Objective: To compare total retinal blood flow in diabetes patients with no or mild non-proliferative diabetic retinopathy and healthy control subjects and to investigate in patients whether there is a difference between retinal blood flow before morning insulin and under normoglycemic conditions using glucose clamp.

Research Design and Methods: 20 patients with type 1 diabetes with no or mild diabetic retinopathy were included in this open parallel group study. 20 healthy age- and sex-matched subjects were included as controls. Retinal blood flow was assessed by combining velocity measurements using laser Doppler velocimetry and diameter measurements using a commercially available Dynamic Vessel Analyzer. Measurements were performed before and during a euglycemic clamp.

Results: Total retinal blood flow was higher in diabetic patients (53 ± 16 μl/min) than in healthy subjects (43 ± 16 μl/min, P = 0.034 between groups). When plasma glucose in diabetes patients was reduced from 9.3 ± 1.7 mmol/l to 5.3 ± 0.5 (P < 0.001) retinal blood flow decreased to 49 ± 15 μl/min (P = 0.0003 versus baseline). Total retinal blood flow during the glucose clamp was not significantly different from blood flow in normal control subjects (P = 0.161).

Conclusions: Type 1 diabetes patients with no or only mild diabetic retinopathy have increased retinal blood flow before their morning insulin dosage. Blood flow is reduced towards normal during euglycemic conditions. Retinal blood flow may fluctuate significantly with fluctuating plasma glucose levels, which may contribute to the microvascular changes seen in diabetic retinopathy.