Hearing impairment refers to partial or total loss of hearing. There are three types of hearing loss: conductive hearing loss, which occurs when there’s an obstruction of sound wave transmission; sensorineural hearing loss which occurs when the inner ear or auditory nerve is damaged; and mixed hearing loss which is a combination of the two.
Let’s start with the normal physiology of hearing. The ear is made up of three parts: the external, middle, and inner ear, all of which help process air vibrations as sound. The main role of the external and middle ear is to transfer and amplify sound, while the inner ear also plays a role in balance. Let’s start with the external ear, which is by far the most common anatomical spot to hang earrings from. The external ear is actually a complex structure made of the auricle, also called the pinna; and the external acoustic meatus. Now, at the end of the external acoustic meatus, there’s the thin, oval tympanic membrane, more commonly known as the eardrum, which separates the external ear from the middle ear. When sound waves hit the tympanic membrane, it vibrates and transfers the vibration to the middle ear.
The middle ear, also called the tympanic cavity, is a tiny chamber found in the petrous part of the temporal bone, and it houses the three auditory ossicles, called the malleus, incus and stapes, which are the three smallest bones in the body. These ossicles create a chain that connects the tympanic membrane to the oval window to transfer and amplify air vibrations into the inner ear to be processed as sound. Now, the inner ear contains the bony labyrinth, which contains cavities filled with perilymph, and the membranous labyrinth, which is made of sacs and ducts suspended in the bony labyrinth.
A part of the inner ear called the cochlea contains the sensory cells for hearing. Now, sound waves travel from the air, through the ear canal, and hit the tympanic membrane, causing it to vibrate. The vibrations are then transferred through bony ossicles to the oval window which makes waves in the perilymph. The waves travel through the fluid of the inner ear and are picked up by the sensory cells, which create nerve impulses that travel through the vestibulocochlear nerve, which is cranial nerve VIII, to the brain.
Now, the causes of hearing loss depend on the type. Conductive hearing loss is most commonly caused by an obstruction of the external canal with debris, wax, or foreign bodies. Other causes include perforation of the tympanic membrane, acute otitis media, a defect in the ossicles of the middle ear due to head trauma, and presbycusis, which is a fancy way of saying hearing loss caused by aging. Presbycusis typically begins with loss of high frequency sounds and eventually progresses to involve all frequencies. Conductive hearing loss can also be caused by otosclerosis, which is a genetic disorder of the middle ear that causes a bony overgrowth of the tissue surrounding the ossicles, impairing vibration transmission.
Sensorineural hearing loss, on the other hand, is caused by impaired function of the inner ear, cochlea, or the vestibulocochlear nerve. Sensorineural hearing loss can be caused by congenital disorders, in utero rubella infections, trauma of the inner ear, constant exposure to loud noise, acoustic neuromas, otosyphilis, and some medications such as aminoglycosides.
Conductive hearing loss is when obstructions in the external or middle ear cause problems transferring soundwaves, so they don’t reach the inner ear and the sensory cells. On the other hand, sensorineural hearing loss impairs the transfer of stimuli to the brain.
Now, the early signs of hearing loss are answering questions inappropriately or not responding when called. Other signs include straining to hear, cupping the hand around the ear, reading lips, and an increased sensitivity to noise. Conductive hearing loss is usually sudden and in this case the client speaks softly because their own voice seems loud. They also hear better in a noisy environment. With otosclerosis, the conductive hearing loss is slowly progressive and it’s usually bilateral. Clients with otosclerosis have a ringing or roaring type of constant tinnitus and they hear loud sounds in the ear when chewing. Sensorineural hearing loss is usually progressive and in this case, clients speak loudly and hear poorly in loud environments. It’s also more commonly associated with tinnitus and, occasionally, with dizziness.
Let’s move on to the diagnosis of hearing loss which is based on signs and symptoms, otoscopic examination, tuning fork tests such as the Rinne and Weber Tests, imaging studies such as X-ray, CT, MRI, and audiometry. The otoscopic examination is done in order to determine the patency of the external canal, identify lesions or excessive cerumen in the canal, and assess whether the tympanic membrane is intact or inflamed. In conductive hearing loss, the otoscopic examination may show evidence of obstruction or abnormalities in the tympanic membrane, while in sensorineural hearing loss, the external canal and the tympanic membrane are unaffected.
The Weber and Rinne tuning fork tests are useful screening tests that determine whether the hearing loss is conductive or sensorineural. With the Weber test, a tuning fork is struck and then placed on the vertex of the client’s head. A normal result is when the tuning fork is heard equally in both ears. In conductive hearing loss, the tuning fork will be heard louder in the affected ear, while in a sensorineural hearing loss, the tuning fork will be heard louder in the unaffected ear.
On the other hand, the Rinne test can only detect the presence of conductive hearing loss. First, a struck tuning fork is placed on the mastoid process and the client can hear the sound conducted through the mastoid bone. Under normal conditions, bone conduction is worse than air conduction, so once they stop hearing the sound from the tuning fork, move it immediately 1 cm away from the ear, and they should still hear the sound via air conduction; this is called a positive result. However, if there’s a problem with the middle ear, the client wouldn’t hear the tuning fork next to their ear and this is called a negative result.
Imaging studies can be done to assess some lesions that lead to hearing loss. So, skull X-rays can determine the bony involvement in otitis media, as well as the location of the otosclerotic lesions, while CT and MRI may be used to determine soft-tissue involvement and the presence and location of tumors. Audiometry can confirm the presence of hearing loss and quantify its severity.
Finally in infants, hearing loss can lead to delays in speech, language and cognitive development. Early intervention can decrease this delay, but it’s only effective if started before the age of 6 months. This is why often, newborns are screened for hearing impairment before they even leave the hospital.