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Hearing How we hear. 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. These signals are then sent to the brain to translate into meaningful information, such as speech 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 (see 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 (otitis media). Collection of fluid in the middle ear ("glue ear" in children). Blockage of the outer ear (by wax). Damage to the eardrum 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 rarely, rheumatoid arthritis affects the joints between the ossicles. 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 aspirin, 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. A brain tumour. A stroke. 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 or accident. If you, your friends or your family think that your hearing is deteriorating, you should see your doctor or visit the Hearing Resource Centre. You may have a range of tests at Audiology Department of the General Hospital, or preliminary tests can be carried out at the Hearing Resource Centre. A number of different professionals may be involved in testing and treatment of hearing loss: An ear, nose and throat (ENT) specialist, also called an otolaryngologist. Audiological physician (doctor specialising in hearing problems). Audiologist (a specialist in the testing of hearing and fitting of hearing aids). 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. 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 25-39dBHL Moderate: 40-68 dBHL Severe: 70-94 dBHL Profoundly deaf: People who cannot hear sounds quieter than 95 dB. 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. Some causes of sensorineural hearing loss can also be improved. For example, an acoustic neuroma can be removed surgically. If there is no cure for the hearing loss (as with age-related hearing loss), a hearing aid for one or both ears usually helps most people, whether the hearing loss is the result of conductive or sensorineural problems. Many different types of hearing aid are available and the audiologist will advise as to 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. Although the sounds heard can be of a buzzing or electronic nature, it can be very useful when used in combination with lip reading. It also lets a person hear the volume of their own speech and so makes conversation easier. TOP