Although tumours and other disorders may impair the function of the auditory nerve or of those areas of the brain that
are responsible for our perception and recognition of sounds, most causes of deafness involve a defect in the ear and may
be divided into two types. A conductive hearing loss results from conditions that interfere with the transmission of airborne
sound through the external ear and the middle ear. These structures normally conduct sounds efficiently to the sensory apparatus
within the cochlea of the inner ear. As sound waves pass through the external ear to the eardrum, their amplitude is increased
as a result of the resonant properties of the outer ear and of the ear canal. Vibrations of the eardrum are then coupled by
the middle ear ossicles, the three smallest bones in the body, to the fluid-filled cochlea. Conditions that impair sound transmission
to the inner ear will therefore attenuate the incoming sound. Conductive hearing loss does not involve damage to the receptor
cells or any other nerve cells in the auditory pathway.
The second major type of deafness to originate from abnormalities of the ear is known as a sensorineural hearing loss,
which is sometimes referred to, albeit usually inaccurately, as ‘nerve deafness’. Sensorineural hearing loss most
often arises from defects in the cochlea itself, but also describes the hearing deficit that results from damage to the auditory
nerve.
Hearing sensitivity varies with sound frequency (or pitch) in a very characteristic way. Decreases in sensitivity at particular
frequencies can provide valuable clues as to the nature of the hearing loss. Moreover, although sounds are most effectively
conducted to the cochlea by the external and middle ear, they can also reach the cochlea by bone conduction if a vibrating
object, such as a tuning fork, is applied to the skull. When airborne sounds are attenuated by disorders of the external ear
or the middle ear, hearing by bone conduction should remain normal. On the other hand, sounds transmitted to the cochlea by
bone conduction will be heard less well in cases of sensorineural deafness.
Abnormalities of the external ear that result in conductive hearing loss include obstruction of the ear canal, most commonly
by wax, and inflammation of the skin surrounding the canal. The eardrum can be ruptured as a result of injury or middle ear
disease. Build up of viscous fluid within the normally air-filled middle ear cavity can impede the mobility of the drum and
the ossicles. This condition characterizes otitis media with effusion (glue ear) and is particularly prevalent in infants
and young children. Damage to the middle ear ossicles may follow head injury. And in otosclerosis, bone growth between the
third of these bones — the stapes or stirrup — and the membranous oval window that provides the entrance to the
cochlea will immobilize the ossicles and reduce the amount of sound energy that is transmitted. Most forms of conductive hearing
loss are characterized by more or less the same degree of deafness across all sound frequencies. In general, the causes of
conductive hearing loss can be remedied by drugs or by surgery. Otherwise, hearing aids provide an effective means of amplifying
the sound so that normal hearing can be restored.
