How do we hear?
Your ears collect and process sounds before sending signals to your brain. In the brain, these electrical signals from sound are read as recognisable information - language for example, or music with pitches and tones.
The ear is made up of three different sections: the outer ear, the middle ear, and the inner ear. These parts all work together so you can hear and process sounds.
The outer ear: sound waves
Sounds are collected by the outer ear (the part people can see), which is also known as the pinna or auricle. Earwax is produced in the ear canal, which is part of the outer ear. Earwax traps dirt to help keep the ear canal clean and has chemicals to protect the ear canal from infection.
The middle ear: vibrations
The middle ear receives sound waves that travel through the ear canal from the outer ear. Its job is to collect these sound waves and convert them into vibrations that are sent to the inner ear. For this to happen, the eardrum, a thin piece of skin stretched tight like a drum, is needed.
The eardrum separates the outer ear from the middle ear and the ossicles, which are the three smallest bones in your body. Sound waves reaching the eardrum cause it to vibrate. This impacts the ossicles and these bones help sound move along on its journey into the inner ear.
The inner ear: nerve signals
Sound comes into the inner ear as vibrations and enters the cochlea, a small, curled tube in the inner ear. The cochlea is filled with liquid, which is set into motion, like a wave, when the ossicles vibrate. This vibration causes the cochlea's tiny cells, covered in tiny hairs, to move, creating nerve signals that the brain reads as recognisable sounds.
Hearing loss can be present from birth due to genetic or developmental abnormalities, or arise through trauma or disease during the lifetime of an individual. The degree of hearing impairment can vary from mild to profound and causes significant social and personal difficulties. It results in communication problems and, if left untreated, can lead to depression, confusion and isolation.
How common is hearing loss?
Prevalence in children
Bilateral hearing loss
Approximately one in 1,000 infants born in Ireland will have permanent bilateral (in both ears) hearing impairment of a moderate or greater degree. Late onset and acquired childhood hearing impairment (moderate or greater) diagnosed during the early years results in an increased prevalence of 2 to 2.5 per 1,000.
Unilateral hearing loss
Almost one in 2,000 children in Ireland are born with a unilateral (in one ear) hearing loss of a moderate or greater degree.
Glue ear is a condition where the middle ear fills with glue-like fluid instead of air. This causes dulled hearing. In most cases it clears without any treatment. Temporary childhood hearing impairment due to ‘glue ear' is widespread, with an 80% period prevalence between birth and seven years of age; 3% of 24 year olds have a hearing loss due to glue ear for more than 50% of the time and this group would benefit from surgical intervention.
There is strong evidence that timely diagnosis and intervention for children and adults with permanent and temporary hearing impairment results in improved outcomes at a relatively low health care cost.
School-age children with hearing difficulties
If (so-called) ‘mild' hearing impairments are included the total prevalence at school entry (unilateral and bilateral, mild to profound) is thought to be of the order of 3 to 4 per 1,000; that means between 3,000 to 4,500 preschool and school age children in Ireland will have a permanent hearing impairment, with potential consequences for communication, literacy, social and emotional development, and later employability.
Prevalence in adults
Permanent acquired hearing loss of a significant degree affects some 8% of the adult population in Ireland. In the over 70-age group this rises to some 50%. Thus, about a quarter of a million adults in Ireland will have a permanent hearing impairment, due mainly to ageing and/or noise exposure, which affects their quality of life, communication, social activity and participation to varying degrees.
The types of hearing loss
There are two main types of hearing loss:
Conductive hearing loss
A conductive hearing impairment is caused by disorders of the outer and/or middle ear. These hearing losses are usually temporary and can sometimes be reduced or eliminated by medical intervention or surgery.
Middle ear infections, a perforated eardrum, earwax in the ear canal, unusual bone growth (otosclerosis), fluid in the middle ear ("glue ear" in children) or head injury.
Sensorineural hearing loss
A sensorineural hearing impairment is one resulting from dysfunction in the inner ear, especially the cochlea where sound vibrations are converted into brain signals, or in any part of the brain that subsequently processes these signals. In many cases, sensorineural hearing loss is permanent and may affect both ears.
