There are various types of hearing loss, but generally speaking there are two main categories: conductive and sensorineural hearing loss. Whether a hearing impairment is categorized as conductive or sensorineural depends on the origin of the loss.
Conductive Hearing Loss
If the loss can be localized to the outer or middle ear, the hearing loss is conductive. The specific hearing loss can originate at the pinna (visible ear), ear canal, eardrum, middle ear bones or any combination of these.
Sensorineural Hearing Loss
When the loss is caused by a problem involving the inner ear (cochlea/auditory nerve), it is considered a sensorineural hearing loss. This type of loss occurs when the hair cells, neural fibers or their connections to the cochlea are damaged or do not function optimally.
It is possible for hearing loss to be of both a conductive and sensorineural nature which is then referred to as a mixed loss.
Understanding the Hearing Evaluation
Determination of the origin of hearing loss is accomplished during audiological testing, which includes completion of an audiogram (or graph of one's hearing). The audiogram is a grid with two scales - frequency and intensity.
The horizontal scale is the "frequency parameter". Frequency, which we perceive as pitch, can be described as the different notes on a musical scale and is measured in Hertz (abbreviated as Hz). The audiogram typically shows test results for 6 to 10 frequencies from 250 (left on the scale) to 8000 (right on the scale) Hz.
The vertical scale is the "intensity parameter". Intensity, which we perceive as loudness, can be described as how loud or soft a sound is. Intensity is measured in decibels (abbreviated as dB or dBHL).
During the hearing test, each test frequency is varied by intensity to determine the softest sound that can be heard. This is referred to as determining hearing threshold. Thresholds for each ear are plotted as markings on an audiogram, or sometimes recorded numerically in a table. When plotted, markings for the left ear will often be written with blue ink and/or indicated by an "X" while those obtained for the right ear will often be written with red ink and/or indicated by an "O". The more the intensity level has to be raised (i.e. appearing more towards the bottom of the audiogram), the more diminished the hearing at that particular test frequency.
Causes of Hearing Loss
Many people associate hearing loss with aging, and although it is by far the most common cause, there are many other causes of hearing impairment.
Hearing loss may be congenital and be caused by factors such as illness during pregnancy and complications during birth. For these types of hearing loss, hearing aids may often be a help.
Other types of hearing loss are hereditary and genetically determined. These types may progress gradually and require more powerful hearing aids.
Presbycusis (Hearing Loss Related to Aging)
Hearing loss related to aging is the most common type of hearing impairment. It is caused by the gradual degeneration of hair cells in the inner ear, which is a common part of the aging process.
In the inner ear there are sensory hair cells that each react to different tones. Some react to high frequency tones (treble) and some to low frequency tones (bass).
The hair cells create neural impulses that are sent via the auditory nerve to the brain for processing.
One of the effects of presbycusis is deterioration of the hair cells resulting in a decline in hearing ability. The deterioration first affects hearing of high frequency tones and eventually also low frequency tones.
The characteristic signs of presbycusis are hearing impairment and decreased speech intelligibility.
The degree of hearing loss associated with presbycusis varies from one individual to the other. The degree of loss is also usually affected by other factors such as noise exposure.
It is not possible to treat presbycusis surgically or medically, but this type of hearing loss can be minimized with hearing aids.
Noise Induced Hearing Loss
Noise induced hearing loss, or acoustic trauma, is damage to hearing caused by exposure to loud noise. An extremely loud brief noise, such as an explosion or firecracker at close distance, can cause permanent hearing loss. Long term exposure to noise or loud sound, such as factory noise or loud music, can also lead to permanent loss.
Noise induced hearing loss typically results in the inability to perceive high frequency sounds (treble). Hearing is often normal up until frequencies of around 1000 Hz and significantly reduced at one or more frequencies thereafter. The largest degree of hearing loss is typically seen at around 4000 Hz. The cause of the hearing loss can be long-term exposure to loud noise (e.g. clubs, concerts or noisy workplaces), or can be the result of a brief, very loud sound (such as an explosion or a gunshot). This reduction in hearing is often temporary, but in case of repeated exposure to such sounds the reduction may become permanent. Generally it can be said that the louder the noise is, the quicker the noise may cause damage to the function of the ear.
People with noise induced hearing loss often have a reduced tolerance to loud sound in the same frequency range (sensitivity to sound). The hearing loss resulting from acoustic trauma is due to damaged hair cells in the cochlea (inner ear).
Hearing protectors offer some protection from noise induced hearing loss. Hearing aids can help minimize the effect of the hearing impairment once present.