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How common is glaucoma worldwide?

Harry Quigley

• Glaucoma as a cause of blindness

  Over ten years ago, I first attended a meeting of the World Health Organization (WHO) Blindness Prevention Program in Geneva. As slides were shown on the causes of world blindness, I saw cataract, trachoma, onchocerciasis, and keratomalacia, but not glaucoma. Whispering to a more experienced attendee, I asked: "Where's glaucoma on the list?" "Down under the Other category, I guess," he said. "Besides, it's too hard to diagnose and can't be treated, so they leave it off." I resolved to put the available data together and found that glaucoma was actually second only to cataract in causing disability among world eye conditions. At this year's American Academy of Ophthalmology meeting, I was asked to give a brief update on the impact of glaucoma.
  Cataract is the dominant cause of blindness at 30 million or more persons affected - to all our shame. Yet, recently updated calculations show that at least 7.6 million persons are bilaterally blind from the two glaucomas, open-angle (OAG) and angle-closure (ACG), by the WHO definition of worse than 3/60. Our data suggest that calculating additional blindness for the 'field blind' isn't helpful, as the vast majority of field blind are also acuity blind.

  Trachoma causes nearly as much blindness as the glaucomas, but is thankfully declining, due to economic development and eradication programs. Glaucoma will do the opposite as populations add more senior citizens everywhere, unless we act now. Other causes affect fewer than two million persons each, including macular disease and diabetes, which are infrequent where there are fewer older citizens and less obesity.
  

  ACG and OAG

  Among the 50 million persons affected by glaucoma, recent prevalence data suggest that 33 million have OAG and 17 million ACG. Detailed studies of OAG from a variety of countries have been supplemented by ACG studies carried out in Singapore and Mongolia. These suggest that bilateral blindness affects <10% of those with OAG, but 25-30% of ACG sufferers. Consequently, there are 4.3 million estimated blind from ACG and 3.3 million from OAG.

  Racial differences

  The prevalence of OAG is highest among African-derived persons, whether living in east Africa or east Baltimore. Black persons have nearly four times more OAG than other ethnicities, among whom it affects 2% of those over the age of 40 years. The proportion of OAG to ACG differs dramatically by race, with Chinese persons having five times more ACG than Europeans and Africans. Chinese have nearly as much ACG as OAG. Among all other ethnicities, OAG is five times more common than ACG. Our recent study of Hispanic Americans suggests more OAG in this group than in Europeans, with a rapidly rising prevalence in older Hispanics-equalling that of African-derived Americans by the age of 75 years. The different age distribution of those in developing and developed countries leads to interesting figures for the geographic location of glaucoma. China now has 21% of the world's population, but 30% of world glaucoma. Four other world regions have about 16-20% of the world's population each, and 16-20% of world glaucoma each (these are South Asia, India, Africa, and Europe/USA). This interesting uniformity derives from different combinations of the important variables: age distribution, total population, and glaucoma prevalence. The median age for those affected by OAG in Africa is 57 years, while among European-derived persons it is 77 years. This has important implications for case identification programs in the various areas.

  Half the cases of glaucoma go undetected

  All recent population surveys have used the disc and field to define glaucomatous neuropathy, and each finds that OAG persons have higher IOP (18.7 compared to 15.6 mmHg in Hispanic normals), but 50% or more have IOP of less than two standard deviations above the population mean IOP. The continued use of IOP screening (overt or subliminal) frustrates the proper identification of OAG, leading to a worldwide, estimated rate of disease diagnosis of less than 20%. Of course, in many areas, glaucoma simply isn't part of health care delivery, so almost all cases go undiagnosed. But, there is no excuse for the continued failure to diagnose glaucoma in almost HALF those affected in the USA, Australia, and Europe.

  Gender

  Another important risk factor is gender, with women being three times more likely to have ACG. There is no gender difference consistently found for OAG. Adding in the longer life span of women, this means that more than two-thirds of the glaucoma blind are women. Since cataract also affects women disproportionately, our mothers are crying out for action against these two treatable conditions.

  Small eyes

  The predilection for ACG among Chinese is particularly intriguing. Since population data suggest that ACG occurs more in small eyes, it was assumed that Chinese populations have more small-eyed individuals. This is NOT true, so factors other than small eye size must explain the higher prevalence. Physiological responses within small eyes must contribute to anatomical crowding.

  Visual field progression

  Every recent controlled clinical trial reports a visual field progression rate of about 3% per year for treated OAG. Since the average person in the USA with OAG has the disease for 15 years from initial field loss to actuarial death, then only 45% (15 x 3%) will measurably worsen under present management. This is consistent with my estimates that only 4% of Europeans and 8% of Africans become bilaterally blind from OAG. Estimates from clinic-based data in one USA study that are somewhat higher can be explained by the non-representative nature of the data and the lack of consistent diagnostic criteria. It is clear that screening programs should be designed to look for those who are most likely to be visually impaired in their lifetime, and not for every single 'case' of glaucoma.

  Rural areas and mortality

  Blindness is 100 times more common in rural, undeveloped areas of the world than in urban, developed settings. In these locations, it is not a quality-of-life issue, but a life and death one. For example, the mortality rate for the blind in Africa is four times that for the sighted.

  Action

  We must soon begin to implement the Action Plan outlined in the meeting Worldwide Glaucoma 2000, co-sponsored by the WHO. The overall report is available at www.wilmer.jhu.edu/research (look under 'Dana Center'). Among other recommendations, it points to the need for a portable glaucoma screening device. Several designs exist, but the probable lack of financial profit presently precludes their development. In addition, glaucoma blindness could be greatly reduced by determining who merits laser iridotomy for ACG, and how effective it is at preventing blindness at various stages of the disease. ACG is rapidly becoming, on its own, the second leading cause of world blindness. While it affects persons who are invisible to most ophthalmologists in developed countries, the solutions for this situation demand that we take some attention away from tube-shunts and laser imaging and put our brains and resources into efforts that allow the invisible to be seen and to see.