In order for a person to hear, sound must first travel from the outer ear to the eardrum. The eardrum works with the small bones of the middle ear known as the hammer, anvil, and stirrup to amplify sound as it travels to the inner ear. The amplified sound passes through fluid in the cochlea, a spiral-shaped structure of the inner ear. The nerve cells of the cochlea have thousands of tiny hairs that convert sound vibrations into electrical signals that are sent to the brain. Different vibrations affect the tiny hairs in different ways and enable the brain to distinguish one sound from another.
Retrocochlear hearing loss is a hearing loss that occurs beyond the inner ear. The issue lies with the transmission from the auditory nerve to the brain. This type of hearing loss cannot be evaluated by clinical symptoms or traditional hearing tests. MRI and other types of imaging must be used to diagnose the secondary disorder affecting the auditory nerve. Imaging allows the physician to observe if there is a tumor present, degeneration of the sheath surrounding the nerve, or brain lesions affecting the hearing centers of the brain.
Auditory brainstem response is a second diagnostic tool. This records neural responses in the brainstem by measuring electrical activity on the scalp. This type of testing is considered to be the most objective because it does not require any behavioral responses from the patient.
The auditory brainstem implant (ABI) is an avenue for regaining some hearing ability by re-establishing a connection between the cochlea and the brain. ABI involves implanting a device that places an electrode on the cochlear nucleus, the first area of the brain stem that processes sound. From that point an array of 21 electrodes is distributed at points throughout the cochlear nucleus which will extract information from the nerve stimulation. This information is sent through the auditory nerve to the brain.