Oxygen distribution in the human eye: relevance to the etiology of open-angle glaucoma after vitrectomy

Abstract

Purpose: Vitrectomy, when followed by cataract surgery, increases the risk of open-angle glaucoma. This study was conducted in patients to determine whether these procedures are associated with increased exposure of the trabecular meshwork to oxygen.

Methods: Oxygen distribution was recorded with a fiberoptic probe in patients undergoing surgery for cataract, glaucoma, or retinal disease. pO(2) was measured beneath the central cornea, in the mid-anterior chamber, and in the anterior chamber angle. In patients who were pseudophakic or were scheduled for cataract extraction, pO(2) was also measured in the posterior chamber and near the lens.

Results: Eyes with no previous cataract or vitrectomy surgery had steep oxygen gradients in the aqueous humor between the cornea and lens. pO(2) was low in the posterior chamber and near the lens. Previous vitrectomy was associated with significantly increased pO(2) in the posterior chamber. Eyes with previous cataract surgery had significantly elevated pO(2) only in the posterior chamber and in front of the intraocular lens (IOL). Eyes that had both vitrectomy and previous cataract surgery had increased pO(2) in the posterior chamber, anterior to the IOL, and in the anterior chamber angle. pO(2) in the posterior chamber and the anterior chamber angle correlated strongly.

Conclusions: Oxygen metabolism by the lens and cornea establishes oxygen gradients in the anterior segment. Vitrectomy and cataract surgery increase pO(2) in the anterior chamber angle, potentially damaging trabecular meshwork cells. We propose that oxygen levels in the anterior chamber angle are strongly influenced by oxygen derived from the ciliary body circulation.

Figure 1.

Oxygen distribution in the anterior of the human eye. (A) Oxygen distribution in the reference group (no previous cataract surgery or vitrectomy), (B) after previous vitrectomy, (C) previous cataract surgery, (D) or previous vitrectomy and cataract surgery. Red dots: oxygen measurements made by corneal entry during glaucoma and/or cataract surgery; yellow dots: oxygen measurements made through pars plana entry at the beginning of vitrectomy surgery. In the table (E), the significant differences in pO₂ are identified in different regions of the anterior segment when the reference group and the different surgical groups are compared. Statistical analysis was by ANOVA, with Bonferroni correction for multiple comparisons.

Figure 2.

Oxygen distribution in the nonsurgical eye and the proposed effect of vitrectomy and/or cataract surgery on oxygen delivery to the anterior chamber angle. (A) In the nonsurgical eye, oxygen enters from the retinal vasculature through the vitreous, across the ciliary epithelium from the ciliary body vasculature, and into the anterior chamber across the cornea. Oxygen is consumed by the lens and the ciliary epithelium. A small amount of oxygen enters the anterior chamber angle by diffusing across the ciliary body and iris stroma (curved red arrow). This is the same pathway as that of the plasma proteins (gold arrow). (B) After vitrectomy, more oxygen reaches the posterior chamber. This supplies more oxygen to the “aqueous surface” of the ciliary epithelium, reducing the amount of oxygen that the ciliary epithelium removes from the blood and slightly increasing the amount of oxygen available to enter the anterior chamber angle from the ciliary body stroma. (C) Cataract surgery reduces oxygen consumption by the lens, thereby increasing the pO₂ anterior to the lens and in the posterior chamber. The increased oxygen on the aqueous surface of the ciliary epithelium reduces the amount of oxygen that the ciliary epithelium removes from the blood, thereby slightly increasing the amount of oxygen available to enter the anterior chamber angle from the ciliary body stroma. (D) After vitrectomy and cataract surgery, significantly more oxygen is available on the aqueous surface of the ciliary epithelium, resulting in the removal of significantly less oxygen from the blood. This process increases the amount of oxygen available to diffuse from the ciliary body stroma, across the iris stroma, and into the anterior chamber angle, exposing the outflow system to a large excess of oxygen and/or oxygen metabolites. (Diagrams modified with permission from an illustration by Keith Kasnot for Merck & Co., Inc. ©2010. Phoenix, AZ: Kasnot Medical Illustration, Inc. All rights reserved.)