**3.6 OCTA and choroid**

The choroid is the main source of oxygen and nutrients for external retina to understand DR pathogeny. Increased choriocapillaris (CC) non-perfusion has been described already in patients without signs of DR [37]. OCTA surely represents a significant advance compared with FA for studying the CC, but the deep localization of this complex vascular structure makes it difficult to get high-quality images. The perfusion defect (PD) is the area where it is not possible to identify flow between the OCTA limits of detection [44, 45]. PD could be an interesting parameter to study CC involvement in DR and is significantly higher in DM patients compared with normal eyes [45]. New prospective studies are needed to clarify the importance of the CC in DR.
