Conductive hearing loss

Conductive hearing loss
Classification and external resources
Anatomy of the human ear.
ICD-10	H90.0-H90.2
ICD-9	389.0
DiseasesDB	3043
MeSH	D006314

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 or alone.

The Weber test, in which a tuning fork is touched to the midline of the forehead, localizes to the affected ear in people with this condition. The Rinne test, which tests air conduction vs. bone conduction is negative (abnormal result).

Causes of conductive hearing loss

External ear

Common

• Cerumen (earwax)
• Otitis externa (ear infection)

Uncommon

• Foreign body in the external auditory canal (not always)
• Exostoses
• Tumour of the ear canal
• Congenital atresia

Tympanic membrane

• Tympanic membrane perforation
• Membrane tension by different pressures in the external and middle ear.^[1] 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 maneuvers to equalize the pressures.

Middle ear

Common

Fluid accumulation is the most common cause of conductive hearing loss in the middle ear, especially in children.^[2] Major causes are ear infections or conditions that block the eustachian tube, such as allergies or tumors.^[2] Blocking of the eustachian tube leads to increased pressure in the middle ear relative to the external ear, and this causes decreased motion of both the ossicles and the tympanic membrane.^[1]

• Acute otitis media
• Serous otitis media

Uncommon

• Cholesteatoma
• Otosclerosis
• Middle ear tumour
• Temporal bone trauma

Inner ear

Common

Severe Otosclerosis, form of mechanical conductive hearing loss most commonly found in people who have been subjected to intense noise. Occurs when there is an obstruction in either the oval window and/or the round window. This type of hearing loss can usually be repaired by surgical opening of the blockage.

Uncommon

• Superior canal dehiscence syndrome

Differentiating conductive and sensorineuronal hearing loss

When a Weber test is carried out, sound localizes to the ear affected by the conductive loss. A Rinne test, in which air conduction is normally greater than bone conduction, is usually negative (abnormal – note unusual terminology here compared with other medical tests), and shows greater bone conduction than air conduction.

Table 1. A table comparing 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 inner 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)

References

1. ^ ^{a b} Page 152 in:Rex S. Haberman (2004). Middle Ear and Mastoid Surgery. New York: Thieme Medical Pub. ISBN 1-58890-173-4. 
2. ^ ^{a b} Merck manuals > Hearing Loss and Deafness Last full review/revision April 2007 by Robert J. Ruben