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To determine the causes of blindness and visual impairment in adults (30 years old) in Pakistan, and to explore socio‐demographic variations in cause.
A multi‐stage, stratified, cluster random sampling survey was used to select a nationally representative sample of adults. Each subject was interviewed, had their visual acuity measured and underwent autorefraction and fundus/optic disc examination. Those with a visual acuity of <6/12 in either eye underwent a more detailed ophthalmic examination. Causes of visual impairment were classified according to the accepted World Health Organization (WHO) methodology. An exploration of demographic variables was conducted using regression modeling.
A sample of 16507 adults (95.5% of those enumerated) was examined. Cataract was the most common cause of blindness (51.5%; defined as <3/60 in the better eye on presentation) followed by corneal opacity (11.8%), uncorrected aphakia (8.6%) and glaucoma (7.1%). Posterior capsular opacification accounted for 3.6% of blindness. Among the moderately visually impaired (<6/18 to 6/60), refractive error was the most common cause (43%), followed by cataract (42%). Refractive error as a cause of severe visual impairment/blindness was significantly higher in rural dwellers than in urban dwellers (odds ratio (OR) 3.5, 95% CI 1.1 to 11.7). Significant provincial differences were also identified. Overall we estimate that 85.5% of causes were avoidable and that 904000 adults in Pakistan have cataract (<6/60) requiring surgical intervention.
This comprehensive survey provides reliable estimates of the causes of blindness and visual impairment in Pakistan. Despite expanded surgical services, cataract still accounts for over half of the cases of blindness in Pakistan. One in eight blind adults has visual loss from sequelae of cataract surgery. Services for refractive errors need to be further expanded and integrated into eye care services, particularly those serving rural populations.
Pakistan, the sixth most populous country in the world,1 is a developing country situated in the World Health Organization's (WHO) Eastern Mediterranean Region. The country ranks 135 in the United Nations Human Development Index,2 and a recent report has suggested that the country is facing significant hardship; a declining growth in gross domestic product (GDP) and a near doubling of the proportion of the population living below the poverty line between 1987 and 2003.3 The geography and climate of Pakistan are extremely diverse; the eastern and southern parts are dominated by the Indus River and its tributaries, the northern parts by the snow‐covered Himalayan mountain range. The country's four provinces are Punjab, Sindh, North West Frontier Province (NWFP) and Balochistan.
The evidence base on national blindness and visual impairment in Pakistan prior to this survey is extremely limited. One study, estimating the main cause of blindness to be cataract (66.7%),4 led the National Committee for the Prevention of Blindness (NCPB) to develop a Five Year National Plan for the Prevention of Blindness (1994–1999) with a particular focus on large‐scale expansion of cataract surgical services.
The aim of this second national survey (conducted between 2002 and 2004) was to apply more rigorous survey methodologies to produce accurate data. Details of the prevalence of blindness among adults (aged 30 years) have been published5 and we now report on the causes of blindness and visual impairment, providing estimates of the magnitude of the major causes and exploring their demographic associations.
A detailed description of the methods used in the survey for sampling and training, and ocular examination protocols has already been published.6 A brief summary of the key methodological details is provided below.
Based on an assumed prevalence of blindness of 1.8%, a random sampling error precision of 0.3%, a design effect of 2.0 and a 10% increase for non‐response, the total sample size was calculated as 16600.
Multi‐stage stratified cluster random sampling, with probability proportional‐to‐size (PPS) procedures, was used. Enumeration, using the random walk method, was undertaken until the target number of adults was attained in each cluster. All enumerated individuals were asked to attend the survey station, set up in their community, for ophthalmic examination in the following days. Enumerated individuals who were unable to attend were examined in their home whenever possible. Three visits were made to homes before marking the subject as a non‐responder.
Ethical approval was provided by the Pakistan Medical Research Council (PMRC) in March 2002. This study followed the tenets of the Declaration of Helsinki.
