Types of Hearing Loss

When describing hearing loss we generally look at three attributes: type of hearing loss, degree of hearing loss, and the configuration of the hearing loss.

Hearing loss can be categorized by where or what part of the auditory system is damaged. There are three basic types of hearing loss: conductive hearing loss, sensorineural hearing loss and mixed hearing loss.

Conductive Hearing Loss

Conductive hearing loss occurs when sound is not conducted efficiently through�the outer ear canal to the eardrum and the tiny bones, or ossicles, of the middle ear.�Conductive hearing loss usually involves a reduction in sound level, or the ability to hear faint sounds. This type of hearing loss can often be medically or surgically corrected.

Examples of conditions that may cause a conductive hearing loss include:

• Conditions associated with middle ear pathology such as fluid in the middle ear from colds, allergies (serous otitis media),�poor eustachian tube function, ear infection (otitis media), perforated eardrum, benign tumors
• Impacted earwax (cerumen)
• Infection in the ear canal (external otitis)
• Presence of a foreign body
• Absence or malformation of the outer ear, ear canal, or middle ear

Sensorineural Hearing Loss

Sensorineural hearing loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear (retrocochlear) to the brain. Sensorineural hearing loss cannot be medically or surgically corrected. It is a permanent loss.

Sensorineural hearing loss not only involves a reduction in sound level, or ability to hear faint sounds, but also affects speech understanding, or ability to hear clearly.

Sensorineural hearing loss can be caused by diseases, birth injury, drugs that are toxic to the auditory system, and genetic syndromes. Sensorineural hearing loss may also occur as a result of noise exposure, viruses, head trauma, aging, and tumors.

Mixed Hearing Loss

Sometimes a conductive hearing loss occurs in combination with a sensorineural hearing loss. In other words, there may be damage in the outer or middle ear and in the inner ear (cochlea) or auditory nerve. When this occurs, the hearing loss is referred to as a mixed hearing loss.

Degree of Hearing Loss

Degree of hearing loss refers to the severity of the loss. There are�five broad categories that are typically used. The numbers are representative of the patient's thresholds, or the softest intensity that sound is perceived:

Normal range or no impairment = 0 dB to�20 dB

Mild loss = 20 dB to 40 dB

Moderate loss = 40 dB to 60 dB

Severe loss = 60 dB to 80 dB

Profound loss =�80 dB or more

Configuration of Hearing Loss

The configuration or shape of the hearing loss refers to the extent of hearing loss at each frequency and the overall picture of hearing that is created. For example, a hearing loss that only affects the high frequencies would be described as a high-frequency loss. Its configuration would show good hearing in the low frequencies and poor hearing in the high frequencies. On the other hand, if only the low frequencies are affected, the configuration would show poorer hearing for low tones and better hearing for high tones. Some hearing loss configurations are flat, indicating the same amount of hearing loss for low and high tones.

Other descriptors associated with hearing loss are:

• Bilateral versus unilateral. Bilateral hearing loss means both ears are affected. Unilateral hearing loss means only one ear is affected.
• Symmetrical versus asymmetrical. Symmetrical hearing loss means that the degree and configuration of hearing loss are the same in each ear. An asymmetrical hearing loss is one in which the degree and/or configuration of the loss is different for each ear.
• Progressive versus sudden hearing loss. Progressive hearing loss is a hearing loss that becomes increasingly worse over time. A sudden hearing loss is one that has an acute or rapid onset and therefore occurs quickly, requiring immediate medical attention to determine its cause and treatment.
• Fluctuating versus stable hearing loss. Some hearing losses change -- sometimes getting better, sometimes getting worse. Fluctuating hearing loss is typically a symptom of conductive hearing loss caused by ear infection and middle ear fluid, but also presents in other conditions such as Meniere's disease.

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)
• M�ni�re'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 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, usually at an audiology clinic of the local hospital or health 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:

Profoundly deaf people, who cannot hear sounds quieter than 95 dB, usually communicate using sign language and lip reading.

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 aids are available.