Diabetic Retinopathy

Diabetic Retinopathy Biomarker Found in Tears

A recent study showed that patients with diabetes have significantly lower substance P levels in their tears compared with patients without diabetes. The research indicates that evaluating substance P levels in patients with diabetes might be useful in diagnosing diabetic neuropathy.

For their cross-sectional study, the researchers recruited 9 participants with diabetes and 17 participants without diabetes.

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Total protein content and substance P concentrations were determined in participants’ flush tears. The researchers evaluated corneal nerve morphology by capturing the corneal sub-basal nerve plexus in the central cornea via the Heidelberg Retinal Tomograph II with the Rostock Corneal Module.

Additionally, corneal nerve fiber density (CNFD) was determined with ACCMetrics on 8 captured images. The researchers compared groups using independent samples t-tests and used Pearson’s correlations to analyze correlations between parameters.

Results of the study showed that participants without diabetes had significantly higher substance P concentrations in their tears compared with participants with diabetes. The researchers noted no significant difference in total protein content between the groups. CNFD was significantly lower in participants with diabetes vs those in the control group. There was a moderate correlation between substance P and CNFD.

“Substance P is expressed at a significantly lower level in the tears of people with diabetes compared with healthy controls,” the researchers concluded. “The positive correlation between substance P and corneal nerve density indicates that substance P may be a potential biomarker for corneal nerve health.”

—Christina Vogt


Markoulli M, You J, Kim J, et al. Corneal nerve morphology and tear film substance P in diabetes. Optom Vis Sci. 2017;94(7):726-731. doi:10.1097/OPX.0000000000001096.