Aims To detect collateral vessels using optical coherence tomography angiography (OCTA) in eyes with branch retinal vein occlusion (BRVO) and to investigate the associations with visual outcomes and macular oedema.

Methods Eyes with macular oedema secondary to BRVO that underwent OCTA at baseline and were followed up for more than 6 months were enrolled. The presence of collaterals, whether the collaterals were leaky or not, and the associations with visual outcomes and macular oedema were investigated.

Results Twenty-eight eyes of 28 patients (8 men and 20 women; mean age, 68 years) were enrolled. Collaterals were detected in 23 eyes (82%) and already existed at the initial visit. Collaterals were more frequently detected in eyes with major BRVO or ischaemic type. One-third of the collaterals were leaky and all of the leaky collaterals had microaneurysms (MAs) inside. Macular oedema in eyes with collaterals was more quickly and frequently resolved than that in eyes without collaterals, but there were no significant differences. Collateral vessel formation did not seem to impact on visual outcomes, but the mean baseline central retinal thickness (CRT) was significantly higher in eyes with collaterals, and the mean CRT reduction at 6 months after treatments was significantly greater than in eyes without collaterals.

Conclusions These results suggest that collateral vessels are formed at the acute phase in eyes with BRVO. In addition, the presence of collaterals might be associated with absorption of macular oedema, but MAs formed in collaterals sometimes can cause macular oedema.