2.4. Ophthalmic examination

Nonmydriatic fundus photography (TRC-NW6S; Topcon, Tokyo, Japan) was performed in all KNHANES participants. In participants with a history of diabetes mellitus or a random blood glucose level of ≥200 mg/dL and/or suspicion of diabetic retinopathy on nonmydriatic photography, 7 standard field photographs were obtained from each eye after pharmacological pupil dilation as per the Early Treatment for Diabetic Retinopathy Study (ETDRS) protocol.^[20] Diabetic retinopathy was identified as defined by the ETDRS severity scale according to the presence of microaneurysms (MAs), hemorrhages, hard exudates (HEs), cotton wool spots, intraretinal microvascular abnormalities, venous beading, and retinal new vessels. A diabetic retinopathy severity score was assigned to each eye according to the modification of the Airlie House Classification system, as follows: level 10, no retinopathy present; level 14, any combination of definite HEs, cotton-wool spots (CWS), intraretinal microvascular abnormalities (IRMAs), or venous loops in the absence of definite MA; level 15, hemorrhage present without any definite MA; level 20, MA only with no other diabetic lesion present; level 31, MA and one or more of the following: hemorrhage or MA < standard photograph 2A, HE, venous loops, questionable CWS, IRMA, or venous beading; level 41, MA and one or more of the following: CWS, IRMA < standard photograph 8A; level 51, MA and one or more of the following: venous beading, hemorrhage or MA > 2A, IRMA > 8A; level 60, fibrous proliferation with no other proliferative lesion; levels 61 to 64, laser scatter photocoagulation scars with retinopathy levels 31 to 51; level 65, proliferative diabetic retinopathy without high-risk characteristics, as defined in the Diabetic Retinopathy Study; level 70, proliferative diabetic retinopathy with several high risk characteristics; and level 80, total vitreous hemorrhage. The level of retinopathy was graded based on the worse eye. The eyes were graded according to the following criteria: no diabetic retinopathy (levels 10–13), nonproliferative diabetic retinopathy (levels 14–51), and proliferative diabetic retinopathy (level >60).

Clinically significant macular edema (CSME) was defined according to the ETDRS criteria as follows: retinal thickening at or within 500 μm of the center of the macula of the retina; HEs at or within 500 μm of the center of the macula, if associated with thickening of the adjacent retina (but not HEs remaining after disappearance of retinal thickening); and one or more zones of retinal thickening and at least 1 large-size disk area large, any part of which is within 1 disc diameter of the center of the macula.^[20]

Vision-threatening diabetic retinopathy (VTDR) was defined as the presence of severe nonproliferative diabetic retinopathy, proliferative retinopathy, or CSME.^[21]