Oxygen Levels and Distribution in the Anterior Chamber: Impact of Cataract Surgery and Vitrectomy

Abstract

Purpose: : Previous studies suggest that oxidative stress plays a role in glaucoma development and that the lens may protect the outflow system from oxidative damage post-vitrectomy (Chang, AJO 2006). To explore the balance of oxidants and anti-oxidants in the anterior segment, we measured levels and distribution of oxygen (pO₂) and ascorbic acid (AsA) in patients undergoing intraocular surgery.

Methods: : Consenting patients undergoing cataract and/or glaucoma surgery were included. An Oxylab pO₂^TM optical oxygen sensor (Oxford Optronix) was introduced via 30G corneal paracentesis. In all patients, pO₂ was sampled in the anterior chamber (AC) angle, mid-AC, and near corneal endothelium. In pseudophakes and eyes undergoing lens extraction, the probe was introduced into the posterior chamber (PC; between lens and peripheral iris) and near the anterior lens surface. Aqueous humor samples were collected and AsA measured by colorimetric analysis.

Results: : 92 patients participated. In the reference group (eyes undergoing initial surgery), pO₂ was highest adjacent to cornea (22.1 ± 1.1 mmHg; ± SE), and decreased in a linear gradient to 3.1 ± 0.4 mmHg near the lens surface. pO₂ was also low in the PC (3.8 ± 0.6 mmHg). An intermediate pO₂ was measured in the AC angle (13.6 ± 1.0 mmHg). In the subgroup of eyes with prior pars plana vitrectomy scheduled to undergo cataract extraction (n=10), there was a significant increase in pO₂ in the PC (6.5 ± 1.0 mmHg; p=0.02) and a trend toward increased oxygen in the AC angle (17.8 ± 4.3 mmHg). A significant increase in pO₂ at the anterior lens surface (p=0.002) and in the PC (p=0.038) was noted in pseudophakes. Multivariate analysis revealed a strong correlation of pO₂ in the PC and the AC angle in all eyes (p<10^-4). A significant inverse relationship (p=0.03) was noted between central corneal thickness (CCT) and AC angle pO₂ levels. AsA levels in diabetics were significantly lower than non-diabetics.

Conclusions: : These in vivo measurements are the first to define oxygen gradients in the anterior segment of the human eye. The effect of vitrectomy and lens extraction on pO₂ levels in the PC and AC angle suggest an important influence of both vitreous and the natural lens in regulating oxygen distribution/metabolism within the eye and provide further evidence for the importance of these structures and of oxygen distribution in the development of glaucoma. The inverse relationship between CCT and pO₂ may account for the increased risk of glaucoma in persons with thinner corneas, a previously unexplained association.

Keywords: oxygen • oxidation/oxidative or free radical damage • antioxidants