Compound in tears may be a biomarker for diabetic nerve health: A discussion with Dr. Maria Markoulli

Background:
Diabetic peripheral neuropathy affects an estimated 60% to 70% of people with diabetes. So earlier detection—and prevention—of peripheral neuropathy could not only improve the health of affected individuals, but also relieve a significant economic burden on patients, families, and the health care system.

But "the lack of a sensitive, non-invasive, and repeatable endpoint to measure changes in small nerve fibers is a major factor holding back clinical trials for the treatment of diabetic peripheral neuropathy,"wrote researchers in Australia in a recent article.

However, these researchers point out that diabetes affects both the cornea and the tear film of the eye. To that end, they investigated whether the nerve fiber density in the corneas of patients with diabetes demonstrates an effect on the tear film. They found that it does.

A new test of the tear film could potentially find diabetic corneal neuropathy earlier and easier than the current method of confocal microscopy.

Furthermore, "recent studies have shown that morphological changes in the corneal sub-basal nerve plexus correlates with changes in the peripheral nerves and may, therefore, be a good surrogate measure for diabetic peripheral neuropathy,"? they wrote.

In this interview, researcher Dr. Maria Markoulli explains how a neuropeptide in the tear film can be an indicator of corneal nerve damage, and how that might (it's only a possibility, she emphasizes) serve as a biomarker for diabetic peripheral neuropathy.

MDLinx: What did your research attempt to investigate, and what did you conclude?

Dr. Markoulli: People with diabetes have been shown to have significantly fewer corneal nerves compared with healthy controls. Corneal nerves are responsible for the production of neuropeptides, including substance P. We hypothesized that with this reduction in nerve density, there would be an accompanying reduction in substance P. So we set out to characterize the relationship between tear film substance P and the structural integrity of the sub-basal nerve plexus in diabetes.

We found that people with diabetes had lower levels of substance P compared with healthy controls. There was also a positive correlation between substance P and corneal nerve density, indicating that substance P may be a potential biomarker for corneal nerve health.

MDLinx: Why would it be helpful to have a biomarker test for corneal neuropathy?

Dr. Markoulli: First, corneal neuropathy can result in poor wound healing and recurrent corneal ulcers, which can be both painful and potentially sight-threatening. Being able to detect corneal neuropathy in patients with diabetes may help us implement systemic management sooner to prevent more serious sequelae. The current method of exploring corneal neuropathy is with in vivo confocal microscopy, a rather specialized instrument usually only available in research settings. Developing an instrument that can be used clinically to detect neuropathy would be more practical.

Second, given that corneal neuropathy has been shown to precede peripheral neuropathy in diabetes, this could potentially help us predict peripheral neuropathy. This is a big call at this point, however, and we are currently in the process of studying the association between tear film neuropeptides and peripheral neuropathy measures.

MDLinx: What is substance P, and how did you collect it and measure it?

Dr. Markoulli: Substance P is a neuropeptide that can be measured in the tear film. It has a role in wound healing, but also maintains corneal integrity as it promotes migration, proliferation, and differentiation of epithelial cells. We collected tears from our participants and analysed these using competitive enzyme-linked immunosorbent assays.

MDLinx: What led you to investigate substance P in the tears as a potential indicator of corneal neuropathy?

Dr. Markoulli: It seems like an obvious gap in the literature–corneal nerve density decreases with diabetes–so what downstream effect does this have on the factors that contribute to the wound healing process? It seemed like a logical thing to do: to look at the factors that nerves produce and see what happens in the presence of diabetes.

MDLinx: If your results are confirmed and this proves to be a valid biomarker, at what stage in the disease process would a patient be tested?

Dr. Markoulli: I really need to emphasise that these are early days and our sample size is small. We are now running a larger study and exploring other neuropeptides as well as the link with peripheral neuropathy.

Ideally, when a patient presents to his/her GP, optometrist or ophthalmologist with diabetes, a screening tool will be used to determine the risk of corneal neuropathy on a yearly basis. If neuropathy is determined to be present, further specialized tests could be implemented to confirm neuropathy and appropriate management could be instigated. At this stage, however, this is all speculation.

MDLinx: You write that it's not yet certain whether substance P correlates with the presence or the absence of peripheral neuropathy. What's your hunch?

Dr. Markoulli: From what we know, corneal and peripheral neuropathy correlate quite well. My hunch is that tear film neuropeptides will correlate with peripheral neuropathy, but that's all that is at this stage.

MDLinx: What's your next step in this line of research?

Dr. Markoulli: We are now running a larger study looking at a broader range of neuropeptides in the tear film as well as various measures of peripheral neuropathy in diabetes.

About Dr. Markoulli: Maria Markoulli, PhD, MOptom, FAAO, is Senior Lecturer at the School of Optometry and Vision Science at the University of New South Wales, in Sydney, Australia.