Hearing impairment comes from different biologic causes. Most commonly, the ear is the affected part of the body.
Conductive hearing loss occurs when sound is not conducted properly through the outer ear, middle ear, or both. It is generally a mild to moderate impairment, because sound can still be detected by the inner ear.
More severe impairments can occur, particularly in Otosclerosis. Generally, with pure conductive hearing loss, the quality of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard.
Conductive hearing loss has a variety of causes:
- Ear canal obstruction
- Middle ear abnormalities:
- Tympanic membrane
- Inner ear abnormalities:
- Superior canal dehiscence syndrome
II. Sensorineural hearing loss
Sensorineural hearing loss is due to insensitivity of the inner ear, the cochlea, or to impairment of function in the auditory nervous system. It can be mild, moderate, severe, or profound, to the point of total deafness.
The great majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve, the Vestibulocochlear nerve or the auditory portions of the brain.
In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech cannot be understood.
Most sensory hearing loss is due to poor hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and intrinsic abnormalities, like deafness genes.
Sensorineural hearing loss that results from abnormalities of the central auditory system in the brain is called Central Hearing Impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from a central cause is unusual.