Purpose The aim of this study was to analyse choroidal structures in normal patients and patients with diabetes with various severities of diabetic retinopathy (DR).

Methods This is a retrospective observation case control study. Three hundred and forty-two diabetic eyes, and age-matched 112 eyes without diabetes mellitus (DM) were enrolled in this study. Patients with DM were classified into no DR, mild/moderate non-proliferative DR (mNPDR), severe NPDR and proliferative DR (PDR). Patients with DM were further divided into two groups based on information regarding systemic DM treatment situation: DM-treated and untreated groups. Central choroidal thickness (CCT), and total choroidal area (TCA), luminal area (LA) and stromal area (SA) were determined using enhanced depth imaging optical coherence tomography and a binarisation method, respectively. The ratio of LA in the TCA was defined as L/C ratio.

Results The haemoglobin A1c (HbA1c) value was significantly higher in the DM-untreated than in the DM-treated subjects. L/C ratio was significantly lower in all the diabetic eyes than control eyes (p<0.05). TCA, LA, L/C ratio and CCT were significantly greater in the DM-untreated than treated group (each p<0.05). In the DM-untreated group, TCA and LAs (p<0.05) and L/C ratio (p<0.01) were significantly lower in mNPDR subjects than normal controls (p<0.05). PDR in the DM-untreated group showed significantly larger SA and LA, and greater CCT than normal controls (each p<0.05).

Conclusions These results suggest that choroidal vasculature was initially involved at an early DR, whereas thickened LA and SA were noted in advanced DR.