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Hearing Loss Is Common in People with Diabetes

June 16, 2008 -- Hearing loss is about twice as common in adults with diabetes compared to those who do not have the disease, according to a new study funded by the National Institutes of Health (NIH).

"Hearing loss may be an under-recognized complication of diabetes. As diabetes becomes more common, the disease may become a more significant contributor to hearing loss," said senior author Catherine Cowie, Ph.D., of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), who suggested that people with diabetes should consider having their hearing tested. "Our study found a strong and consistent link between hearing impairment and diabetes using a number of different outcomes."

The researchers discovered the higher rate of hearing loss in those with diabetes after analyzing the results of hearing tests given to a nationally representative sample of adults in the United States. The test measured participants’ ability to hear low, middle, and high frequency sounds in both ears. The link between diabetes and hearing loss was evident across all frequencies, with a stronger association in the high frequency range. Mild or greater hearing impairment of low- or mid-frequency sounds in the worse ear was about 21 percent in 399 adults with diabetes compared to about 9 percent in 4,741 adults without diabetes. For high frequency sounds, mild or greater hearing impairment in the worse ear was 54 percent in those with diabetes compared to 32 percent in those who did not have the disease.

Adults with pre-diabetes, whose blood glucose is higher than normal but not high enough for a diabetes diagnosis, had a 30 percent higher rate of hearing loss compared to those with normal blood sugar tested after an overnight fast.

The study, published early online June 17, 2008, in the Annals of Internal Medicine, was conducted by researchers from the NIDDK, the National Institute on Deafness and Other Communication Disorders (NIDCD), components of the NIH, and Social & Scientific Systems, Inc., which provides support on public health topics to NIH and other government agencies.

The researchers analyzed data from hearing tests administered from 1999 to 2004 to participants in the National Health and Nutrition Examination Survey (NHANES) conducted by the National Center for Health Statistics, part of the Centers for Disease Control and Prevention (CDC). Half of the 11,405 survey participants aged 20 to 69 were randomly assigned to have their hearing tested, and nearly 90 percent of them completed the hearing exam and the diabetes questionnaire. The hearing test, called pure tone audiometry, measures hearing sensitivity across a range of sound frequencies.

"Using the data from the hearing tests, we measured hearing impairment in eight different ways. Also, participants responded to questions about hearing loss in the questionnaire, which asked whether they had a little trouble hearing, a lot of trouble hearing, or were deaf without a hearing aid," said Cowie. In addition, 2,259 of the participants who received hearing tests were randomly assigned to have their blood glucose tested after an overnight fast.

Earlier U.S. studies that examined diabetes and hearing loss found a weaker association or no association, but these studies were based on smaller samples of older adults, and they were not nationally representative, according to co-author Howard Hoffman, an epidemiologist at NIDCD. "This is the first study of a nationally representative sample of working age adults, 20 to 69 years old, and we found an association between diabetes and hearing impairment evident as early as ages 30 to 40."

"The link between diabetes and hearing loss has been debated since the 1960s or before, and our results show that a relationship exists even when we account for the major factors known to affect hearing, such as age, race, ethnicity, income level, noise exposure, and the use of certain medications," noted Kathleen Bainbridge, Ph.D., of Social & Scientific Systems, Inc.

Diabetes may lead to hearing loss by damaging the nerves and blood vessels of the inner ear, the researchers suggest. Autopsy studies of diabetes patients have shown evidence of such damage.

Diabetes is a group of diseases marked by high levels of blood glucose resulting from defects in insulin production, insulin action, or both. Afflicting nearly 21 million people in the United States, it is a major cause of heart disease and stroke and the most common cause of blindness, kidney failure, and lower limb amputations in adults. Pre-diabetes, which causes no symptoms, affects about 54 million adults in the United States, many of whom will develop type 2 diabetes in the next 10 years. Pre-diabetes raises the risk of a heart attack or stroke even if diabetes does not develop. People with pre-diabetes can often prevent or delay diabetes if they lose a modest amount of weight by cutting calories and increasing physical activity. People with diabetes also benefit from diet and exercise as well as medications that control blood glucose, blood pressure, and cholesterol.

Most NHANES participants with diabetes had type 2 diabetes, which accounts for up to 95 percent of diabetes cases in the United States. Type 2 diabetes usually appears after age 40, and is more common in overweight, inactive people and in those with a family history of diabetes.

Hearing loss is a common problem caused by aging, disease, heredity, and noise. About 17 percent of American adults — 36 million people — report some degree of hearing loss. There is a strong relationship between age and reported hearing loss: 8 percent of American adults 18 to 44 years old, 19 percent of adults 45 to 64 years old, and 30 percent of adults 65 to 74 years old report trouble with hearing.

The NIDDK conducts and supports research in diabetes and other endocrine and metabolic diseases; digestive diseases, nutrition, and obesity; and kidney, urologic, and hematologic diseases. Spanning the full spectrum of medicine and afflicting people of all ages and ethnic groups, these diseases encompass some of the most common, severe, and disabling conditions affecting Americans. For more information about NIDDK and its programs, see http://www.niddk.nih.gov/.

