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Editors Selection IGR 12-2

Population Genetics: Glaucoma in Africans versus Europeans

Comment by George Lambrou on:

26213 African Descent and Glaucoma Evaluation Study (ADAGES): III. Ancestry differences in visual function in healthy eyes, Racette L; Liebmann JM; Girkin CA et al., Archives of Ophthalmology, 2010; 128: 551-559

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It is established and well-documented by now that African ancestry is not only a risk factor for developing glaucoma, but also that glaucoma patients of African descent are more prone to visual impairment and to faster progression towards severe field loss and blindness, than patients of European descent. But are these differences limited to glaucomatous eyes? What happens before the disease develops? And do visual function differences exist in the normal population? These are the questions that Racette et al. (545) set out to explore, using data collected from 760 participants of African (n = 393) or European (n = 367) descent, recruited in two major epidemiological studies, ADAGES and DIGS.

The sample analyzed here consisted exclusively of 'healthy' patients, defined as having no history of glaucoma treatment, IOPs below 22 mmHg and normal appearance of the disc on stereo photographs. All included eyes had a t least two SITA visual fields and, if possible, two SWAP fields and two FDTs. The authors, however, excluded on purpose these functional tests from the criteria of 'healthy' eyes, in order to avoid a selection bias. The data were analyzed first unadjusted, then after adjusting for potential covariates such as age, C/D ratio, disc size, central corneal thickness and blood pressure. The results are unambiguous: small, but statistically significant differences exist, showing ‐ in the absence of any detectable structural change to the disc ‐ a slightly worse visual performance for persons of African, vs. those of European descent. These differences are more prominent when the aforementioned covariates are taken into account.

In the absence of any detectable structural change to the disc a slightly worse visual performance for persons of African, vs. those of European descent is apparent

What can be the causes of those differences? With all past evidence of earlier and faster progression of glaucomatous visual impairment in African-ancestry patients, a potential explanation springs to mind: the study may be detecting early signs of visual field loss in subjects recruited as healthy, but who are in fact in the preclinical stage of the glaucoma continuum. The validity of this hypothesis can only be tested by longitudinal follow up of these patients and the authors have already undertaken this task. Another possible explanation is that the normal range of the visual function is different in the two ethnic groups and that this shows as 'poor' performance of African-ancestry patients evaluated against normative databases built predominantly from visual fields of European patients. Finally, experiential, educational and socioeconomic factors may have introduced a bias, although the authors have taken special care to avoid this, e.g., by analyzing separately the first and second of the SAP fields and finding similar results in both cases.

Prospective research may shed light on differences in ethnic ancestry and perhaps explain the higher risk of glaucomatous visual loss in patients of African descent

Overall, this is the first study designed and powered to investigate ethnic ancestry differences in visual function. The results are unambiguous and highlight the need for additional prospective research to shed light on these differences and perhaps offer an explanation for the higher risk of glaucomatous visual loss in patients of African descent.

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