May 2008

 

OPHTHALMOLOGY NEWS

 

Making the glaucoma ancestry connection


by Maxine Lipner Senior EyeWorld Contributing Editor

 

 

New study delves into differences that may put those of African descent at increased glaucoma risk

Source: Christopher A. Girkin, M.D.

For those of African ancestry, the threat of glaucoma looms about four or five times larger than for those of European extraction, according to Christopher A. Girkin, M.D., professor of ophthalmology, University of Alabama at Birmingham. While this has been well known for some time, now investigators are gaining insight into what may be behind this. In a recent study e-published in the February 2008 issue of Clinical Ophthalmology, Dr. Girkin reviewed the literature to help bring to light anatomical differences between the eyes of glaucoma patients of African or European ancestry.

“For some reason the eyes in people of African descent or ancestry are at a greater risk to develop IOP-related injury,” Dr. Girkin said. “One of the things that had been previously described by several groups is morphologic differences in the structure of the optic nerve in individuals of African descent.”

Morphological differences

Dr. Girkin and fellow investigators at the University of Alabama previously showed in a January 2005 issue of Ophthalmology that for normal individuals of African descent the optic nerve is larger and the optic cup is deeper than for those of European extraction. Dr. Girkin believes. “A larger nerve with a deeper cup may possibly be more vulnerable at a given level of IOP; however, this needs clinical conformation in a longitudinal study,” he said. “The cup is larger, but there’s a similar neuro-retinal rimmed area in those of African versus European descent,” Dr. Girkin said. “If you take into account the larger disk, African Americans have a larger cup disk ratio just because the nerve is larger and thus there is a larger cup.” Once you adjust for the larger disk, the rim area is the same between the two groups. “There are similar gross amounts of neural tissue within the disc,” Dr. Girkin said. “It’s just entering and exiting the eye through a larger neural canal.”

Investigators here also determined that the cup was deeper in African Americans. “This may have some biomechanical significance,” Dr. Girkin said. “Either it could be that there’s more posterior insertion of the lamina cribrosa into the neural canal, or it could be that the lamina cribrosa is thinner.” In both cases this would show up as a deeper cup, he points out. “Both may have significant biomechanical impact that needs to be looked at,” he said.

In addition, some computational models suggest that the larger optic disc may be more vulnerable to stress. “There’s some preliminary evidence at least in computational modeling that a thinner lamina cribrosa and potentially a larger lamina cribrosa due to the larger optic disk may exhibit more strain and tissue deformation under a given level of stress,” Dr. Girkin said.

To address the impact of these issues, a group of investigators are conducting a multicenter longitudinal study sponsored by the National Eye Institute. The African Descent and Glaucoma Evaluation Study is coordinated by Pam Sample, Ph.D., and colleagues at the Hamilton Glaucoma Center, San Diego, with Jeffrey M. Liebmann, M.D., and his imaging research group at the New York Eye and Ear Infirmary, New York, and Dr Girkin’s groups at the University of Alabama.

Clinical impact

These structural differences may increase vulnerability of those of African descent to glaucoma. Dr. Girkin points to an engineering model on theoretical eyes described by Bellezza and colleagues in the September 2007 issue of Investigative Ophthalmology and Visual Science (IOVS) to help explain what may be taking place1.

“They found that if you increased the size of the nerve, in theory the lamina should experience more deformation or strain at a given load,” Dr. Girkin said. “It is sort of like a longer bridge that bends more than a shorter one.” If it turns out that the lamina cribrosa is truly thinner in those of African descent, this would in effect reduce the connective tissue support, resulting in a thinner, less structurally sound nerve,” Dr. Girkin explained.

In screening for glaucoma, keeping ancestry in mind can be important depending upon normative databases used. “It means that different normative cutoffs would be needed when you start using these devices to detect glaucoma in individuals of African decent, or you need to adjust for differences in rim area,” Dr. Girkin said.

He urges practitioners to keep in mind that some of the diagnostic algorithms are not as well suited to those of African-American ancestry. “You need to make sure that if you’re using the instruments to detect disease in a specific population that either you’re using the parameters that best work in that population, or, that novel detection algorithms are developed specific to that population that take into account these morphologic differences seen in racial groups,” Dr. Girkin said.

Overall, Dr. Girkin believes that there are two take-home points to consider from the research. “There is racial variation in the size and depth of the optic cup within the nerve fiber layer that may affect the ability to use these instruments within different racial groups—both to detect glaucoma and also with regard to the progression,” Dr. Girkin said. “The other issue which I think can drive basic research is that these morphologic differences within the optic nerve may relate to the biomechanical behavior of the optic disk and may explain some of the higher rates of glaucoma in this population.”

Editors’ note: Dr. Girkin has no financial interests related to his comments.

Contact Information

Girkin: 205-325-8110, cgirkin@uab.edu

References

1. Bellezza AJ, Hart RT, Burgoyne CF: The optic nerve head as a biomechanical structure: initial finite element modeling. Invest Ophthalmol Vis Sci 2000;41:2991-3000.

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