The NIDCD supports and conducts research and research training on the normal and disordered processes of hearing, balance, smell, taste, voice, speech and language and provides health information, based upon scientific discovery, to the public. For more information about NIDCD programs, see http://www.nidcd.nih.gov/.

The National Institutes of Health (NIH) — The Nation's Medical Research Agency — includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. It is the primary federal agency for conducting and supporting basic, clinical and translational medical research, and it investigates the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov/.

Mothers' High Normal Blood Sugar Levels Place Infants at Risk for Birth Problems

May 7, 2008 -- Pregnant women with blood sugar levels in the higher range of normal — but not high enough to be considered diabetes — are more likely than women with lower blood sugar levels to give birth to babies at risk for many of the same problems seen in babies born to women with diabetes during pregnancy, according to a study funded in large part by the National Institutes of Health.

These problems included a greater likelihood for Caesarean delivery and an abnormally large body size at birth. Infants born to women with higher blood sugar levels were also at risk for shoulder dystocia, a condition occurring during birth, in which an infant’s shoulder becomes lodged inside the mother's body, effectively halting the birth process.

The study authors declined to make recommendations for acceptable blood sugar levels for pregnant women. The researchers were unable to identify a precise level where an elevation in blood sugar increased the risk for any of the outcomes observed in the study. Rather, the chances for the outcomes were observed to increase gradually, corresponding with increases in the women’s blood sugar levels.

It is well known that high blood sugar levels characteristic of the diabetes that occurs during pregnancy present risks for expectant mothers and the infants born to them. The current study is the first to document that higher blood sugar levels, not high enough to be considered diabetes, also convey these increased risks. Furthermore, when the researchers mathematically adjusted for other potential causes of these risks — such as older maternal age, obesity, and high blood pressure — the increased risks due to higher blood sugar levels were still present.

"These important new findings highlight the risks of elevated blood sugar levels during pregnancy," said Duane Alexander, M.D., director of the NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development, which provided much of the funding for the study. "NIH-supported studies now in progress will provide guidance on how to manage them. Until the results of those studies are available, all pregnant women should consult a health care professional about being screened for diabetes during pregnancy."

Additional NIH funding was provided by the National Institute of Diabetes and Digestive and Kidney Diseases, and the National Center for Research Resources.

Diabetes results from difficulty transferring sugar (glucose) from the blood to the body’s tissues. It occurs in roughly 5 percent of all pregnancies in the United States. Mothers with diabetes during pregnancy are also at increased risk for preeclampsia, a potentially fatal disorder involving dangerously high blood pressure. Babies born to mothers with diabetes — when they reach adulthood — are at higher risk for obesity as well as diabetes, high blood pressure, and heart disease.

The seven-year study involved more than 23,000 pregnant women at 15 centers in 9 countries.

The results of the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study appear in the May 8 New England Journal of Medicine. The researchers were led by Boyd E. Metzger, M.D. Professor of Medicine at the Northwestern University Feinberg School of Medicine in Chicago.

Dr. Metzger explained that before the current study, physicians were not sure at which point elevated maternal blood sugar posed a risk for the baby. Frequently, high maternal blood sugar levels accompany such conditions as obesity, high blood pressure and older maternal age — all known to increase the likelihood for Caesarean delivery. For this reason, it wasn’t known whether the increased risk for Caesarean delivery and other problems seen with mild elevations in blood sugar during pregnancy were caused by the elevated blood sugar levels, or by these accompanying conditions. In their study, however, the researchers made adjustments for these accompanying conditions and found that the higher blood sugar levels still conveyed increased risks.

To conduct the study, the researchers performed an oral glucose tolerance test on each woman, from the 24th through the 32nd week of pregnancy. For the test, the women fasted, after which their blood glucose level was measured. Next, the women drank a glucose solution, and then their blood glucose was measured at predetermined intervals. Women with blood sugar levels high enough to raise safety concerns were referred for treatment and were not included in the study. The remaining women were observed throughout the study until they gave birth.

The researchers found that the higher the mother's blood sugar levels, the greater the chances that they would deliver by Caesarean section. In addition, the higher the mother's blood sugar levels, the more likely the infants were to have high insulin levels and low blood sugar levels at birth. Both conditions indicate exposure to high glucose levels in the womb. Moreover, the higher the mother’s blood sugar levels, the more likely the women were to develop preeclampsia, and the more likely their infants were to be born prematurely, and to experience shoulder dystocia. So, for example, women with the lowest fasting blood sugar levels gave birth to abnormally large babies roughly 5 percent of the time, while women with the highest blood sugar level gave birth to large babies 26 percent of the time.

"These relationships are continuous and generally increase incrementally over the range of blood glucose levels we saw in the study," he said.