The Need for a Revised Approach to Epidemiological Monitoring of the Prevalence of Visual Impairment

The distribution of the leading causes of world blindness has evolved over the years. Available estimates of the causes of blindness throughout the world suggest a remarkable reduction in the proportion of blindness due to the causes of blindness most readily addressed through public health oriented programs, such as onchocerciasis, Vitamin A deficiency, and trachoma.^1,2 In contrast, conditions that require individual level ophthalmic services to prevent blindness—cataract, glaucoma, diabetic retinopathy and others—increasingly predominate as the major causes of blindness worldwide.¹ Scaling up of the training and retention of ophthalmologists may be the challenge of the day for the second decade of the Vision 2020 initiative and beyond. As we go forward, we will continue to require unbiased data regarding the major causes of blindness to evaluate where the problem of blindness stands and what progress is being made throughout the world.

Kuper, Polack and Limburg, among others, have taken the lead in providing us with a valuable practical tool for assessing the prevalence of major blinding eye conditions, the Rapid Assessment of Avoidable Blindness (RAAB).³ This approach makes the conduct of a population-based survey that is representative of a population achievable for a motivated ophthalmic leader, by providing software to walk the user through the statistical and epidemiological complexities which otherwise might shipwreck a noble vision to collect data regarding vision loss. Indeed, the number of surveys completed has expanded greatly since the RAAB methodology became available.

Standardization of the method also provides interesting opportunities to combine (or compare) data across surveys in a sort of meta-analysis. For instance, pooling of these data potentially could be used to assess what the “other” causes of visual impairment are which in aggregate make up the second leading non-refractive cause of vision loss worldwide after cataract,¹ but which individually have a prevalence too low to estimate in a single survey of achievable scale. The downside of the approach is that the analytic methods are pre-specified, and users rely upon a fixed algorithm for determining the proportion of blindness attributable to various causes, based on the WHO/PBL Eye Examination Record (Version III) method endorsed in 1988 by WHO. This approach is “…not designed to collect information about every eye condition likely to be encountered in field surveys. It provides for the recording of blinding eye conditions which are of public health significance.”⁴ It may be time for a revision and/or clarification of this method.

The major area in which the WHO/PBL Eye Examination Record (Version III) method needs attention is with regard to attribution of cause of blindness: While the system correctly indicates that if one disorder is secondary to another (e.g., cataract due to anterior uveitis), the underlying condition (anterior uveitis) should be identified as the cause of blindness at the level of the eye. The problem comes when there are co-existing primary disorders (such as one in each eye), when the disorder which is “most readily curable or, if not curable, that which is most easily preventable” [emphasis in the original] is to be counted as the cause of visual loss.⁴ I am concerned that in using this method we are guilty of a misclassification bias that overestimates the true extent of avoidable blindness. If we explicitly attribute blindness to the most treatable or preventable among various conditions in 1988, we “bake in” a misclassification bias based on 1988 clinical and public health capabilities, rather than providing unbiased data on which to base future policy. What is treatable and preventable changes over time, and the data from these surveys should be pointing out the need for strategies that make problems such as diabetic retinopathy and glaucoma more preventable or treatable, rather than de-emphasizing them systematically.

Other remediable problems I would like to suggest for the next revision of RAAB methodology include the lack of ability to compare results with “blindness” data using the other widespread cutoff defining blindness as 6/60 or worse often used in US studies; while US standards need not be used, it would be valuable to be able to report data in such a way that they could be compared to these studies if desired, which would not be difficult to achieve. Use of “pinhole” visual acuity as the definition of refractive error also is a problematic compromise, as non-refractive conditions (notably cataract) may improve with pinhole as well, potentially leading to overestimation of the extent of refractive errors at the expense of more complicated conditions. As the cost of equipment for estimating refractive errors comes down, it should become more feasible to demonstrate that vision loss suspected as being due to refractive errors is in fact reversible by refraction. Finally as mentioned previously, it would be very useful if the method allowed the coalescing of data regarding the prevalence of less common conditions (e.g., pediatric cataract, amblyopia, uveitis, retinal degenerations, and others). While these conditions individually contribute less to the burden of disease than cataract, some of them may now contribute more to blindness than conditions still ascertained for historical reasons and because of their susceptibility to public health interventions, such as onchocerciasis and Vitamin A deficiency. We may never know the extent to which these conditions can be addressed efficiently, unless prevalence data are collected, prompting the development of practical solutions. Also, promoting a culture which, de facto, dismisses them as unimportant in blindness alleviation is not appropriate if we are to embark upon efforts to promote clinical excellence, which we will need to do if we are to scale up our ability to address the new leading causes of blindness (most of which require individual-level clinical care by clinical experts).

