Hearing Disorders Flashcards
Here you can see the anatomy of the normal ear which is traditionally divided into three sections, the outer, middle and inner ear.
How is sound transduced through the ear?
Sound waves propagate in the air, enter the external auditory canal and bounce off the tympanic membrane like a trampoline. Set into motion, the tympanic membrane moves the malleus, incus and stapes. Movement of the stapes footplate changes the pressure in the fluid-filled inner ear and that triggers a traveling wave in the basilar membrane of the cochlea.
Hearing can also occur by bone conduction when the sounding source, such as a vibrating tuning fork, is in contact with the bones of the skull resulting in their vibration. When the sound is transmitted through air or bone, the traveling wave in the basilar membrane moves from the base to the apex of the cochlea.
Inside the cochlea, hair cells with stereocilia are found in the organ of Corti, which rests on the basilar membrane. These hair cells are in contact with the tectorial membrane and are deformed by the traveling wave of sound.
High frequency tones maximally displace the basilar membrane near the base of the cochlea. As the frequency of the sound decreases, the point of maximal membrane displacement moves toward the cochlear apex. The distortion of the stereocilia on the hair cells causes depolarization of the cell and that results in increased electrical activity along the auditory nerve.
How big a problem is hearing loss?
According to World Health Organization estimates, almost 300 million people worldwide have moderate to profound hearing loss affecting both ears. Hearing loss affects all ages and presents special challenges in childhood. The problem will only get bigger as the population grows older.
If the transmission of sound through the air fails, hearing can still occur through bone conduction. This is called conductive hearing loss and its causes are multiple.
You can give yourself a conductive hearing loss by sticking your finger in your ear to block out sound. Try speaking and you will note that your voice lateralizes to the side of the occluded ear.
There are many causes of conductive hearing loss and those listed here are a small sample. It is remarkable how many patients are surprised by the improved hearing when ear wax is cleaned out of their ear canals. In children a peanut or other foreign object can lodge in the external auditory canal. Infection, inflammation and perforation of the tympanic membrane all cause loss of hearing conducted by air. In otosclerosis bony overgrowth of the stapes, at its point of attachment to the cochlea, causes the ossicles to stiffen and fail to transmit sound vibrations from the tympanic membrane.
What is sensorineural hearing loss?
Sensorineural hearing loss arises principally from damage to hair cells in the organ of Corti. This can occur following a very loud noise, viral infection, ototoxic medication, fractures of the temporal bone and Ménière’s disease.
It can be a congenital problem and commonly occurs with advancing age.
What is Presbyacusis?
refers to age-related hearing loss that is gradually progressive and involves mostly high frequency sounds.
More causes of sensorineural hearing loss
Drug induced ototoxicity occurs with aminoglycoside antibiotics such as gentamicin, but can also occur with Lasix, salicylates, quinine and cancer drugs such as cisplatin.
Sensorineural hearing loss occurs with acoustic neuroma and other tumors of the cerebellar pontine angle that compress the auditory nerve.
Theoretically hearing loss can also occur with stroke, demyelination, trauma or any other structural disease affecting the central auditory pathways. However, hearing loss is less often seen with disease of the brain parenchyma because the central auditory pathways cross over at many way-stations along their journey to the medial geniculate nucleus of the thalamus and ultimately to Heschl’s gyrus. This redundancy in neural circuitry helps to protect the individual from gross lateralizing hearing loss which is not the case for other sensory systems such as vision, pain and temperature. Occasionally one does find hearing loss in multiple sclerosis patients with brainstem plaques.
A person can have both conductive and sensorineural hearing loss, which is termed:
mixed hearing loss.
What causes mixed hearing loss?
This is due to pathology that can affect the middle and inner ear simultaneously. The causes include otosclerosis, temporal bone fractures, chronic middle ear infections (otitis media), middle ear tumors (cholesteatoma) and some inner ear malformations.
What is Meniere’s disease?
Ménière’s disease produces vertigo, fluctuating sensorineural hearing loss and tinnitus.
What causes Meniere’s disease?
Decreased resorption of endolymphatic fluid causes fluid build-up or hydrops with high pressure, bowing and subsequent rupture in the membranes separating the endolymph from the perilymph.
