Chronic Kidney Disease Has No Impact on Tear Film Substance P Concentration in Type 2 Diabetes

Purpose: The study aimed to ascertain the potential effects of chronic kidney disease (CKD) on substance P concentration in the tear film of people with type 2 diabetes. Methods: Participants were classified into two groups: type 2 diabetes with concurrent chronic kidney disease (T2DM–CKD (n = 25)) and type 2 diabetes without chronic kidney disease (T2DM–no CKD (n = 25)). Ocular surface discomfort assessment, flush tear collection, in-vivo corneal confocal microscopy, and peripheral neuropathy assessment were conducted. Enzyme-linked immunosorbent assays were utilized to ascertain the levels of tear film substance P in collected flush tears. Correlation analysis, hierarchical multiple linear regression analysis, and t-tests or Mann–Whitney U tests were used in the analysis of data for two-group comparisons. Results: There was no substantial difference between the T2DM–CKD and T2DM–no CKD groups for tear film substance P concentration (4.4 (0.2–50.4) and 5.9 (0.2–47.2) ng/mL, respectively; p = 0.54). No difference was observed in tear film substance P concentration between the low-severity peripheral neuropathy and high-severity peripheral neuropathy groups (4.4 (0.2–50.4) and 3.3 (0.3–40.7) ng/mL, respectively; p = 0.80). Corneal nerve fiber length (9.8 ± 4.6 and 12.4 ± 3.8 mm/mm(2), respectively; p = 0.04) and corneal nerve fiber density (14.7 ± 8.5 and 21.1 ± 7.0 no/mm(2), respectively; p < 0.01) were reduced significantly in the T2DM–CKD group compared to the T2DM–no CKD group. There were significant differences in corneal nerve fiber density (21.0 ± 8.1 and 15.8 ± 7.7 no/mm(2), respectively; p = 0.04) and corneal nerve fiber length (12.9 ± 4.2 and 9.7 ± 3.8 mm/mm(2), respectively; p = 0.03) between the low- and high-severity peripheral neuropathy groups. Conclusion: In conclusion, no significant difference in tear film substance P concentration was observed between type 2 diabetes with and without CKD. Corneal nerve loss, however, was more significant in type 2 diabetes with chronic kidney disease compared to type 2 diabetes alone, indicating that corneal nerve morphological measures could serve greater utility as a tool to detect neuropathy and nephropathy-related corneal nerve changes.