Hearing Loss

Hearing loss is when your hearing worsens, as opposed to deafness, which is when hearing loss becomes total.  Hearing loss can occur very gradually, as in age-related hearing loss, or it can be very sudden, as in some viral infections of the inner ear, or trauma.

The ear receives sounds from the environment and transmits them to the brain.  It consists of three parts:

• Outer Ear: The outer ear includes those parts that are visible on the sides of the head, as well as the canals that enter the head.
• Middle Ear: The canals lead to the eardrum, which is attached to three small bones – the 'ossicles' – that amplify and conduct sound to the inner ear.
• Inner Ear: This consists of an organ called the 'cochlea', which is shaped like a snail shell.  The cochlea contains tiny cells called 'hair cells' (which vibrate when a sound is conducted to them via the eardrum and ossicles of the middle ear).  The movement of the hair cells transmits electrical impulses down the auditory nerve to the brain.  That is when you 'hear' the sound.

Causes and Development; Contributing Risk Factors

Two types of hearing loss occur, conductive and sensorineural, depending on which part of the ear is affected.  Conductive hearing loss is caused by anything that blocks the conduction of sound from the outer through to the inner ear.  As well as those mentioned below, causes include:

• 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 trauma
• otosclerosis, a condition in which the ossicles of the middle ear become immobile because of growth of the surrounding bone

Sensorineural hearing loss refers to damage to the pathway for sound impulses from the hair cells of the inner ear to the auditory nerve and the brain.  As well as those mentioned below, causes include:

• age-related hearing loss
• acoustic trauma (such as a loud noise) to the hair cells
• viral infections of the inner ear (may be caused by viruses such as mumps, measles, chickenpox)
• certain drugs, such as aspirin, quinine and some antibiotics, affect the hair cells
• acoustic neuroma (a benign – non-cancerous – tumor of the auditory nerve)
• viral infections of the auditory nerve (caused by viruses such as mumps and rubella)
• strokes.

If a pregnant woman has rubella, the fetus is at risk of being born deaf.  For this reason, girls are vaccinated against rubella before child-bearing age.  Some children are born deaf because of different problems with their development.

Diagnosis and Tests

Hearing loss and deafness are symptoms of the disorders that cause them.  Depending on the cause, they may be associated with other symptoms.  For instance, disorders that involve the inner ear and auditory nerve often also cause dizziness and tinnitus (ringing or whistling sounds in the ear).

Many tests can be carried out to find the cause of hearing loss.  A doctor or audiologist will first listen to the symptoms and perform a thorough examination of the nerves and ears.  Tuning forks and a special electronic device with headphones (an audiometer) are used to test the degree of hearing loss and whether it is conductive or sensorineural.  Other more complex tests may be carried out to check the function of the eardrum and ossicles.

If a sensorineural cause is suspected, the person may have tests called 'electrocochleography' and 'auditory brainstem responses', which measure the activity of the cochlea, auditory nerve and brain when a sound is heard.  None of these tests are uncomfortable.  If the cause of the hearing loss seems to be within the brain, the person may have a magnetic resonance imaging (MRI) scan of the brain.

Treatment and Prevention

The treatment of hearing loss depends on the cause.  Infections are treated with antibiotics; blockages of the outer and middle ears can be manually cleared; damaged eardrums can be repaired surgically and otosclerosis can be treated effectively by replacement of the affected ossicles with tiny artificial bones.  Some causes of sensorineural hearing loss can also be cured.  For example, an acoustic neuroma (a benign tumour of the auditory nerve) can be removed surgically.

If there is no cure for the hearing loss (as with age-related hearing loss), a hearing aid usually helps most people, whether the hearing loss is conductive or sensorineural.  Many different types of hearing aid are available and a doctor / audiologist will advise as to which type is best.  Most people manage very well with a hearing aid and lead perfectly normal lives.

For profound deafness, a cochlear implant may help.  This device transmits sound directly into the auditory nerve via an electrical wire implanted into the cochlea.  Although the sounds heard are rather 'buzzing' or 'electronic', it can be very useful when used in combination with lip reading.  It also lets a person hear the loudness of their own speech and so makes conversation easier.  This is most important in deaf young children, as it helps them to develop good speech.

Prognosis

The outcome depends on the cause.  It may be temporary as with glue ears in children, curable as with otosclerosis or incurable but manageable as with age-related hearing loss.  Only very rarely is hearing loss caused by a more serious condition that affects the brain, such as a tumor.