PURPOSE: The purpose of this study was to evaluate deficits at varying levels of visual system in diabetes without clinical retinopathy (NoDR) and to explore the optimal method for detecting early diabetic visual disorders among functional and retinal structural assessments included. METHODS: This cross-sectional study examined eyes by the Early Treatment of Diabetic Retinopathy Study (ETDRS) charts, visual psychophysical tests, optical coherence tomography (OCT), and OCT angiography (OCTA). Visual psychophysical metrics included grating acuity (GA), and contrast sensitivity to first-order motion stimuli (1stM), second-order contrast-modulated stationary stimuli (2ndS), and second-order motion stimuli (2ndM). Generalized linear mixed effect (GLME) models were applied to assess group effects and linear relationships between measurements. The receiver operating characteristic (ROC) analysis was utilized to identify the optimal classifier for detecting NoDR. RESULTS: Fifty-three eyes of 33 patients with NoDR and 40 eyes of 27 healthy controls were included. The NoDR group showed significant reductions in various visual functions, including ETDRS acuity, GA, 2ndS, and 2ndM (P values <
0.001), and microvascular changes in foveal vascular density (FD-300), the acircularity index (AI) of the foveal avascular zone, and the parafoveal superficial capillary plexus density (P values <
0.05). GLME models revealed these retinal variations were not significantly correlated with early diabetic visual function abnormalities. ROC analysis demonstrated the integration of GA and FD-300 (area under the curve [AUC] = 0.911) is the most effective classifier for detecting early diabetic visual dysfunctions. CONCLUSIONS: In addition to retinal defects, both low- and higher-order visual function disorders along the visual pathway exist in patients with NoDR. Combining functional and structural measurements may provide more accurate assessments for detecting early diabetic visual disorders. TRANSLATIONAL RELEVANCE: Sophisticated visual psychophysical measurements, including grating acuity and second-order function, could be applied for detecting early diabetic visual disorders.