Ethnic variations factor into OAG

Various issues impact the effectiveness of population screening for open-angle glaucoma (OAG).

However, use of ethnicity-specific normative databases for structural tests may improve their diagnostic performance, said Dr Mingguang He, PhD.

"Available evidence indicates that [more than] 50% of OAG in industrialized countries is undetected," said Dr He, deputy director and professor, Zhongshan Ophthalmic Center, Sun Yat-Sen University, Guangzhou, Peoples Republic of China. "Identification of these people is a challenge and indicates the need for screening tests that are accurate and reproducible."

Age-specific prevalence of the disease was highest in Africa and Latin America, followed by China. The prevalence was similar in India, Japan and Europe.

Considerations on screening method

No one diagnostic test is considered a gold standard for OAG diagnosis.

Whereas optic disc stereo assessment for optic disc damage and standard automated perimetry to identify visual field loss represent the usual methods employed in clinical trials, the best criterion may be progressive change in the optic nerve, Dr He explained.

A number of structural tests exist for assessing the optic disc and retinal nerve fibre layer (RNFL), and there are also several methods to assess function.

Results of a study published in 2005 by Wollstein et al. evaluating the diagnostic efficacy of these different techniques and technologies indicated that optical coherence tomography was better than frequency-doubling technology (FDT) and scanning laser polarimetry followed by short-wave automated perimetry.

In a systematic review and meta-analysis of screening tests for detecting OAG, Mowatt et al. concluded FDT, oculokinetic perimetry and scanning laser tomography were the most promising tests.

Weighing efficacy

The efficacy of a screening test must be considered in the context of population prevalence, he noted.

Assuming a test has 95% sensitivity and specificity - even in a population where the prevalence of glaucoma is relatively high, such as among older Africans and Latin Americans where the prevalence is about 5% - the test would still have a 50% false positive rate.

If used in a population where the prevalence of OAG is only 3% (e.g., Chinese), the false positive rate is 72%, and when the prevalence rate drops to 2%, as in older Japanese, European and Indian cohorts, the false positive rate rises to 84%.

"These false positive rates represent the proportion of screened individuals who will be referred unnecessarily for healthcare," Dr He said.

Further complicating the effectiveness of community screening is the fact that a high proportion of undiagnosed OAG in the population is a mild form of the disease. Distribution data on structural features show there is a lot of overlap between these individuals and the eyes of normal persons without glaucoma.

"Looking at RNFL thickness, for example, it is easy to differentiate eyes with advanced glaucoma from those that are 'super normal'," Dr He said. "However, it can be challenging to separate early glaucoma from normal eyes... (which) will result in a lot of false positives and false negatives."

The databases for some structural diagnostic platforms are also based on 'normal' eyes defined using arbitrary criteria, but what is normal may differ in different ethnic groups.

However, after adjusting for the latter, there were no significant ethnicity-related differences for other disc parameters.

Dr He noted that the latest generation of a retinal tomographer (Heidelberg Retina Tomograph III, Heidelberg Engineering, Heidelberg, Germany) integrates data from various ethnic groups in its normative database and that research using this platform with the Moorfields regression analysis and glaucoma probability score found that it performed similarly in detecting glaucoma across different ethnicities.