Conductive hearing loss (CHL) is a type of hearing impairment that occurs when sound waves are unable to efficiently travel through the outer ear, tympanic membrane (eardrum), or middle ear structures such as the ossicles.
This blockage or dysfunction prevents sound from being effectively conducted to the inner ear, resulting in reduced hearing ability. Common causes include ear infections, fluid in the middle ear, earwax buildup, damage to the eardrum, or abnormalities in the ossicles.
CHL can occur alone or alongside sensorineural hearing loss, in which case it is classified as mixed hearing loss. Depending on the underlying cause, conductive hearing loss is often treatable and sometimes reversible through medical interventions, such as medication, surgery, or assistive devices like hearing aids. However, chronic or permanent cases may require long-term management to improve hearing and communication abilities.
Signs and symptoms
Conductive hearing loss makes all sounds seem faint or muffled. The hearing loss is usually worse in lower frequencies. Congenital conductive hearing loss is 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. A 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 nasopharynx. Earwax is a very common cause of a conductive hearing loss, which may present suddenly when the wax blocks sound from getting through the external ear canal to the middle and inner ear.
Causes
Common causes of conductive hearing loss include:
External ear
- 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).
- Ear canal stenosis & atresia can exist independently or may result from congenital malformations of the auricle such as microtia or anotia.
- Acquired stenosis (narrowing) of the external auditory canal following surgery or radiotherapy
Middle ear
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. 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.
Inner ear
The third window effect is caused by:
- Superior canal dehiscence – which may require surgical correction
- Enlarged vestibular aqueduct
- Labyrinthine fistula
Diagnosis
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.
Otoscopy
Examination of the external ear canal and eardrum is important and helps identify problems located in the outer ear up to the tympanic membrane.
Differential testing
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 than 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:
{| cellspacing=0 cellpadding=10 |
|bgcolor=#cccccc align=center|Criterion
|bgcolor=#eeeeee align=center|Sensorineural hearing loss
|bgcolor=#eeeeee align=center|Conductive hearing loss
|-
|bgcolor=#eeeeee|Anatomical site
|Inner ear, cranial nerve VIII, or central processing centers
|Middle ear (ossicular chain), tympanic membrane, or external ear
|-
|bgcolor=#eeeeee|Weber test
|Sound localizes to the normal ear
|Sound localizes to the affected ear (ear with conductive loss)
|-
|bgcolor=#eeeeee|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
Tympanometry, or acoustic impedance testing, is a simple objective test of the ability of the middle ear to transmit sound waves from the outer ear to the middle ear and to the inner ear. This test is usually abnormal with conductive hearing loss. A type B tympanogram reveals a flat response, due to fluid in the middle ear (otitis media) or an eardrum perforation. A type C tympanogram indicates negative middle ear pressure, which is commonly seen in eustachian tube dysfunction. 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.