The two fluids have very different compositions and their mixture produces a sudden change in vestibular cochlear pressure and in their electrical firing properties.Attacks can last 30 minutes to several hours, until equilibrium is reestablished.
Disease progression with Meniere’s disease
In the early stages, the symptoms remit between attacks and patients may be symptom free for a year or more. With disease progression, the hearing loss persists and gradually worsens. Tinnitus may become continuous.
How in Meniere’s disease tx?
Treatment with a low salt diet and diuretics can be helpful. Vestibular sedatives including anticholinergics, antihistamines and benzodiazepines may be helpful when the patient is stricken with an attack.
Hearing loss can be quantified and further characterized with audiometry. The patient listens with each ear to sound at specific frequencies and specific intensities presented via air or bone conduction. Sound frequencies from 250 to 8000 Hertz are used and responses are measured in decibels. With hearing loss, the threshold for detecting a sound at a specific frequency rises, and it can rise markedly. This is an audiogram of the right ear in a patient with normal hearing. The vertical axis shows hearing thresholds in decibels. Note that as the threshold increases, the hearing worsens, hence the reason for the higher numbers to project downwards rather than up on the y axis.
The audiometric pattern of hearing loss can be very helpful in diagnosis. Conductive hearing loss typically shows a fairly equal threshold elevation for each frequency as shown in the this slide. However, early otosclerosis can cause fixation of the footplate of the stapes and result in a greater threshold elevation in the lower frequencies. With middle ear effusions, there may be a greater elevation of thresholds in the higher frequencies. The tympanogram measures the impedance of the middle ear to sound and can be useful for the identification of middle ear effusions in children.
Sensorineural hearing loss typically shows steadily rising threshold elevations with increasing frequencies. In other words, the higher the sound frequency, the worse the hearing. However, noise-induced hearing loss often shows a selectively greater loss at 4000 Hertz when compared to higher frequencies. This is illustrated on the next slide. Another exception is Ménière’s disease in its early stages. There the thresholds are elevated more in lower than in higher frequencies. In other words hearing is initially worse for sounds in the lower frequency range.
Here are a series of audiograms taken over the years in an individual with hearing impairment caused by noise. This might have involved chronic exposure to loud sounds in a machine shop. Note the disproportionate loss of hearing at 4000 Hertz especially early on.
Perforation of the tympanic membrane is common and usually a consequence of infection. Trauma, including barotrauma, can perforate the membrane as well. The degree of hearing loss depends on the size of the hole.
Otitis media with effusion is very common in children. The diagnosis can be difficult and tympanometry may help.
Growth of abnormal bone around the otic capsule leads to fixation of the stapes resulting in a conductive hearing loss. This is often accompanied by a progressive sensorineural component. The disorder typically starts in the 20s to 30s and is bilateral. The family history is often positive. Treatment is observation, hearing aid or stapedectomy.
The arrows point to a contrast enhancing lesion in the right cerebellar pontine angle. This most often turns out to be a vestibular schwannoma, formerly called an acoustic neuroma. Since its origin is from Schwann cells, the former designation of the tumor is preferable. Meningiomas also occur in this spot. However, the patients typically present with sensorineural hearing loss and not with vertigo. The compression of the eight cranial nerve is so slow and gradual that the brain has more than adequate time for central compensation when the vestibular input decreases on one side. By blocking visual fixation and provoking the semicircular canals, one can still detect the vestibular abnormality but that is a topic covered in a separate module on vertigo.
Here is another MRI of the posterior fossa showing a large contrast-enhancing tumor of the cerebellar pontine angle. The tumor is compressing the brainstem. Such a patient is likely to display deficits of neighboring cranial nerves, especially the facial motor nerve or cranial nerve 7. And, ipsilateral cerebellar dysfunction is likely due to compression of the cerebellar peduncles. Note that the 4th ventricle is partially obstructed. The patient is at risk for acute hydrocephalus due to obstruction of CSF flow at this level.
In conclusion, there are many causes for hearing loss. A thorough clinical assessment includes a good history, a careful otologic and neurologic exam, audiometry and appropriate imaging. A variety of treatments are available to improve hearing but they are beyond the scope of this talk. You should consult your local ENT doctor for advice.
