Aug. 07, 2014
Angle-closure glaucoma (ACG) differs from many eye diseases in that it is a preventable cause of permanent blindness if caught soon enough. ACG is also the second-leading cause of blindness in Asian populations throughout the world.
A new study focuses on the prevalence of asymptomatic angle closure in a predominantly Caucasian population and the efficacy of laser peripheral iridotomy (LPI) in treatment. "ACG has been regarded as an uncommon disease in non-Asian ethnic groups due to the low prevalence reported in large population-based research — 0.04 percent in the Beaver Dam study and 0.06 percent in the Melbourne study," says Syril K. Dorairaj, M.D., with the Department of Ophthalmology at Mayo Clinic's campus in Jacksonville, Florida. "Based on those results, ophthalmologists have tended to underuse gonioscopy during routine eye examinations for Caucasian populations."
In those studies, either gonioscopy was not performed or no details were reported regarding the method of diagnosis of angle status. In contrast, when 4,297 study participants from northern Italy were screened by ophthalmologists and diagnosis was obtained using methods including gonioscopy, primary angle-closure glaucoma (PACG) was reported in 0.6 percent of the patients. In a similar study of 1,636 participants in South Brazil (of whom 72 percent identified as white), prevalence of PACG was 0.7 percent.
"The high prevalence of PACG is significant, as it may be preventable if iridocorneal apposition is diagnosed and reversed early, before intraocular pressure (IOP) is elevated," says Dr. Dorairaj. "For PACG to be prevented by early LPI, however, it is essential that darkroom gonioscopy be performed routinely and asymptomatic angle closure has to be diagnosed."
ACG in Caucasians
It is difficult to formulate guidelines for screening unless the prevalence of asymptomatic angle closure in Caucasians is known. Dr. Dorairaj and his team will screen 5,000 volunteer Caucasian hyperopes. "A large sample is required because previous studies have shown less than 1 percent iridocorneal apposition," says Dr. Dorairaj. "From this study, if only 50 cases are detected and only 25 participants agree to follow-up after LPI surgery, there will be enough data for examining the efficiency of LPI."
All included subjects will undergo detailed ocular examination including:
- Visual acuity
- Estimation of intraocular pressure by Goldmann applanation tonometry
- Darkroom indentation gonioscopy
- Optic nerve examination
Once detected to have appositional angle closure, study participants will undergo ultrasound biomicroscopy (UBM) and axial length estimation.
Peripheral laser iridotomy
Effect of background illumination on occludable angle opening distance
Effect of background illumination on occludable angle opening distance on ultrasound biomicroscopy testing (UBM). A. Open angles with lighted condition. B. Same angle with UBM under dark background illumination, showing appositional angle closure, increasing the chances of angle-closure detection.
Dr. Dorairaj's team will also examine the efficacy of LPI in widening appositionally closed angles in study participants via a combination of surgical and theoretical methods:
- Participants who are identified as candidates of asymptomatic angle closure will undergo LPI after the anterior segment is imaged with ultrasound biomicroscopy (UBM).
- Axial length and keratometry will be measured before and after LPI.
- Pupil dilation and constriction contributes significantly to the changes in the iris contour, so the anterior segment will be imaged over a period of time in which the patient is exposed to different light conditions. The images will be analyzed to characterize the iris contour.
The team will measure iris concavity, cord length, angle opening distance and angle recess area to develop a finite element model based on the images obtained from clinical examination and UBM images. After LPI is performed, the anterior segment will be imaged via a procedure identical to the pre-LPI screening and the images will again be evaluated.
"Our goal is to assess the efficacy of LPI in these patients and identify patient-specific LPI guidelines," says Dr. Dorairaj. "This work will be a step toward development of diagnostic and predictive computational models of the anterior eye, to detect angle closure at its early stages and to treat it effectively."
Test early and often
"Testing for ACG should be provided early and at every patient visit," says Dr. Dorairaj. "Skillful use of darkroom gonioscopy and correct interpretation of its findings may help ophthalmologists to recommend prophylactic LPI only in the eyes of patients who truly need it."
For more information
Contact Syril K. Dorairaj, M.D., at 904-953-2377 or firstname.lastname@example.org.