Expert: Testing for angle-closure glaucoma should be based on physiological factors

Iris volume change and choroidal expansion could be measured to assist in more accurate diagnosis of angle-closure glaucoma, a glaucoma specialist said.

Issue: August 2010

Dynamic processes should be examined when assessing risk of development and progression of angle-closure glaucoma, with an emphasis on physiological factors instead of only anatomical factors, according to a physician.

“By looking at anatomy alone and measuring things, we didn’t learn enough about angle-closure. We have to begin thinking about the dynamic processes in the eye,” Harry A. Quigley, MD, said.

He delivered the Edward Jackson Memorial Lecture at the opening session of the joint meeting of the American Academy of Ophthalmology and the Pan-American Association of Ophthalmology.

Dr. Quigley said physiological factors in addition to anatomy could possibly affect pressure changes in the eye.

Accurate diagnostic factors are increasingly important because of both the prevalence of glaucoma in aging populations and the high numbers of angle-closure glaucoma patients going blind, he said. In addition, there are few tools to assist clinicians in determining which narrow-angle patients will develop the disease.

Iris volume changes in light and dark conditions could be tested to better diagnose angle-closure glaucoma, along with iris dilator insertion by ultrasound biomicroscopy and measurement of provoked choroidal expansion, he said.

“[We should] replace what are now ineffective provocative tests for angle-closure glaucoma with tests that actually tell us whom to treat,” he said. “The stakes here are extremely high. In China, it is the difference between treating 5 million people and 50 million people who are at risk for this disease.”

He said that one of the puzzles of angle-closure glaucoma is why it occurs most commonly in Chinese populations and women. Because small-sized eyes are predisposed for development of angle-closure glaucoma, it had been suggested that Chinese populations had a higher rate of small eyes. However, research has shown that they do not.

“We’re finding that Chinese persons, Asians, Africans and Europeans have essentially the same distribution of anterior chamber depth and axial depth. Therefore, it isn’t that there are more small eyes among those predisposed populations,” Dr. Quigley said. “So, if Chinese persons have the same proportion of small eyes as other ethnicities, small-eyed Chinese persons must have other risk factors that lead to the disease.”

Iris volume change

Changes in iris volume due to light exposure could be an important predictor for angle-closure glaucoma, according to Dr. Quigley. Anterior segment optical coherence tomography can assist clinicians in observing the dynamics of iris change. When exposed to light, the angle opens; when light is removed, the angle closes. Upon dilation, the iris is smaller, enlarging as the pupil shrinks.

With anterior segment OCT, Dr. Quigley showed how the iris most likely loses substantial volume when the pupil is dilated. Because the iris is like a sponge, water moves in and out of it at a rapid pace. The phenomenon of the iris changing in volume would differ between individuals, he said. Narrow angles tend to close more often, losing less volume, which could obstruct the trabecular meshwork.

“When we examined a group of angle-closure and open-angle glaucoma patients, we found that, statistically, those eyes with angle-closure lose less volume when the iris is in a dilated position compared to those with open angle-closure,” Dr. Quigley said. “So, here’s a … physiological risk factor: change in iris volume and change in pupil diameter. Angle-closure eyes have less change.”

Two large longitudinal studies are being conducted in China that are examining iris volume change in relation to angle-closure glaucoma.

Choroidal expansion

Choroidal expansion could be another predictor for angle-closure glaucoma, Dr. Quigley said. Many physicians have observed positive pressure during surgery when the iris has prolapsed. Dr. Quigley noticed that the positive pressure phenomenon occurred most frequently in angle-closure cases.

The phenomenon could be caused in angle-closure cases by choroidal expansion that increases volume and eye pressure. It could occur if the permeability variable in the choroid is altered, causing protein and water to escape out of blood vessels in the eye, he said. That, in turn, could cause swelling of the choroid.

“If it swells by 100%, the eye pressure will rise to 70 mm Hg or 80 mm Hg instantly,” he said. “Sudden volume has been taken up, and something has to equalize it. When the choroid swells in a narrow angle eye, the lens moves slightly forward, occluding the iris lens channel. In a predisposed eye, the iris bows forward and angle-closure, as a phenomenon, occurs.”

Some patient groups with a higher rate of angle-closure glaucoma, such as women, might have a tendency toward choroidal expansion, he said.

“In eyes with a narrow iris lens channel and an expandable choroid, the lens will move slightly forward, close the iris lens channel and initiate primary angle-closure,” Dr. Quigley said.

Testing involving choroidal expansion could assist in determining angle-closure glaucoma.

“We can envision measuring choroidal expansion and provoking it in a controlled situation,” he said. – by Erin L. Boyle

Reference:

Quigley HA. Angle-closure glaucoma — simpler answers to complex mechanisms: LXVI Edward Jackson Memorial Lecture. Am J Ophthalmol. 2009;148(5):657-669.

Harry A. Quigley, MD, can be reached at Wilmer Eye Institute, Johns Hopkins University, 600 N. Wolfe St., Wilmer 120, Baltimore, MD 21287, U.S.A.; +1-410-955-6052; fax: +1-410-955-2542; e-mail: hquigley@jhmi.edu.