In this issue of Ophthalmic Epidemiology, Müller and colleagues report results from a Rapid Assessment of Avoidable Blindness (RAAB) which are instructive at a number of levels for the blindness prevention community, including with regard to these issues regarding the collection and analysis of blindness prevalence data. At the most straightforward level, the results show us that blindness remains a major problem in Eritrea, and that diseases such as cataract and glaucoma which require ophthalmic care are an important cause of blindness, as in several other surveys from Africa and elsewhere in the world.^5–15 While more than half of severe visual loss or blindness was attributed to cataract, only about 1% was attributed to surgical complications. However, in reading further (as noted by the authors), we find that blindness due to complications of cataract surgery must be one of the leading causes of blindness in single eyes, because 182 eyes (39.2% of those operated) present with visual acuity worse than 6/60. Thus, while the WHO/PBL Eye Examination Record (Version III) analysis would suggest that poor surgical outcome is a minor cause of visual loss in the population, it is interesting to consider whether patients who have lost one eye to cataract surgery would be willing to undergo cataract surgery in the other eye. Or would they instead decide they are “too old”, “unaccompanied”, have decided to wait for “maturity” or have a “contraindication” to surgery? Indeed, if half of the patients with eyes presenting with severe visual loss or worse following cataract surgery refused surgery in the second eye—and were coded as having visual loss attributable to cataract, following WHO/PBL attribution procedures—poor outcome of surgery would be a cause of vision loss not much lower than cataract. Here is an important subset of “avoidable” blindness that is not well assessed by our present analytic methodology, yet has profound importance in using surveys to assess the success of interventions to address avoidable blindness. For the reasons previously mentioned, the survey also provides no information about less common causes of blindness that could be pooled with the results of other surveys to develop prevalence estimates.

As stated previously, in most locations, it is likely that the leading causes of blindness in the present and in the decades to come are predominantly conditions which require clinical care. The results of the RAAB in Eritrea corroborate findings elsewhere that suggest the quality of cataract services we have been offering, at least in years past, is not sufficient, which may be part of the reason the cataract problem persists. In a situation like this, operated patients may serve as aphakic (or pseudophakic) de-motivators in their community for decades, hampering blindness alleviation efforts for many years. Acceptance of poor quality because of the urgency of immediate need may therefore greatly hinder long-term efforts to alleviate blindness from a population perspective.

Many other RAAB surveys have found a high prevalence of blindness due to complications of cataract surgery in various parts of the developing world.^{5,6,8–10,14,15} In contrast, Powe and colleagues found that 89.7% of all eyes with cataract operated in the United States mostly during the 1980s—when conventional extracapsular surgery predominated—achieved visual acuity of 20/40 or better.¹⁶ While developing country cases with advanced cataracts and possibly neglected co-morbidities likely are fundamentally more difficult and less likely to lead to good outcomes than US cases were in the 1980s, and the method of ascertaining visual outcome is non-identical, there appears to be a large quality gap. In order for cataract surgery to serve as a driver of a robust eye care system, as it needs to, quality will have to be better. I would argue that a strong emphasis on outstanding training of expert clinician-surgeons, including ongoing follow-up quality control efforts, should be viewed as one of the most important priorities in blindness alleviation. The same consideration presumably applies to conditions requiring individual-level clinical care—diabetic retinopathy, glaucoma, childhood blindness and likely others—which also need to be scaled up.

In health crises, there clearly is a role for both relief (to address acute emergencies) and development (to cultivate a sustainable health system). Drawing upon this perspective, it has been argued that the vast prevalence of cataract blindness argues for an urgent response, in which there is no time to train full-fledged, highly trained ophthalmologists. However, the issues of blindness alleviation primarily require development for long-term population-level impact. I would argue that we would be better off spending a small amount of extra time developing a cadre of robustly trained specialists rather than compromise our training approaches in a way that could compromise the all-important level of quality, and might also impede the future development of a robust ophthalmic profession that would be the desirable outcome of development input. A longer training period will slightly lengthen the lag phase of the growth curve in providing clinical services such as cataract surgery, but failing to develop a robust profession of ophthalmology will be to fail in a primary development objective. If we accept that ophthalmologists are needed to deal with the problem of blindness alleviation, we need to accomplish the goal of developing a flourishing profession within which the best and brightest will be happy to practice. If it is concluded that sub-ophthalmologist cataract surgeons are needed to accomplish relief objectives in the brief lag phase before ophthalmologists could be trained, there should be plans in place to make sure these efforts and activities do not impede the development of the definitive ophthalmology profession.

In summary, several revisions to the 1988 standards for classification of blindness and visual impairment in rapid population surveys are needed to further empower the practical and valuable RAAB methodology to provide needed data worldwide. Relatively simple adjustments could improve the quality of data obtained, and provide additional information that is urgently needed. Results from such surveys indicating a high population prevalence of blindness due to cataract surgery suggests that ensuring high quality surgery should be one of our highest priorities in seeking to scale up blindness alleviation efforts. High quality is critical to fully empower cataract surgery as a driver of the individual patient-level interventions that increasingly are needed to address the leading causes of blindness worldwide.

ACKNOWLEDGMENTS

a. Funding/Support (including none): Dr. Kempen receives research support from the National Eye Institute Grant R01 EY014943, National Eye Institute Grant U10 EY014655, Research to Prevent Blindness, and the Paul and Evanina Mackall Foundation. None of the sponsors had any role in the preparation, review, and approval of this manuscript.

b. Financial Disclosures: John H. Kempen: (Consultant, C) Lux Biosciences, (C) Alcon, (C) Allergan, (C) Sanofi Pasteur.

c. Contributions of Authors: Writing the Article (JHK); Critical Review of the Article (JHK); Final Approval of the Article (JHK)

d. Statement about Conformity with Author Information: This editorial does not constitute original research utilizing human or animal subjects and therefore approval or exemption Institutional Review Board and Institutional Animal Care and Use and HIPAA compliance are not applicable.

e. Other Acknowledgments: None