Diabetes May Lead to Early Aging of the Auditory System

Newswise — Diabetes mellitus (DM) affects at least 16 million Americans, ranks seventh as a cause of death in the United States, and costs the national economy over $100 billion yearly. About 95 percent of persons with DM have type 2, in which the pancreatic beta cells retain some insulin-producing potential, and the rest have type 1, in which insulin is required for long-term survival. Now, preliminary findings of a new study have determined that this disease, approaching epidemic proportions, may lead to premature aging of the body's auditory or hearing system. The results add to the literature that has chronicled diabetes' damaging impact on various organs of the body such as the kidneys, eyes, heart, and nervous system.

The findings come from the Department of Veterans Affairs National Center for Rehabilitative Auditory Research (NCRAR), which is currently conducting a five-year epidemiological study to assess the prevalence and severity of auditory dysfunction in veterans with diabetes. Preliminary results of this study, the largest prospective study of diabetes and auditory function to date, will be presented by Nancy Vaughan, PhD, author of "A Large-Scale Study of Auditory Function in Diabetic Veterans" and Stephen Fausti, Ph.D., Director of the NCRAR, at the Mid Winter Meeting of the Association for Research in Otolaryngology (http://www.aro.org) being held February 22-26, 2004 at the Adam's Mark Hotel, Daytona Beach, FL. Methodology: The study included 694 veterans aged 25 to 85 years old, 342 with diabetes; 352 without. By the completion of the study, 800 participants in all will have been evaluated. Participants were divided into two groups: 60 years old and under and those over 60. Diabetic patients included only those diagnosed for at least five years. All participants completed audiometric testing (hearing loss, otoacoustic emissions, and auditory brain stem response) and a questionnaire. Glucose levels and HbA1c (glycosylated hemoglobin " a measure of overall diabetes control) were tested to indicate metabolic control over the past three months. Cooling and vibration sensitivity were measured (Quantitative Sensory Test) as indicators of peripheral neuropathy (damage to the peripheral nerves in the arms, hands, legs, and feet). Participants received minor compensation for the four to five hour laboratory session.

Results: A variety of tests were used to evaluate auditory function and interim results were analyzed in relation to duration of disease, age, self-reported noise exposure, insulin use, and HbA1c (hemoglobin) results. Data were also collected on microvascular and neuropathy changes typically associated with diabetes. Further analysis is needed to determine the role of these diabetic mechanisms in the outcomes of this study. Significant findings to date include: increased hearing loss in diabetic veterans aged 60 and younger compared to those of the same age without diabetes, a lack of effect on the cochlear amplifier system as shown by otoacoustic emissions testing, and delays in central auditory processing revealed in the auditory brainstem response (ABR) tests. Further investigation is planned to determine the cause and effect of the central auditory processing delays seen in diabetic patients.

Conclusion: Diabetic patients under age 60 exhibited greater hearing loss than non-diabetic study participants of the same age; however, in those over 60, hearing loss was similar between diabetic and non-diabetics. It is theorized that, in older diabetic patients, presbycusis (age-related hearing loss) may obscure any changes in hearing loss due to diabetes. Central auditory processing functions are affected by diabetes to a greater extent than peripheral functions (e.g., hearing loss), indicating that ABR and pure tone testing may be appropriate to reveal changes early in diabetic care. Since hearing takes place not only in the ear, but in higher auditory and cognitive centers responsible for processing the sounds received in the cochlea, early identification of processing changes could be important to verbal communication. The finding that hearing loss is exacerbated in diabetics under 60 years of age is consistent with the theory that diabetes is associated with accelerated aging of the auditory system.