Diabetic Neuropathy: Function-Structure of Corneal Nerves to Assess Injury-Repair

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Peripheral neuropathy (PN) affects about 50% of the diabetic population. The symptoms range from pain, numbness, paresthesia and ulceration in the extremities and PN is the major cause of non-traumatic amputations. In 2010 the VA healthcare system spent $206 million on care of veterans that received lower- limb amputations due to diabetes-induced PN. This single statistic illustrates the impact of this disease and the devastating effect it has on the quality of life and functioning of veterans and their families as well as the large cost burden on the VA healthcare system. We now know that PN can develop before the onset of hyperglycemia and can be detected in subjects with pre-diabetes and impaired glucose tolerance. These subjects are often overweight with symptoms of metabolic syndrome. Recent statistics indicate that 72% of all Veterans are overweight or obese placing them at high risk of developing PN and type 2 diabetes. The diagnosis of PN in its early stages is challenging and no treatment is available, besides glycemic control, which is ineffective for type 2 diabetes. With the prevalence of obesity and type 2 diabetes at epidemic levels in both the veteran and general populations there is a critical need for improving the diagnosis of PN and finding a treatment. The goals of this proposal are to investigate both of these important issues. A key to improving treatment of PN like many disorders is early detection. Presently, the clinical diagnosis for PN is subjective with most veterans receiving a diagnosis of PN only after presenting with symptoms and advanced PN. Earlier diagnosis of PN is needed if new treatments aimed at preserving nerves and stimulating regeneration is to be successful. Recently, loss of sub-epithelial corneal nerves has been promoted as being a surrogate marker of PN. However, utilizing corneal confocal microscopy to evaluate loss of corneal nerves as a routine method to detect PN would be challenging. Thus, we have developed an objective test relying on corneal sensitivity as a simple screening method for detecting PN. The method employs the use of a hyperosmolar eye drop to activate transient receptor channel-8 receptors in the cornea to cause reflex blinking and squinting if the nerves are intact. Even before the onset of hyperglycemia, i.e. pre-diabetes, damaged peripheral nerves lose sensation, and will have less reflex response to corneal stimulation. Development of a method of early detection of PN that can be performed annually during a routine clinical primary care visit or eye examination would improve the standard of care for veterans with diabetes. In this application we will also extend our examination of a safe and cost effective treatment to slow progression and reverse nerve damage caused by PN, thereby providing veterans a solution for this devastating problem. We first introduced fish oil as a treatment of PN in VA supported pre-clinical studies in the last funding period and will continue these studies culminating in a feasibility study for treating human subjects with type 2 diabetes and neuropathy. Our previous studies suggest that long-chain omega-3 (n-3) polyunsaturated fatty acids that are commonly found in fish oils, primarily eicosapentaenoic acid and docosahexaenoic acid, may be an effective treatment for PN associated with pre-diabetes and diabetes. Consumption of n-3 polyunsaturated fatty acids found in fish oils is low in the Western diet due to historically increased consumption of n-6 polyunsaturated fatty acids. Numerous clinical studies suggest potential benefits of n-3 polyunsaturated fatty acid consumption on various diseases. Therefore, there is a tremendous interest in increasing the dietary intake of n-3 polyunsaturated fatty acids as a capsule for the general public and for select clinical populations that may benefit the most. Even though the health benefits of n-3 polyunsaturated fatty acids have been widely examined, their role as a potential treatment for diabetes complications, including PN, have not been thoroughly studied. The studies presented in this application will fill this void and will provide rationale to advance to clinical trials n-3 polyunsaturated fatty acids, i.e. fish oils as a treatment for PN.