[Adapted from BUPA's Health Information Team] Approximately 10% of New Zealanders are hard of hearing or deaf.
Sound and the ear
Sound consists of vibrations of air in the form of waves. The ear is able to pick up these vibrations and convert them into electrical signals that are sent to the brain. In the brain, these signals are translated into meaningful information, such as language or music with qualities like volume and pitch. The volume of sound is measured in decibels (dB).
The ear
The ear consists of three parts: the outer ear, middle ear and inner ear. Please see the diagram. The outer ear is the visible part of the ear on either side of the head and includes the ear canals that go into the head. The fleshy parts of the outer ear act as "collectors" of sound waves, which then travel down the ear canal to the eardrum. This is a membrane of tissue that separates the outer ear from the middle ear.
The sound waves cause the eardrum to vibrate. This vibration is passed on to the middle ear, which consists of three small bones called the "ossicles", which amplify and conduct the vibrations of the eardrum to the inner ear.
The inner ear consists of an organ called the cochlea, which is shaped like a snail's shell. The cochlea contains tiny cells called hair cells which move in response to the vibrations passed from the ossicles. The movement of these hair cells generates an electrical signal that is transmitted to the brain through the auditory nerve.
Causes of hearing loss
There are many possible causes of hearing loss. These can be divided into two basic types, called conductive and sensorineural hearing loss.
Conductive hearing loss is caused by anything that interferes with the transmission of sound from the outer to the inner ear. Possible causes include:
- Middle ear infections (acute otitis media)
- Collection of fluid in the middle ear ("glue ear" in children)
- Blockage of the outer ear (by wax)
- Damage to the eardrum or hearing bones by infection or an injury
- Otosclerosis, a condition in which the ossicles of the middle ear become immobile because of growth of the surrounding bone.
Sensorineural hearing loss is due to damage to the pathway for sound impulses from the hair cells of the inner ear to the auditory nerve and the brain.
Possible causes include:
- Age-related hearing loss - the decline in hearing that many people experience as they get older
- Acoustic trauma (injury caused by loud noise) to the hair cells
- Viral infections of the inner ear (may be caused by viruses such as mumps or measles)
- Ménière's disease (abnormal pressure in the inner ear)
- Certain drugs, such as chemotherapy drugs, quinine and some antibiotics, which can affect the hair cells
- Acoustic neuroma, a benign (non-cancerous) tumour affecting the auditory nerve
- Viral infections of the auditory nerve (such as mumps and rubella)
- Infections or inflammation of the brain or brain covering - eg meningitis
- Multiple sclerosis
- In many cases the exact cause may not be found
Deafness in children
In New Zealand approximately 138 children are born with or acquire hearing loss in the first few years of life. This is most likely to be an inherited condition or the result of an infection such as measles or meningitis. But there are many known other causes.
Diagnosing hearing loss
In adults, hearing loss may be very gradual, as in age-related hearing loss, or it can be very sudden, as in some viral infections of the inner ear. If you, your friends or your family think that your hearing is deteriorating, you should see your GP.
A person who experiences hearing loss can have a range of tests, at an audiology clinic. A number of different professionals may be involved in testing and treatment of hearing loss:
- An audiologist who specializes in hearing testing and rehabilitation
- An ear, nose and throat (ENT) specialist, also called an otolaryngologist, who diagnoses and treats medical conditions of the ear
When examining a person with hearing loss, a doctor will want to know how the hearing loss has developed and what sort of problems it causes.
He or she will also perform a physical examination. Tuning forks and a special electronic device with headphones (an audiometer) are used to test the degree of hearing loss.
Hearing tests
An audiometer produces sounds of different volumes and pitch (frequencies). During testing, you are asked to indicate, usually by pushing a button, when you hear a sound in the headphones. The level at which a person cannot hear a sound of a certain frequency, is known as their threshold.
People of any age can have a hearing test, even babies. Small children aren't able to indicate when they have heard a sound so there are different hearing tests for different age groups. Infants over six months are able to turn their head when they hear a sound, so they will see a puppet or toy. Newborn to six month old babies are tested while they are asleep. The activity of the auditory nerve can be measured in response to sound.
Hearing loss is measured in decibels hearing level (dBHL). A person who can hear sounds across a range of frequencies at 0 to 20 dB is considered to have normal hearing.
The thresholds for the different types of hearing loss are as follows:
- Mild 25-39 dBHL
- Moderate 40-68 dBHL
- Severe 70-94 dBHL
Profoundly deaf people, who cannot hear sounds quieter than 95 dB, sometimes communicate using sign language and lip reading, especially if they have been deaf since childhood.
If a sensorineural cause is suspected a number of tests can be performed to pinpoint where the problem lies. One test is based on otoacoustic emissions. This measures the responses the cochlea makes to sounds produced by a probe placed in the outer ear. Another test looks at the auditory brainstem responses, which measure the activity of the cochlea, auditory nerve and brain when a sound is heard. None of these hearing tests is uncomfortable.
If the cause of the hearing loss seems to be due to a brain abnormality, a magnetic resonance imaging (MRI) scan of the head may be recommended.
Treating hearing loss
The treatment of hearing loss depends on the cause. A bacterial infection of the middle ear can be treated with antibiotics; blockages of the outer and middle ears can be cleared; damaged eardrums can be repaired surgically; and ossicles affected by otosclerosis can be replaced with artificial bones.
If there is no medical treatment for the hearing loss (as with age-related hearing loss), hearing aids usually help most people, whether the hearing loss is the result of conductive or sensorineural problems. Many different types of hearing aids are available and the audiologist will help you choose which type best suits your needs.
When a hearing aid does not give sufficient amplification, as with profound deafness, a cochlear implant may help. This device transmits sound directly into the auditory nerve via electrodes surgically implanted into the cochlea. Cochlear implants can be particularly valuable for deaf children when implanted at a very early age.
