The clinical examination of hearing loss should include differentiating between conductive and sensorineural hearing loss. There are different causes of sensorineural hearing loss, such as inner ear disease or damage to the cochlear nerve. In all cases, the perception of sound waves via both air and bone conduction is impaired. Sounds reaching the affected ear will therefore be perceived to be quieter than on the unaffected side.
Conductive hearing loss on the other hand is caused by diseases of the middle ear, such as otosclerosis or otitis media. Obstruction of the outer auditory canal, as seen in cerumen impaction, for example, can also lead to conductive hearing loss. In this case, air conduction of sound waves from middle to inner ear is impaired. Bone conduction, however, is unaffected and causes sounds on the affected side to appear louder.
There are two hypotheses that attempt to explain this phenomenon. First, it is assumed that impaired sound conduction causes upregulation of the inner ear, which makes it more sensitive towards stimuli received via bone conduction. It also causes these sounds to be perceived as louder. Second, if sound waves cannot easily reach the inner ear, they probably cannot exit it easily either. These trapped sound waves could therefore make patients perceive sounds as louder.
The Rinne and Weber tests are easy and quick methods for differentiating simple forms of conductive and sensorineural hearing loss. More complex illnesses, such as combined conductive and sensorineural hearing loss, often result in test results that are difficult to interpret.
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