The vast majority of human sensorineural hearing loss is associated with abnormalities in the hair cells of the cochlea. This dysfunction may be present from birth due to genetic or developmental abnormalities, or arise through trauma or disease during the lifetime of an individual. Causes include acoustic trauma (loud noise), head injury, stroke, mumps, meningitis, congenital issues, Meniere's disease, brain or auditory nerve tumour or multiple sclerosis.
Warning signs of hearing loss
Most sensorineural hearing losses develop over time. As a result of the gradual nature of the hearing loss, it can be difficult to identify at the early stages. The obvious warning signs are:
• Asking people to repeat what they have said
• Having difficulty following conversations, particularly when in a restaurant, group situation, or when there is background noise
• Feelings of ear fullness or pressure, ear noises (ringing, buzzing), and dizziness
• Turning up the radio or TV volume louder than others prefer
• Do you find yourself ignoring sounds when not facing their source?
• Has your spouse or a friend ever told you that you don't hear well?
• Do you feel frustration with friends and family when you cannot hear them?
• Do you hear some people more clearly than others?
• Do you hear conversation but have difficulty understanding the words?
• Are you withdrawing more from contact with friends and family?
• Do you turn one ear toward a speaker in order to hear better?
Levels of hearing loss
Hearing loss is often classified by means of different levels and these levels are measured in decibels, or dB. The same definitions are not always used, but the following are among the most common.
On average, the quietest sounds that people can hear with their better ear are between 25 and 40dB. People who suffer from mild hearing loss have some difficulties keeping up with conversations, especially in noisy surroundings.
On average, the quietest sounds heard by people with their better ear are between 40 and 70dB. People who suffer from moderate hearing loss have difficulty keeping up with conversations when not using a hearing aid.
On average, the quietest sounds heard by people with their better ear are between 70 and 95dB. People who suffer from severe hearing loss will benefit from powerful hearing aids, but often they rely heavily on lip-reading even when they are using hearing aids. Some also use sign language.
On average, the quietest sounds heard by people with their better ear are from 95dB or more. People who suffer from profound hearing loss are very hard of hearing and rely mostly on lip-reading, and/or sign language. They may hear loud sounds.
Causes of hearing loss
Advancing age is the most common form of hearing loss. Age-related hearing loss, or presbycusis, is the progressive loss of ability to hear high frequencies with increasing age. It begins in early adulthood, but does not usually interfere with ability to understand conversation until much later. Your genes and loud noises (such as from rock concerts or music headphones) may play a large role.
Exposure to loud noise can cause hearing loss. Daily exposure to excessive noise in the workplace is the primary factor in many cases of hearing loss in the working population. Also, the increasing exposure to music through nightclubs, rock concerts, and MP3 players is causing hearing damage in high numbers of listeners, especially among young people.
Some drugs and antibiotics can damage the function of hair cells or the auditory nerve. Drugs that can cause damage to the hairline cells of the cochlea are quinine, aminoglycerides, diuretics, aspirin in large doses and some cancer drugs.
Wax can build up and block sound from passing through the auditory canal. Periodic removal of earwax can be performed by a GP.
People who sustain head injury are especially vulnerable to hearing loss or tinnitus (ringing or buzzing in the ears), either temporary or permanent. Head injuries that cause a reduced blood supply can harm the inner ear structure.
High fever for a prolonged period of time can harm the inner ear structure.
Other causes of hearing loss include Meningitis, Meniere's syndrome, benign growths and tumours in the hearing nerve, and viral infections such as mumps and measles.
Otitis media is an inner ear infection characterised by the build-up of fluid in the middle ear lining. This can be caused by allergies, head colds, inflamed tonsils and adenoids, blocked eustation tubes, sore throats and other viruses.
Perforation of the eardrum can be caused by a blow to the ear, a change in air pressure associated with flying or scuba diving, a foreign object such as a cotton swab used to clean the ears, or pressure caused by a middle-ear infection.
Otosclerosis is an abnormal bone growth in the middle ear that causes hearing loss. This immobilisation of the middle ear bones is caused from deposits forming between the tiny bones of the middle ear. Also, dislocation of the tiny ear bones can occur.
A malformation of the ear canal can sometimes cause a hearing loss.