Oxygen Levels and Distribution in Rhesus Monkeys: Association with Intraocular Pressure and Aqueous Outflow Facility

Abstract

Purpose: Previous studies suggested that oxidative stress plays a role in the development of glaucoma and that the lens may protect the outflow system from oxidative damage after vitrectomy. (Chang S, AJO 2006;141:1033) In human subjects, vitrectomy and cataract surgery significantly increase the exposure of the tissues of the outflow pathway to oxygen, potentially increasing the likelihood of oxidative damage in these tissues. (Siegfried C et al, IOVS 2010;51:5731) To explore the impact of vitrectomy and cataract extraction on the function of the aqueous humor outflow pathway, we measured oxygen (pO₂), intraocular pressure (IOP) and determined aqueous outflow facility in the eyes of rhesus monkeys undergoing pars plana vitrectomy (PPV) followed by cataract extraction (CE).

Methods: Five rhesus monkeys (ages 19-21 years) underwent sequential PPV, CE with lens implantation in one eye and sham procedures in the fellow eye, using standard surgical techniques. Intraocular pO₂ measurements were performed with the Oxylab pO₂^TM optical oxygen sensor (Oxford Optronix) in the anterior chamber (AC), posterior chamber (PC) and vitreous cavity, as previously described. IOP and pneumatonography data were obtained to calculate outflow facility at baseline, post-PPV, and post-CE. Aqueous and vitreous humor specimens were obtained. Comparisons between surgery and sham eyes were performed by the 2-tailed paired t-test.

Results: pO₂ increased in the AC angle following vitrectomy (p=0.01) and CE (p=0.001) as well as at the lens surface (p=0.001) and the PC following CE (p=0.001). IOP increased following PPV and CE, from baseline mean IOP 19.0 mmHg to 22.3 mmHg, (p<0.05). Although outflow facility decreased in both the sham and surgery eyes, the decline was greater in the surgery eyes.

Conclusions: Our preliminary data in this study of the effect of increased oxygen on aqueous outflow facility and IOP suggest a correlation with decreased function of the conventional outflow pathways. The impact of increased exposure to oxygen and potential oxidative damage may be further understood through our pending studies on the antioxidant status of the aqueous and vitreous humor samples as well as histopathological examination of these macaque eyes.

Keywords: 633 outflow: trabecular meshwork • 634 oxidation/oxidative or free radical damage