Importance: Whether neural or vascular defects occur first in the early onset of diabetic retinopathy (DR) is undetermined.

Background: To investigate microcirculation and microstructure differences of optic nerve head (ONH) between diabetic eyes without clinically evident retinopathy and healthy controls using optical coherence tomography angiography (OCTA).

Design: Cross-sectional observational study.

Participants: Sixty eyes of 60 patients with type 2 diabetes and without clinically evident retinopathy and 60 eyes of 60 age-matched healthy controls were included in this study.

Methods: All participants underwent 4.5 × 4.5-mm rectangle scans centred on the ONH using OCTA (RTVue-XR Avanti; Optovue, Fremont, CA).

Main outcome measures: Peripapillary retinal nerve fibre layer (RNFL) thickness and capillary perfusion density inside the ONH and in the peripapillary region were compared between the two groups.

Results: Vessel density values in both peripapillary and inside the disc were significantly lower in diabetic patients without DR compared to normal controls. The reduction of vessel density was prominent in all eight peripapillary sectors in diabetic eyes (all P < 0.05). Thinning of RNFL thickness was significant in the nasal superior (P < 0.001), inferior nasal (P = 0.023) and superior nasal quadrant (P < 0.001) in diabetic eyes in comparison to normal controls.

Conclusions and relevance: ONH perfusion and peripapillary RNFL thickness were significantly decreased in preclinical DR patients compared to normal controls. Microvascular alterations in ONH may occur earlier than peripapillary RNFL defect in the course of DR.