PURPOSE: To evaluate the correlation between hyperreflective foci (HRF), the biomarker of activated microglia, and subretinal fluid (SRF), representing the dysfunction of retinal pigment epithelium (RPE), in treatment-naïve patients with diabetic macular edema (DME). METHODS: Sixty-one treatment-naïve patients (61 eyes) with DME were included in the research. Participants were divided into two categories based on the presence or absence of SRF. Basic characteristics were recorded. The parameters, including the HRF number in inner and outer retina, central macular thickness (CMT), intraretinal cyst (IRC), as well as the width, height and area of SRF, were analyzed with optical coherence tomography angiography (OCTA). The correlations between HRF and the parameters including SRF, IRC and CMT were analyzed accordingly. RESULTS: The mean CMT in DME with SRF group was much thicker than that in DME without SRF group (P <
0.0001). The mean HRF number in the outer retina and whole retina was markedly higher in DME patients with the presence of IRC or SRF when compared to those without IRC or SRF (P <
0.05). Further analysis showed that the width, height and area of SRF were positively correlated with the HRF number in the outer retina and the ratio of outer/whole retina HRFs (P <
0.05). CONCLUSION: The positive correlation between the increased number of HRF, especially in the outer retina, and the formation of SRF in patients with DME supports the hypothesis that microglia activation represented by HRF might cause the dysfunction of RPE and the breakdown of the outer blood-retinal barrier (oBRB), which leads to the increased fluid leakage in subretinal space.