Oxidative Metabolism in the Eye Differs According to Race

Pauline Anderson

July 13, 2011

July 13, 2011 — Researchers have discovered that patients of African descent with glaucoma have higher oxygen levels in different parts of their eyes, which might help explain their higher rate of this eye disorder compared with whites.

"The significantly higher intraocular PO2, detected in African American patients is likely to reflect an important underlying physiological difference" write the authors of the new study. "The amount and distribution of oxygen in the eye are an indication of the metabolic activity of the ocular tissues."

The study, led by Carla J. Siegfried, MD, professor of ophthalmology and visual sciences, Washington University, St. Louis, Missouri, appears in the July issue of the Archives of Ophthalmology.

To test the hypothesis that oxygen or an oxygen metabolite, such as superoxide anion or hydrogen peroxide, damages cells and leads to increased intraocular pressure, researchers at Washington University School of Medicine measured the PO2 in the eyes of 72 patients (53 white, 19 of African descent) undergoing surgery for glaucoma and/or cataracts.

Oxygen Measurements

The researchers introduced an optical oxygen sensor into the anterior chamber and positioned the tip of the flexible fiber optic probe to get 3 measurements: near the central corneal endothelium, in the midanterior chamber, and in the anterior chamber angle. If fluid does not drain properly from the anterior chamber angle, pressure builds up, causing optic nerve damage and, eventually, vision loss from glaucoma.

The researchers took 2 additional measurements in patients having cataract extraction: at the anterior lens surface and in the posterior chamber just behind the iris.

The study found significantly increased PO2 in all 5 sites in the eyes of patients of African descent compared with white patients. For example, the mean PO2 in the anterior chamber angle was 11.7 mm Hg among whites and 16.6 mm Hg in patients of African descent (P = .009).

After adjusting for age, the differences between the racial groups became more significant at all 5 locations, revealing an effect of age on ocular oxygen metabolism. Adjusting for race revealed a significant age-dependent increase in PO2 beneath the central cornea and in the midanterior chamber.

Glaucoma Prevalence

Because all the patients in this study had cataracts, glaucoma, or both, the results could not determine whether the elevated PO2 in the eyes of patients of African descent actually predisposed them to either disease. However, people of African descent have a significantly greater risk of developing glaucoma compared with white individuals; the prevalence among people of African descent is 6 times greater in some age groups. Primary open-angle glaucoma is the leading cause of blindness among patients of African descent and is 16 times more likely to result in blindness in them than in whites, according to background information included in the study.

Exposure to increased intraocular oxygen could increase oxidative stress, elevate pressure, and damage the drainage system in the eye, contributing to this increased glaucoma risk among patients of African descent, write the authors. "Oxygen reacts with the high ascorbate in the ocular fluids, leading to the production of hydrogen peroxide," which could potentially overwhelm normally protective ocular mechanisms.

Excess oxygen could directly increase production of reactive oxygen species in the cells of the lens and the aqueous outflow system or the trabecular meshwork, the cells of which are damaged in patients with open-angle glaucoma, state the authors.

The differences in oxygen use could also reflect differences in systemic physiology, they add.

Race Self-Reported

The study was not designed to measure racial differences. The number of patients of African descent in the study was relatively small. Another limitation was that race was self-reported. "In future studies, genetic classification would provide a more objective and accurate definition of racial backgrounds to evaluate disease association," write the authors.

Future plans include further analysis of corneal oxygen consumption using noninvasive methods. "If such noninvasive tests correctly predict intraocular PO2 in patients, they could be useful for larger-scale clinical epidemiologic and genetic analysis," the authors note.

The oxygen metabolism differences identified in this study may be significant in areas other than the eye. "Understanding the biochemical and genetic basis of differences in oxygen metabolism could be relevant to racial variation in the prevalence of systemic disease or drug metabolism and function," the authors write.

The study was supported by an American Health Assistance Foundation National Glaucoma Research Grant, the National Eye Institute, and an unrestricted grant from Research to Prevent Blindness. The authors have disclosed no relevant financial relationships.

Arch Ophthalmol. 2011;129:849-854. Abstract