The cochlear implant, How do they work? Who needs them? How have they evolved over the years?

The cochlea is a hollow, spiral-shaped bone found in the inner ear needed for auditory transduction. Sound waves are converted into electrical impulses that the brain can interpret as individual sound frequencies. The spiral shape of the cochlea allows for differing frequencies to stimulate specific areas along the spiral. Specific areas along the cochlea are stimulated by vibrations carried within a fluid known as endolymph found in the cochlear duct. The vibrations are then converted to electrical impulses in the cochlear duct through mechanical stimulation of hair cells within a particular structure known as the organ of Corti. The vestibulocochlear nerve carries these impulses from the cochlea to the brain for interpretation. If these hair cells (cilia) or the auditory nerve are damaged, the cochlear cannot perform its function, resulting in deafness. (1)

 

 

 

 

 

 

There are three types of hearing loss: conductive, sensorineural and mixed (element of both conductive and sensorineural). However, not all kinds result in the need for a cochlear implant. Conductive hearing loss is when sounds do not pass from your outer to the inner ear, this can be due to a blockage or a problem with the eardrum or bones in the ear. Some conductive hearing losses are permanent; others can be treated. Causes of sensorineural hearing loss are the damage of cilia and/or the auditory nerve. It can be congenital or acquired; the causes include ageing, illness, exposure to loud sounds, prematurity, and associations with some syndromes. You may need a hearing aid or a cochlear implant depending on the severity. However if your auditory nerve is not intact a cochlear implant won’t work as signals won’t be able to be transferred to the brain. (2)

The first cochlear implant was implanted by William House and John Doyle of Los Angeles, California, in 1961. It is a small electronic device that electrically stimulates the cochlear nerve. It consists of two parts, an external and internal part. The outer part sits behind the ear; the inner part is placed under the skin behind the ear during surgery. The outer part has a microphone that picks up on sounds; the audio processor then converts these sounds into digital information and is then transported through the coil to the implant. The implant then transmits electrical signals down a thin wire (the electrode) into the cochlear. The hearing nerve fibres pick up the signals and send them to the brain, which interprets them as sound, producing a hearing sensation. Although normal hearing isn’t restored, with appropriate therapy and practice, the improved hearing experience can increase awareness of sounds in the environment and better communication through easier lip reading and listening. It is important to understand a cochlear implant is different from a hearing aid. A hearing aid amplifies but may not significantly improve speech understanding. (3)

 

 

 

 

 

 

 

 

Cochlear implants have evolved dramatically over the years and stemmed from a researcher named Alessandro Volta around 1790, who placed metal rods in his ears and connected them to a 50-volt circuit as the first attempt to hear using electricity. During the depression of the 1930s, researchers found that putting a current near the ear can create auditory sensations before even reaching the inner ear. Also, scientists had learned more about how the cochlea works at this time.

In 1957 Djourno and Eyries brought the first stimulation of an acoustic nerve with an electrode which meant you could hear background noise. In the 60s, they made a significant advance when researchers learned that specific auditory nerves must be stimulated with electrodes in the cochlea to reproduce sound. Dr William House implanted three patients in 1961. All three found they could get some benefit from these implants. 1964 -1966, they placed an array of electrodes in cochleae. Researchers learned even more about the positioning of electrodes and the position results.

In 1984, the cochlear implant was no longer deemed experimental and was given the stamp of FDA approval for implantation in adults.4 The first bilateral implant implanted in an adult was in 1966. One wire goes into one inner ear and the other under the scalp from the single implant into the other ear. Allowing for bilateral hearing, so it is clear where sounds are coming from. Nowadays, implants are so advanced we can use Bluetooth to listen to music and take hands-free calls.

3) Cochlear Implant Surgery | Johns Hopkins Medicine 4) History of the Cochlear Implant (verywellhealth.com)