may have depends on the part of the ear that is affected. The five types of hearing loss are conductive hearing loss, sensorineural hearing loss, mixed hearing loss, neural hearing loss and auditory processing disorder.
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Conductive hearing loss (CHL): Conductive hearing loss occurs when there is a problem with the outer or middle ear. CHL causes a weaker signal to travel to the normal inner ear resulting in hearing loss. The above graph shows a conductive hearing loss in the left ear. The air conduction results are the blue Xs. The bone conduction results are the blue >s. Note that the BC results are closer to the top of the graph! The hallmark of conductive hearing loss is normal hearing with bone conduction and hearing loss with air conduction. This occurs because bone conduction does not test the affected middle ear! Sometimes this type of hearing loss can be corrected or improved with surgery or medical intervention. Common causes of conductive hearing loss include:
Holes in the eardrum (perforations)
Fluid in the middle ear (otitis media)
Disconnection or stiffening of the middle ear bones
The stapes getting stuck in its opening to the cochlea (otosclerosis)
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Sensorineural hearing loss (SNHL): Sensorineural hearing loss occurs when there is a problem with the inner ear. In this case, the signal from the other and middle ear gets to the cochlea at the correct level, but because of damage to the sensory cells of the inner ear, the cochlea cannot stimulate the hearing nerve normally to get the signal to the brain. The above graph shows a high frequency sensorineural hearing loss in the left ear. Here, the AC and BC results are the same, because the cochlea is tested by AC and BC testing. This type of hearing loss is generally permanent and cannot be corrected or improved with surgery or medical intervention. Common causes of sensorineural hearing loss in children include:
Family history of childhood hearing loss
Different diseases or syndromes
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Mixed hearing loss (MHL): Mixed hearing loss is a combination of both conductive and sensorineural hearing loss and indicates a problem with the outer/middle ear and the inner ear. In this case, the middle ear will send a weaker signal to the inner ear through air conduction, and in combination with that, the inner ear will also be unable to stimulate the auditory nerve normally because of damage to the inner ear. Some of the bone conduction responses may be normal or close to normal, depending on what is affected in the middle ear. The above graph shows a mixed high frequency hearing loss in the left ear. In the lower frequencies, bone conduction is close to normal, but as the frequency increases, the bone conduction gets worse and matches up with the air conduction results.
Neural hearing loss (NHL): Neural hearing loss occurs when there is a problem with the nerve transmission of sound from the cochlea to the brain. In this case, the outer, middle and inner ears may respond normally, but because of damage to the auditory nerve or anywhere along the neural pathway to the brain, the signal that gets to the hearing part of the brain is altered. A neural hearing loss can have almost any type of audiogram (normal to profound), normal middle ear function, normal otoacoustic emissions, but will generally have decreased speech understanding scores, elevated or poor acoustic reflexes and abnormal auditory brainstem response testing.
Auditory Processing Disorder (APD): Think of auditory processing as what the brain does with what it hears. There are two parts to hearing: detection and comprehension. Some children can have normal detection of sounds (ie. a normal audiogram) yet still have trouble understanding speech, especially in background noise situations or if the speech signal isn’t ideal. These children can also have trouble following directions or staying organized. An auditory processing disorder will most likely have a normal audiogram, tympanograms, reflexes, OAEs and ABRs, but may show poorer than expected speech test scores. There are several special tests that can be performed to help diagnose an auditory processing disorder. Ask your audiologist if you have concerns about auditory processing disorder with your child.