Hearing impairment, deafness, or hearing loss refers to the inability to hear things, either totally or partially.
Symptoms may be mild, moderate, severe or profound. A patient with mild hearing impairment may have problems understanding speech, especially if there is a lot of noise around, while those with moderate deafness may need a hearing aid. Some people are severely deaf and depend on lip-reading when communicating with others.
People who are profoundly deaf can hear nothing at all. In order to communicate spontaneously and rapidly with people, they are totally reliant on lip-reading and/or sign language. People who are born deaf find lip-reading much harder to learn compared to those who became hearing impaired after their had learnt to communicate orally (with sounds).
Some diseases or circumstances that can cause deafness include:
- Chicken pox
- Sickle cell disease
- AIDS - offspring of mothers who had AIDS during pregnancy have a much higher risk of being deaf by the age of 16 years
- Lyme disease
- Diabetes - studies have shown that up to 40% of diabetes patients suffer from some kind of hearing loss
- Tuberculosis (TB), experts believe that the medication, streptomycin, used to treat TB may be the key risk factor
- Hypothyroidism - and underactive thyroid gland
- Some cancers
- Second hand smoke exposure can increase hearing loss risk in teenagers
Many people globally have untreated hearing loss. It is estimated that 23 million Americans live with untreated impaired hearing.
The difference between hearing loss and deafness
Hearing loss refers to a diminished ability to hear sounds like other people do, while deafness refers to the inability to understand speech through hearing even when sound is amplified. Profound deafness means the person cannot hear anything at all; they are unable to detect sound, even at the highest volume possible.
Degree of hearing impairment - a person's severity of hearing impairment is categorized by how much louder than "usual levels" sound volumes need to be set at before they can detect a sound.
Degree of deafness - any degree of deafness means the person cannot understand speech through hearing at any level of amplification. If a person is profoundly deaf, they cannot detect sounds at any volume. Some people define profoundly deaf and totally deaf in the same way, while others say totally deaf is the end of the hearing spectrum.
How do we hear things?
Sound waves enter the ear, go down the ear canal (auditory), and hit the eardrum, which vibrates. The vibrations from the eardrum pass to the three ossicles (bones called the malleus (hammer), incus (anvil) and stapes) in the middle ear. These occicles amplify the vibrations, which are then picked up by small hair-like cells in the cochlea; they move as the vibrations hit them, the movement data is sent through the auditory nerve to the brain. The brain processes the data, which we interpret as sound.
A diagram of the human ear
Three types of hearing loss
There are three different types of hearing loss:
1) Conductive hearing loss
This means that the vibrations are not passing through from the outer ear to the inner ear, specifically the cochlea. It can be due to an excessive build-up of earwax, glue ear, an ear infection with inflammation and fluid buildup, a perforated eardrum, or a malfunction of the ossicles (bones in the middle ear). Also, the eardrum may be defective.
Ear infections can leave scar tissue which damages the functioning of the ear drum.
The ossicles may be impaired due to infection, trauma, or their fusing together (ankylosis).
2) Sensorineural hearing loss
Hearing loss is caused by dysfunction of the inner ear, the cochlea, auditory nerve, or brain damage. Usually, this kind of hearing loss is due to damage of the hair cells in the cochlea. As humans get older, the hair cells lose some of their function, and our hearing gets worse. In Western Europe and North America it is estimated that over half of all people over 70 years of age have hearing impairment caused by degenerated hair cells in the cochlea.
Long-term exposure to loud noises, especially high frequency sounds, is another common reason for hair cell damage. Damaged hair cells cannot be replaced. Currently, research is looking into using stem cells to grow new ones.
Sensorineural total deafness may be due to birth defects, inner ear infections, or head trauma. If the ear drum and middle ear are functioning properly, patients may benefit from a cochlear implant - a thin electrode is inserted in the cochlea, it stimulates electricity through a tiny microprocessor which is placed under behind the ear, under the skin.
3) Mixed hearing loss
This is a combination of conductive and sensorineural hearing loss. Long-term ear infections can damage both the ear drum as well as the ossicles. Sometimes, surgical intervention may restore hearing, but it does not always work.
On the next page we look at prelingual deafness, post-lingual deafness and unilateral and bilateral deafness. We also discuss the symptoms of hearing impairment. On the final page we take a look at how deafness is diagnosed and the available treatments for deafness and hearing impairment. We also discuss sign language and lip-reading.