Does neuronal damage precede vascular damage in subjects with type 2 diabetes mellitus and having no clinical diabetic retinopathy?

AIM: To investigate the occurrence of neuronal damage, as the earliest change occurring, before the clinical evidence of diabetic retinopathy.
METHODS: 70 eyes of subjects with type 2 diabetes mellitus and with no evidence of diabetic retinopathy (cases) and 40 eyes of subjects with no diabetes mellitus (controls) were studied using spectral-domain OCT and microperimetry. The influence of age and gender on the outcome measures was also analyzed.
RESULTS: Age- and gender-matched subjects showed a decreased mean retinal nerve fiber layer thickness in cases when compared to the controls (27 vs. 33 μm; p=0.018). Among the cases, subjects between 40 and 45 years of age showed a reduced mean central foveal thickness (175.1 vs. 198.1 μm; p=0.05), mean retinal thickness in the central 6-mm fundus (260.5 vs. 275.3 μm; p=0.006) and mean retinal nerve fiber layer thickness (29 vs. 39 μm; p=0.036) when compared to the controls. However, no differences were noted in the microperimetry outcomes in cases when compared to the controls. The duration of diabetes and the glycemic control did not show any significant changes on the outcome measures in cases, except for a significantly lower mean retinal sensitivity in diabetics with glycosylated hemoglobin values<7% as compared to those with glycosylated hemoglobin≥7% (14.1±2.9 vs. 15.4±1.7 dB; p=0.027).
CONCLUSION: The results suggest that there is some evidence of early neuronal damage particularly on spectral-domain OCT, before the clinical evidence of diabetic retinopathy, in subjects with type 2 diabetes mellitus.