Sensorineural Hearing Loss

Sensorineural hearing loss is a common disorder that affects millions of people. Hearing loss has many different presentations, ranging in severity from mild to profound, including low and high frequency patterns, and can affect people of any age.

The source for this picture is; http://s3.images.com/huge.101.506931.JPG  This picture is a cross section of the cochlea and shows the hair cells of the inner ear.  As you can see, there if one row of inner hair cells and three rows of outer hair cells.   A sensorineural hearing loss involves damage, or lack of formation of the microscopic hair cells in the inner ear.  The hair cells are located on the basilar membrane within the cochlea.  The pictures below are scanning electron micrographs of a healthy rodent cochlea, and a cochlea damaged from noise. This picture is from the House Ear Institute in Los Angeles.

Again, you can see the one row of inner hair cells and the three rows of outer hair cells in the picture of the intact cochlea.  It is very easy to see the extensive damage to the hair cells of the cochlea in the picture to the right.  This damage is irreversible and leads to significant hearing impairment.  Humans cannot regenerate the hair cells once they are damaged.  That is why it is so important to protect your hearing when exposed to noise.

The Causes of Sensorineural Hearing Loss:

Noise:

Noise is perhaps the most common occupational and environmental hazard. As many as 30 million Americans are exposed to potentially harmful sound levels in their workplaces. Outside of work, many persons pursue recreational activities that can produce harmful noise. Sixty million Americans own firearms, and many use them without adequate hearing protection. Other non-occupational sources of noise include chain saws and other power tools, amplified music, and recreational vehicles such as snowmobiles and motorcycles. Some types of toys for children can produce sounds capable of causing permanent hearing damage

Ototoxic Drugs:

A common cause of hearing loss, especially in developing countries, is ototoxicity. An ototoxic hearing loss happens when someone takes or is given a drug that causes hearing loss as one of its side effects. Sometimes the drugs are needed to save lives, and hearing loss is the price paid for being able to live.

Sometimes the drug-induced hearing loss is temporary and can be reversed or stopped. Other times it is permanent. People with hearing loss need to be especially aware of the potential for ototoxic effects, as an ototoxic drug can make an existing hearing loss worse.

Genetic Factors:

There are many genetic causes of hearing loss (e.g., genes can mutate or a child can inherit genes), hundreds are known, and over time the genes are being identified which may lead to genetic therapies. As much as half of deafness may have a genetic cause.

 Viral Infections:

One viral infection that can jeopardize the hearing of an unborn child is cytomegalovirus (CMV). In fact, a congenital cytomegalovirus infection is one of the most common causes of hearing impairment in children.  Some childhood diseases caused by viruses have been linked to hearing loss. Mumps, chicken pox, and influenza are examples of viral infections that can cause a hearing impairment.  Some researchers believe that sudden hearing loss during adulthood is due to an unidentified viral infection. Sudden hearing loss without an identifiable cause is called idiopathic hearing loss. Sometimes, the person’s hearing returns as mysteriously as the hearing loss started.

Aging:

Age related hearing loss is often called presbycusis.  As a person ages, their cochlear hair cells may become damaged. This results in a high-frequency hearing loss that can start as early as middle age. The adult male of the species appears to be at greater risk, particularly if he is over 40 years old. In addition, there may be a genetic component to presbycusis. Noise exposure is also known to play a role.

The impact of presbycusis can be seen in age-hearing loss charts. As a person ages, the middle ear also ages, going through physical changes that make it more difficult for a person to discriminate sound.

Statistics show a large percentage of older folks develop presbycusis. As many as half over 75 may have hearing loss. According to the a Profile of Older Americans from the federal Department of Health and Human Services Administration on Aging, hearing loss was one of the most common condition experienced by mature people in a survey.

Diabetes:

Diabetics have twice the risk of developing hearing loss as are nondiabetics, according to researchers at the American Diabetes Association 67th Scientific Sessions (ADA).  “The pathologic changes that accompany diabetes could plausibly affect the vasculature or the neural system of the inner ear, resulting in sensorineural hearing impairment,” Dr. Cowie explained in a presentation on June 24th.  After adjusting for age, 31.6% of self-reported diabetics had hearing impairment at the lower frequency range versus 14.5% of the nondiabetics subjects. The figures were 56.8% and 35.8% for the two groups, respectively, at the higher frequency range.

Meniere’s Disease:

Meniere’s disease is a disorder of the inner ear that affects balance and hearing and includes odd sensation of movement (vertigo), dizziness, loss of hearing in one or both ears and ringing in the ear (known as tinnitus). Meniere’s disease happens when there is a change in the fluid volume of a part of the inner ear called the “labyrinth.” Possible causes include ear infection and head injury.

Head Trauma:

Direct head trauma, particularly trauma severe enough to cause unconsciousness, can cause inner ear concussions and hearing loss. Fluctuation in hearing after head trauma may take place within the first year after trauma, after which the associated hearing loss should be considered to be stabilized.

Acoustic Tumors:

An acoustic neuroma is a common tumor of the acoustic nerve, the nerve responsible for hearing. It generally starts in the internal auditory canal, the bony channel through which the nerve courses as it goes from the ear to the brain. Acoustic neuromas grow, compressing the brainstem and other structures, including the facial nerve. Patients with an acoustic neuroma often complain of hearing loss, tinnitus and problems with their balance.

Ordinarily, these tumors can be cured with surgery. They are managed best by neurotologists, inner ear subspecialists with particular interest and expertise in treatment of this specific tumor.