Conductive hearing loss
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|Conductive hearing loss|
|Anatomy of the human ear.|
|Classification and external resources|
Conductive hearing loss occurs when there is a problem conducting sound waves anywhere along the route through the outer ear, tympanic membrane (eardrum), or middle ear (ossicles). This type of hearing loss may occur in conjunction with sensorineural hearing loss (mixed hearing loss) or alone.
Common causes of conductive hearing loss include:
- Cerumen (earwax) or foreign body in the external auditory canal
- Otitis externa, infection or irritation of the outer ear
- Exostoses, abnormal growth of bone within the ear canal
- Tumor of the ear canal
- Congenital stenosis or atresia of the external auditory canal (narrow or blocked ear canal).
- Acquired stenosis (narrowing) of the external auditory canal following surgery or radiotherapy
Fluid accumulation is the most common cause of conductive hearing loss in the middle ear, especially in children. Major causes are ear infections or conditions that block the eustachian tube, such as allergies or tumors. Blocking of the eustachian tube leads to decreased pressure in the middle ear relative to the external ear, and this causes decreased motion of both the ossicles and the tympanic membrane.
- acute or serous otitis media
- otitis media with effusion or 'glue ear'
- Perforated eardrum
- Chronic suppurative otitis media (CSOM)
- Otosclerosis, abnormal growth of bone in or near the middle ear
- middle ear tumour
- ossicular discontinuity as a consequence of infection or temporal bone trauma
- Congenital malformation of the ossicles. This can be an isolated phenomenon or can occur as part of a syndrome where development of the 1st and 2nd branchial arches is seen such as in Goldenhar syndrome, Treacher Collins syndrome, branchio-oto-renal syndrome etc.
- Barotrauma unequal air pressures in the external and middle ear. This can temporarily occur, for example, by the environmental pressure changes as when shifting altitude, or inside a train going into a tunnel. It is managed by any of various methods of ear clearing manoeuvres to equalize the pressures, like swallowing, yawning, or the Valsalva manoeuvre. More severe barotrauma can lead to middle ear fluid or even permanent sensorineural hearing loss.
Third window effect caused by:
Conductive hearing loss makes all sounds seem faint or muffled. The hearing loss is worse in low frequencies. Congenital conductive hearing loss is usually identified through newborn hearing screening or may be identified because the baby has microtia or other facial abnormalities. Conductive hearing loss developing during childhood is usually due to otitis media with effusion and may present with speech and language delay or difficulty hearing. Later onset of conductive hearing loss may have an obvious cause such as an ear infection, trauma or upper respiratory tract infection or may have an insidious onset related to chronic middle ear disease, otosclerosis or a tumour of the naso-pharynx. Earwax is a very common cause of a conductive hearing loss which may present suddenly when water gets behind the wax and this blocks the ear canal.
Diagnosis requires a detailed history, local examination of the ear, nose, throat and neck, and detailed hearing tests. In children a more detailed examination may be required if the hearing loss is congenital.
Examination of the external ear canal and ear drum is important and may identify the problem.
For basic screening, a conductive hearing loss can be identified using the Rinne test with a 256 Hz tuning fork. The Rinne test, in which a patient is asked to say whether a vibrating tuning fork is heard more loudly adjacent to the ear canal (air conduction) or touching the bone behind the ear (bone conduction), is negative indicating that bone conduction is more effective that air conduction. A normal, or positive, result, is when air conduction is more effective than bone conduction. With a one-sided conductive component the combined use of both the Weber and Rinne tests is useful. If the Weber test is used, in which a vibrating tuning fork is touched to the midline of the forehead, the person will hear the sound more loudly in the affected ear because background noise does not mask the hearing on this side.
The following table compares sensorineural hearing loss to conductive:
|Criteria||Sensorineural hearing loss||Conductive hearing loss|
|Anatomical site||Inner ear, cranial nerve VIII, or central processing centers||Middle ear (ossicular chain), tympanic membrane, or external ear|
|Weber test||Sound localizes to normal ear||Sound localizes to affected ear (ear with conductive loss)|
|Rinne test||Positive Rinne; air conduction > bone conduction (both air and bone conduction are decreased equally, but the difference between them is unchanged).||Negative Rinne; bone conduction > air conduction (bone/air gap)|
Tympanometry, or acoustic immitance testing, is a simple objective test of the ability of the middle ear to transmit sound waves across it. This test is usually abnormal with conductive hearing loss.
Pure tone audiometry, a standardized hearing test over a set of frequencies from 250Hz to 8000Hz, may be conducted by a medical doctor or audiologist or audiometrist, with the result plotted separately for each ear on an audiogram. The shape of the plot reveals the degree and nature of hearing loss, distinguishing conductive hearing loss from other kinds of hearing loss.
Most causes of conductive hearing loss can be identified by examination but if it is important to image the bones of the middle ear or inner ear then a CT scan is required. CT scan is useful in cases of congenital conductive hearing loss, chronic suppurative otitis media or cholesteatoma, ossicular damage or discontinuity, otosclerosis and third window dehiscence. Specific MRI scans can be used to identify cholesteatoma.
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Management falls into three modalities: surgical treatment, pharmaceutical treatment, and supportive, depending on the nature and location of the specific cause.
In cases of infection, antibiotics or antifungal medications are an option. Some conditions are amenable to surgical intervention such as middle ear fluid, cholesteatoma, otosclerosis. If conductive hearing loss is due to head trauma, surgical repair is an option. If absence or deformation of ear structures cannot be corrected, or if the patient declines surgery, hearing aids which amplify sounds are a possible treatment option. Bone conduction hearing aids are useful as these deliver sound directly, through bone, to the cochlea or organ of hearing bypassing the pathology. These can be on a soft or hard headband or can be inserted surgically, a bone anchored hearing aid, of which there are several types. Conventional air conduction hearing aids can also be used.
- "Hearing Loss". HealthCentral. Retrieved 8 June 2013.
- Ruben, Robert J. (April 2007). "Hearing Loss and Deafness". The Merck Manual. Retrieved 8 June 2013.
- Page 152 in:Rex S. Haberman (2004). Middle Ear and Mastoid Surgery. New York: Thieme Medical Pub. ISBN 1-58890-173-4.
- "Types, Causes and Treatment". Hearing Loss Association of America. Retrieved 8 June 2013.