Can something in a patient’s tears tip you off as to whether they have diabetic peripheral neuropathy? According to an Australian research team, the answer is yes. The same investigators previously linked risk of diabetic corneal neuropathy to low levels of substance P—a neuropeptide found in tears that can be gathered non-invasively. Now, their new study, published in The Ocular Surface, shows that substance P can also be a biomarker for diabetic peripheral neuropathy.

The team looked at 63 patients with Type I or Type II diabetes and 34 age-matched controls. They assessed the patients’ neuropathy using the Total Neuropathy Score and evaluated the concentration of substance P—as well as calcitonin gene-related peptide—in tears. They also imaged patients’ corneal sub-basal nerve plexus using corneal confocal microscopy and quantified the corneal nerve fiber length, fiber density, branch density, total branch density, nerve fractal dimension and inferior whorl length.

They found the median concentration of substance P in tears was significantly reduced in those with diabetic peripheral neuropathy compared with those without it and with the control group. Patients with Type II diabetes showed no difference from the control group in neuropeptides, whether they were positive or negative for any neuropathy.