Aims To determine the effect of diabetes on inner and outer retinal function in persons with diabetes and no clinically detectable retinopathy or with non-proliferative diabetic retinopathy (NPDR).

Methods Visual function was assessed in 18 adults with normal retinal health, 23 adults with diabetes and 35 adults with NPDR and normal visual acuity. Contrast sensitivity and frequency doubling technology (FDT) sensitivity were used to assess ganglion cell function. Acuity, dark adaptation, light-adapted visual sensitivity and dark-adapted visual sensitivity were measured to evaluate cone and rod photoreceptor visual function. The presence and severity of diabetic retinopathy was determined by grading of 7-field stereoscopic fundus photographs using the Early Treatment Diabetic Retinopathy Study grading system.

Results Participants with NPDR exhibited impairment of all measured visual functions in comparison with the normal participants. Inner retinal function measured by FDT perimetry was the most impaired visual function for patients with NPDR, with 83% of patients exhibiting clinically significant impairment. Rod photoreceptor function was grossly impaired, with almost half of the patients with NPDR exhibiting significantly impaired dark-adapted visual sensitivity.

Conclusion Both inner retinal and outer retinal functions exhibited impairment related to NPDR. FDT perimetry and other visual function tests reveal an expanded range of diabetes induced retinal damage even in patients with good visual acuity.