4.1 Aims of chapter
This is the last section of my literature review. In this chapter I summarise findings from studies reporting the prevalence and incidence of diabetic retinopathy (DR) and diabetic maculopathy in African countries in light of the rising prevalence of diabetes mellitus. In the final part of the chapter I bring together findings from the literature review and present the hypothesis and aims of my thesis.
4.2 Introduction
The International Diabetes Federation (IDF) has estimated that the number of adults with diabetes in Africa will increase by 98%, from 12.1 million in 2010 to 23.9 million in 2030 [207] a presumed consequence of poor diet, sedentary lifestyles, obesity, population growth and ageing (in part due to successes in combating communicable diseases) [199]. 31 of the 48 least economically developed countries, as defined by the UN, are situated in Africa [208]. The epidemic rise in diabetes therefore poses significant public health and socioeconomic challenges for the continent.
As described in Chapter 1, diabetes causes visual impairment through cataract and DR, a progressive disease of the retinal microvasculature. DR can be graded on the basis of the clinical features as detailed in Chapter 2. The grades of retinopathy correlate with likelihood of development of proliferative DR and can be
standardised by standard retinal photographs as used in the Early treatment of Diabetic Retinopathy Study (ETDRS) [19]. DR is the sixth leading cause of global blindness [8], and is one of nine target disease areas of the 'Vision 2020 action plan' a joint program of the WHO and the International Agency for the Prevention of Blindness. It is the leading cause of blindness in the working age population in the USA and in many European countries (and was until recently in the UK) [209]. With
69 increasing prevalence of diabetes in Africa, it is likely that DR will become an
important cause of blindness. On this continent genetic factors, limited access to healthcare, and high rates of malnutrition, infectious disease, HIV and anaemia are likely to affect the spectrum of pathology encountered.
An overall assessment of current levels of DR in Africa has not been performed previously. In order to estimate the current and future burden of disease, to provide data to enable the assessment of changes that may result from service development, and to inform future research a systematic review of the
epidemiological literature was required. The aim of this systematic review was therefore to summarise findings from reliable research studies of estimates of the prevalence and incidence of diabetic retinopathy and maculopathy in African countries. I completed this review under the supervision of Professor Paul Garner at Liverpool School of Tropical Medicine (LSTM); the results have been published [210].
4.3 Methods
4.3.1 Data sources and search strategy
I performed a systematic narrative review of published literature according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [211]. Relevant studies published between 1948 and February 2011 were identified by searching, using a pre-defined strategy, the following electronic databases: Medline (Pubmed), Embase (OVID) and Embase Classic, Science Citation index and Conference Proceedings Citation index (ISI Web of Science). The
following were also searched: the African regional database 'African Index Medicus', the grey literature database 'OpenSIGLE', the WHO International Clinical Trials Registry and the meta-Register of Controlled Trials (mRCT). I developed customised searches with the help of Vittoria Lutje, a Cochrane Collaboration trained trials co-ordinator at the Cochrane Infectious Diseases group, LSTM. Search
70 histories are reproduced in Appendix 2. No language, publication status, time limits or language restrictions were applied to electronic searches. Search results were merged using reference management software (Endnote, Thomson Reuters) and duplicate records removed. The reference lists of articles identified, including existing reviews, were hand-searched.
4.3.2 Selection criteria
The following were included: studies reporting prevalence or incidence or
progression of DR or diabetic maculopathy; cross sectional or cohort study design; studies of subjects with diabetes mellitus resident in African countries. Exclusion criteria were: studies with fewer than 50 subjects; studies of populations of African origin residing outside the continent; reports not published in English; case series and conference abstracts. To improve the current relevance of the review those reports published before 1990 were excluded.
The method used to apply selection criteria was as follows. I examined titles and abstracts and removed obviously irrelevant reports. I retrieved full text copies of the potentially relevant reports. Multiple reports of the same study were linked together. Full-text reports were examined independently by me and a colleague (Ian MacCormick, specialist trainee in ophthalmology and clinical PhD student at the Malawi-Liverpool-Wellcome clinical research programme) for compliance with eligibility criteria. Disagreements were resolved by discussion.
4.3.3 Data extraction and assessment of risk of bias
Major outcome variables were extracted independently by Ian MacCormick and me into a spreadsheet (Excel, Microsoft) with a standardised approach. The main outcome variables extracted were the prevalence of DR, proliferative diabetic retinopathy (PDR) and diabetic maculopathy and the incidence of DR, PDR and diabetic maculopathy. Prevalence of grades of retinopathy were recorded by
patient according to the worse eye and, unless stated, are presented as such below. Studies were stratified by the source of the population sample (with community
71 studies more likely to give a more accurate population based assessment of
prevalence); and risk of bias was assessed by seeking evidence of incomplete outcome data (missing data, subjects excluded from report, subjects lost to follow up in cohort studies).
72
4.4 Results
4.4.1 Literature search
The literature search yielded 380 citations of which 142 were reviewed in full text; 71 met the inclusion criteria and reported on a total of 62 studies (Figure 4.1) [2,212-281]. The literature search report is shown in Table 4.1.
Table 4.1 Literature search report for articles reporting prevalence, incidence or progression of DR or diabetic maculopathy in African countries. Four further articles were identified through hand searching and personal communication.
Source Date range searched Retrieved (before duplicate removal) ELECTRONIC DATABASES
Medline (Pubmed) 1948 – 6/2/2011 204
Embase (OVID) + Embase classic 1947 – 6/2/2011 333 Science citation index + Conference
proceedings citation index (ISI web of science)
1900 – 7/2/2011 199
Total number of records in Endnote database after deleting duplicates 370 OTHER DATABASES
African Index Medicus database Searched 8/2/2011 5
OpenSigle Searched 8/2/2011 0