It's a debate that has been raging for 150 years: Does diabetes contribute to hearing loss? Even though we know that the high-sugar environment typical of diabetes damages a host of other bodily systems – including vision, kidney function, dental health and nervous system function – until recently, a connection between diabetes and hearing loss could not be clinically supported. However, a recently concluded large-scale study indicates that diabetics under the age of 60 may be wise to add an audiologist to their diabetes management team.
We at the Veterans Affairs (VA) National Center for Rehabilitative Auditory Research (NCRAR) at the VA Medical Center in Portland, Oregon, conducted a study over a five-year period of 791 patients, approximately half of whom had diabetes. The diabetic patients aged 60 or younger had more high-frequency hearing loss than those without diabetes in the same age group, yet there was no difference in hearing loss between patients with and without diabetes after age 60. These results not only build the case for a clear connection between diabetes and hearing loss, they also implicate diabetes in the acceleration of normal aging processes throughout the body.
High Sugar Wreaks Havoc
Diabetes mellitus is a condition characterized by excess levels of blood sugar (called glucose), a necessary fuel used by our bodies' cells to produce energy. Type 1 diabetes occurs because the body can't make enough insulin to use the available glucose. The other and most common form, Type 2 diabetes, results from a partial or complete resistance of the body to its own insulin – a hormone naturally produced in the pancreas that helps convert sugar and starches into the energy needed for daily life. Insulin helps the cells to break down and metabolize glucose. When glucose levels in the blood are not regulated properly, significant damage can occur to the tiny blood vessels within various organs, such as the eyes and kidneys. This damage is called microangiopathy. Nerve cells can also be damaged by excess glucose, perhaps because of the damage to the blood supply caused by microangiopathy or perhaps because of some effect of high glucose levels directly on the nerve cells.
Among the most common complications of diabetic microangiopathy are damage to the retina of the eye, which causes vision to deteriorate, and damage to the kidneys, which can lead to kidney failure. Diabetes also affects nerve cells, often causing numbness or tingling in the hands and feet. This tingling is called peripheral neuropathy because it affects the peripheral nerves that are found in the extremities. Nerve cells in the central nervous system (brain and spinal cord) may also be affected by diabetic changes but that damage is more difficult to document as it happens over a long period of time. The more often cells are exposed to abnormal glucose levels, the more damage is caused by diabetes. For example, about 80 percent of diabetic patients have some degree of retina damage after 15 years.
A Tough Case to Prove
Given the insidious effects of diabetes, particularly the effects on small blood vessels, it could be expected that the delicate structures involved in hearing would be affected also. The inner ear, called the cochlea, has an intricate network of small blood vessels that supply nourishment to the microscopic structures that receive and analyze sound. However, normal processes of aging and other factors, such as the effects of nutrition and noise, have made it very challenging to isolate diabetes as a clear culprit.
Animal studies have shown changes in some of the capillaries of specific cochlear structures in diabetic animals. Although damage to the tiny blood vessels of the cochlea is a common finding in these animal studies, the effect of this damage on hearing is inconclusive. Some animal studies have found that cochlear damage in diabetes is accelerated if noise exposure or obesity is also present but these findings also are inconsistent.
Autopsy examination of the human cochlea suggests that changes in cochlear structures also occur in people who have had diabetes; however, unlike damage to the blood vessels of the eye which results in vision loss, the microvascular damage in the cochlea has not been clearly established as a cause of hearing loss in people with diabetes. One theory is that there is such an extensive network of small blood vessels in the cochlea that hearing may not be affected by limited amounts of damage or by damage in only certain parts of its complex structures.
Contributing to the controversy is age-related hearing loss. We know that the prevalence of diabetes increases with age, but since hearing loss also increases with age, it is difficult to identify which of these factors plays a greater role in the hearing loss of older people with diabetes.
Why is it that many of the changes in the body caused by diabetes are similar to changes that accompany normal aging? Byproducts of glucose metabolism, called advanced glycation endproducts (AGEs), are often the culprits in disorders associated with both diabetes and aging. AGEs create deposits on certain types of cells causing various complications, such as the thickening of blood vessel walls, plaque formation and other types of tissue damage. The body normally has the ability to deal with these accumulations but diabetes accelerates the damage so that it is more difficult for the body's defenses to keep pace with the barrage of AGE deposits. The fact that differences in hearing loss between diabetic and non-diabetic patients in the NCRAR study were confined to higher frequency sounds, similar to age-related hearing loss, may indicate that the diabetic patients under age 60 were experiencing the aging type of hearing loss earlier than would normally be expected.
Apart from the damage to blood vessels in the cochlea, another potential and relatively serious effect of diabetes on hearing is direct damage to the auditory nerve pathway (neuropathy). The auditory nerve is the beginning of the nerve pathway that takes sound from the cochlea to the brainstem where it is transmitted to other nerve centers and on into the auditory areas of the brain. Some studies have shown delays in the speed of transmission in the brainstem, the very beginning of that central pathway. After sound is analyzed in the inner ear, it is converted to electrical impulses that are sent to the brain for interpretation. If the impulses are delayed, it may cause difficulty in the interpretation of complex sounds like speech. Speech sounds occur rapidly in succession and the central nervous system needs to transfer the messages coded as electrical impulses fast enough to keep pace with the incoming information.
The NCRAR is currently conducting a second diabetes study to investigate whether some people with diabetes have more difficulty understanding speech – difficulty that might be the result of damage to the central auditory nervous system. If we can resolve the question of whether diabetes damages the hearing organ and leads to communication difficulties, it may be possible to head off some of the damage, as with other diabetes-related complications. The key to successfully managing and even reducing ongoing damage is identifying diabetic complications as early as possible. With early identification, appropriate treatment can be prescribed, such as making changes in nutritional routines or other daily activities like exercise.
Nutrition is an important part of good overall health and it is especially critical in the prevention and management of diabetes. Studies to determine how hearing loss is affected by nutrition are rare but there has been a recent focus on a possible connection between reduced levels of calcium, vitamin B-12 and folate and hearing loss in older women (see FYI, p. 31). More research is needed in this important area but it is a fair assumption that any nutritional deficiency that affects blood supply or nerve transmission might also exacerbate hearing loss in a diabetic condition.
Whereas the role of nutrition is still debated, avoiding loud noises is inarguably an important step in protecting hearing, whether factors for disease are present or not. We are still uncertain as to whether noise damage makes the ear more susceptible to other agents of hearing loss, such as aging, diabetes and diseases of the ear, such as Meniere's disease.
Although annual hearing tests are not currently a recommended part of the standard of care for diabetic patients, it is highly recommended that diabetics maintain a record of their hearing status from the date of diabetes diagnosis. This will provide an audiologist with a baseline against which future hearing tests can be compared to determine if, and how fast, hearing may be changing. While the hearing loss may not be preventable, an audiologist can help develop a strategy for dealing with the social and emotional effects of hearing loss on the patient, family and friends. While we await more conclusive research results on hearing loss and diabetes and information on how to deal with them medically, we can still take preventive measures and assume personal responsibility to protect our hearing.
