BDNF and non-proliferative diabetic retinopathy: Serum levels of brain derived neurotrophic factor (BDNF) were shown to be significantly lower in NPDR and PDR patients compared to healthy controls in a trial analyzing a total of 114 patients (27 NDR, 30 NPDR, 30 PDR, and 27 healthy controls; Kaviarasan et al., 2015).