Conductive Hearing Loss

Conductive hearing loss is hearing impairment caused by a problem in the outer or middle ear that interferes with the mechanical transmission of sound to the cochlea. Because the inner ear and auditory nerve are intact, sound that does reach the cochlea is processed normally; the deficit is one of conduction rather than perception. Common causes include earwax impaction, middle-ear fluid or infection, tympanic-membrane perforation, and fixation of the ossicles as in otosclerosis.

Definition

Conductive hearing loss is a reduction in hearing resulting from impaired mechanical transmission of sound through the external or middle ear to the cochlea, with preserved cochlear and neural function, classically producing an air-bone gap on audiometry.

Scope

This topic covers conductive hearing loss as a clinical category defined by the site of the lesion (external auditory canal, tympanic membrane, or middle ear and ossicular chain) and by its characteristic findings on hearing assessment, including an air-bone gap. It treats the entity as a reference concept, describing causes and mechanisms; it does not provide diagnostic algorithms or treatment instructions.

Key concepts

• Air-bone gap
• Sound conduction through the outer and middle ear
• Ossicular chain (malleus, incus, stapes)
• Tympanic membrane perforation
• Middle-ear effusion
• Otosclerosis and ossicular fixation
• Cerumen impaction
• Tuning-fork tests (Rinne and Weber)

Mechanisms

Sound normally passes from the external auditory canal across the tympanic membrane and is amplified by the lever action and area ratio of the ossicular chain before entering the cochlea. Any process that attenuates this mechanical pathway produces conductive loss: obstruction of the canal (cerumen, foreign body), perforation or scarring of the tympanic membrane, fluid or pus in the middle-ear space (as in otitis media with effusion), or stiffening and fixation of the ossicles. Otosclerosis, an abnormal remodelling of the otic capsule bone that fixes the stapes footplate, is a characteristic cause in adults. The hallmark on audiometry is an air-bone gap, in which sound delivered by air conduction is more impaired than sound delivered directly to the cochlea by bone conduction.

Clinical relevance

Recognising the conductive pattern of hearing loss is part of how the auditory system is assessed and how the many causes affecting the outer and middle ear are conceptualised. This entry describes the category and its mechanisms for reference and education; it is not a basis for individual diagnosis or for selecting medical or surgical management.

Epidemiology

Conductive components contribute substantially to the global burden of hearing loss, much of it potentially reversible. Middle-ear effusion is extremely common in childhood and is a leading cause of conductive loss in children, while otosclerosis and chronic middle-ear disease are notable causes in adults. The Global Burden of Disease Study 2019 documented the large overall prevalence of hearing loss to which conductive causes contribute.

History

The separation of hearing loss into conductive and sensorineural types was made possible by tuning-fork tests in the nineteenth century and formalised with the development of pure-tone audiometry, which allowed air and bone conduction to be measured separately and the air-bone gap to be defined. This distinction remains the foundation of clinical hearing assessment.

Seminal works

• schilder-2016
• markou-2009
• rosenfeld-ome-2016

Frequently asked questions

What is an air-bone gap?

It is the difference on audiometry between hearing thresholds for sound delivered by air conduction (through the ear canal and middle ear) and by bone conduction (vibration applied to the skull, bypassing the middle ear). A gap indicates that the conductive pathway is impaired while the cochlea is relatively preserved, the signature of conductive hearing loss.

Is conductive hearing loss permanent?

Many causes, such as earwax or middle-ear fluid, are transient or reversible, while others, such as otosclerosis or chronic ossicular damage, are more persistent. The course depends on the underlying cause; this entry describes the category in general terms and is not individual medical advice.