Review of The Epidemiology of Eye disease

In the next several blogs I would like to review and summarize : Epidemiology Of Eye Disease 3rd edition by Gordon Johnson, Darwin Minassian, Robert Weale , and Sheila West. Imperial College Press. ISBN-13 978-1-84816-625-7.
This book is a must for anyone interested in epidemiology of the eye and developing world ophthalmology --- what we know, where we are currently for the reduction of blindness worldwide, and the way forward. As usual I will not give credit to some authors, paraphrase freely and add in my comments. If you are interested in developing world ophthalmology then here’s a book worth purchasing --- “an instant classic”.

Al Sommer in the foreword to the second edition defines epidemiology as, at its heart, mere counting ---- but ‘ counting ‘ of a practical and insightful sort. Establishing the magnitude and distribution of disease is fundamental to designing effective intervention strategies. He also makes reference to Ida Mann who is considered by many to be the father of ophthalmic epidemiology in the developing world. An ophthalmic leader / giant before her time. Truly we see so much from standing on her shoulders.
Hugh Taylor states in the foreword to the third edition that one of his mentors used to say that epidemiology was “ only organized common sense “ [ about vision ]. The trick is to learn how to do that organizing --- to be mindful of all the potential biases in the inclusion or exclusion of those data to be counted. Epidemiology is important to ophthalmology because it gives us clear indications as to how to manage the problems we encounter in delivering eye care as well as in managing individual patients. The epidemiologic aspects of a wide range of ophthalmic diseases and conditions are beautifully presented and each chapter is completed with a list of issues for further research. In addition, this work provides the wider framework for fashioning eye health care policy and practice that eventually leads us to the noble goal of Vision 2020 and the Right to Sight for All.
Section 1 Introduction ( Chapter 1 ) talks about the prevalence, incidence, and distribution of visual impairment. This chapter defines magnitude as the size of the problem of a condition or disease. The two measures used are prevalence and incidence. Prevalence is a static measure that provides a snapshot of the disease in the population at a particular point in time, and includes all cases of disease regardless of duration. Incidence is a more dynamic measure of the magnitude of the problem, and measures the number of new cases of disease in the population at risk ( that is, the population that does not already have the disease ) over a defined period of time.
Blindness is defined internationally as a VA of less than 3/60 ( < 20/ 400 ) in the better eye with the best possible correction , or a visual field loss in each eye to less than 10 degrees from fixation. In reality severe visual field loss is rarely used to designate blindness. Low vision is defined as VA of less than 6/18 ( < 20 / 60 ) but equal to or better than 3/60 in the better eye with best possible correction. Many countries have varying definitions for blindness which can make comparing apples to apples difficult. For example, in the USA the definition of blindness is based on vision less than 20/200 ( < 6/60 ). The highest prevalence of blindness is concentrated in sub-Saharan Africa, the Indian sub-continent, and SE Asia.
WHO 2002 data estimated the # of visually impaired people ( best corrected vision < 6/18, < 20/60 ) as 161 million of whom 37 million were blind ( < 3/60, <20/ 400 ) --- cataract blind being 47%, glaucoma 12%, AMD 9% , and diabetic retinopathy 5%. Fortunately blindness due to the poverty issues have decreased –-- onchoceriasis, trachoma, and xerophthalmia / childhood blindness. Blindness from corneal scarring / opacity ( unilateral or bilateral ) is much more common in the developing world than in the developed world. Blindness from diabetes has increased worldwide.
Childhood blindness is problematic to say the least. Often difficult to even identify these kids. With an average prevalence of less than one blind child per 1000 children aged 15 years or under in many developing countries, it is possible to examine several thousand children without finding a single case of blindness. Blindness is strongly age related.
From many glaucoma surveys, it has been shown that much glaucoma is not diagnosed and also many patients that do not have glaucoma but ocular hypertension or a large symmetrical C/D ratio are on treatment. That certainly has been my experience in subSaharan Africa, the Caribbean, and Central America. Although not always reliable, one of the best things you can do with glaucoma patients is to encourage them to have their family members checked for glaucoma. Unfortunately I think this is not routinely done. In the developing world, many glaucoma patients show up quite late with severe advanced bilateral disease.
If the presenting vision is considered then uncorrected refractive errors becomes a significant cause of low vision. In reality it’s often not the best corrected vision but the presenting vision which is important. Recently WHO has recommended that instead of best corrected vision , presenting vision should be used to define blindness. In addition, it is estimated that globally there are 1. 04 billion people with presbyopia and half of those have no or inadequate spectacles. My experience is donated used glasses from North America are not wanted nor useful. What can be helpful is reading glasses and myopic glasses. I once stayed at an ophthalmic guest house in a west African country and in one of the larger closets, I ‘discovered’ many boxes of abandoned used glasses that had been brought over from the States by well meaning volunteers. Usually those glasses eventually will get thrown out.
The rest of this chapter deals with prevalence data derived from various surveys, limitations of prevalence surveys, incidence of blindness, causes of blindness, patterns of distribution of blindness, geography variations, socio-economic factors, the concept of avoidable blindness, conclusions and epidemiological data.
The conclusion noted that the causes of blindness vary widely from region to region, within regions and even within countries. Globally, uncorrected cataract is the leading cause of blindness. The estimate for trachoma is now lower than for glaucoma. The populations affected are quite different, and it is likely that cases of glaucoma will continue to increase in the future whereas trachoma cases are expected to decrease further.
The high incidence of blindness after cataract surgery is a concern and is probably due to both poor ( no ) pre-op evaluation and also surgical complications. In many developing world clinics, the ophthalmic surgeon does not evaluate the pre-op cataract patient. All white / dense cataracts should not automatically be taken to the O.R. As I have written about previously, if you do that you will have some post-op disappointments. Checking for color ( red light ) perception, RAPD, and light projection will often avoid unnecessary / not useful surgery in dense, white cataracts.
Interesting, many studies have used literacy as a proxy indicator for socio-economic status. Many studies have shown that literacy is one of the most important determinants of the prevalence of blindness. The association of poverty with blindness has been studied extensively. A national survey in Pakistan showed that the prevalence of blindness caused by cataract, glaucoma , and corneal opacity was lower in affluent clusters and households. It has been stated that a combination of factors like poverty, lack of education, and inadequate health services were responsible to a great extent for the burden of blindness in Africa. Rural areas have significant more blindness than urban ones.

Thank you. To be continued.


Baxter McLendon


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