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Purpose To measure the levels of vascular endothelial growth factor ( VEGF) in aqueous humor in patients with unilateral acute primary angle closure ( APAC) and retinal oxygen saturation ( SO2) after trabeculectomy. Methods Twelve patients, with unilateral trabeculectomy-required APAC, were recruited as the study group. Aqueous humor samples were collected prospectively for every subject. VEGF concentrations were analysed via enzyme-linked immunosorbent assay ( ELISA), and retinal SO2 was measured using oximeter (Reykjavik, Iceland) at 1 month after trabeculectomy. Results In the APAC eyes, the mean aqueous humor concentration of VEGF was 388.4 ± 260.1 pg/ml and positively correlated with preoperative intraocular pressure ( ρ = 0.658, p = 0.020). In the normal controls, the mean SO2 levels in the larger arterioles and venules were 93.0 ± 5.9% and 59.5 ± 5.4% and the arteriovenous [A-V] difference was 33.5 ± 6.3%. In the study group, the arteriolar SO2 at 1 month after surgery (97.6 ± 19.4%, p = 0.147) did not differ significantly from the values in the control group. However, the mean venular SO2 was significantly lower than the control eyes (50.0 ± 9.1% versus 59.5 ± 5.4%, p = 0.004), and a remarkable increased A-V difference (47.6 ± 22.7% versus 33.5 ± 6.3%) was found in the APAC eyes (p = 0.037). Conclusions Lower venular SO2 and increased A-V difference existed in the APAC eyes after surgery, and it is possible that the saturation was even lower during the attack. Together with the elevated VEGF concentrations in aqueous humor, these provided indirect evidence that ocular hypoxia was constantly present during APAC attack. [ABSTRACT FROM AUTHOR]