The WHO categories of visual impairment were used in this study.7 Blindness (BL) was defined as a presenting visual acuity (ie, with glasses for distance if normally worn, or unaided) of <3/60 (<20/400 (logarithm of minimum angle of acuity (logMAR) >1.30) in the better eye. Severe visual impairment (SVI) was defined as <6/60 to 3/60, and moderate visual impairment (MVI) as <6/18 to 6/60. A “near normal” category was also included as <6/12 but 6/18. Any person with an acuity of <6/12 in the better eye was regarded as visually impaired. As visual fields were only assessed on a subset of the sample, constricted visual fields were not included in the definition of blindness. The Snellen notation for visual acuity has been used herein for ease of comparison.
This was defined as a participant presenting with 6/12 in one eye and <6/60 in the other eye.
Distance visual acuities were measured in all subjects using a reduced logMAR tumbling “E” chart.8 All study participants had a basic eye examination and all also underwent autorefraction (Nikon Retinomax K‐plus II Nikon, Tokyo, Japan). Individuals with <6/12 presenting visual acuity in one or both eyes (“red card holders”) were subject to a more detailed examination. All “red carders” had their visual acuity retested with the autorefraction result in a trial frame and all had a slit‐lamp examination with a dilated posterior segment examination. All “red carders” presenting with <6/18 with a treatable condition were referred to the nearest eye care facility.
The survey ophthalmologist, epidemiologist and the three clinical ophthalmologists determined the cause(s) of visual loss, following the principles outlined in the WHO Prevention of Blindness Proforma (Version III). For each eye, all pathological findings were recorded at the time of examination. Any degree of improvement in visual acuity when retested with a refractive correction was deemed evidence of refractive error present in that eye. One main cause was then selected for each eye, the WHO recommendations stipulating that (1) if any pathology is secondary to another, the primary pathology should be selected (eg, if the pathology was band keratopathy secondary to uveitis, uveitis should be selected), and/or (2) conditions amenable to treatment or (3) which could have been prevented are preferentially selected over and above unavoidable causes. Following this, the main cause in the right eye or the main cause in the left eye was chosen to represent the principal cause for the individual. If the main causes in the right eye and the left eye differed, the principal disorder for the individual was selected as the one most readily treatable or, if not treatable, the one which was more amenable to prevention (eg, if the main causes were right eye cataract and left eye optic atrophy, cataract was selected as the principal cause). Refractive error was considered more amenable to treatment than cataract.9 If refractive error and cataract co‐existed in the same eye, cataract was given as the main cause if refractive error correction did not improve the visual acuity 6/18.
All data were double entered by two trained data processors. Conditions were subgrouped as preventable or treatable (ie, avoidable), or unavoidable. Cause‐specific proportions of blindness and visual impairment were determined by age group, gender, province, location of household (rural/urban) and level of literacy. Univariate and age/gender‐adjusted logistic regression modelling was used to explore associations of the major causes (eg, cataract vs all other causes) with demographic variables. Generalised estimating equations were used in the models to adjust for dependency in the data due to clustered sampling. Estimation of the cause‐specific magnitude in Pakistan was calculated from age‐ and gender‐standardised prevalence data (using the latest population data).1 Extrapolations for the year 2020 were calculated using projected population estimates derived from the US census bureau.10
A total of 17314 adults (30 years) were enumerated, 16507 (95.3%) of whom were examined and included in this study. Of the study sample, 53.1% were women, their mean age being significantly lower than that of males (45.9 vs 48.9 years, respectively, p<0.001). The demographics of responders and non‐responders, and details of the prevalence of visual impairment have been described in detail elsewhere.5 A total of 4416 subjects (27%, 95% CI 26.1% to 27.4%) were identified with a presenting visual acuity <6/12. Of these, 561 subjects (crude prevalence 3.4%, 95% CI: 3.1% to 3.7%) presented blind.
Initially, all possible pathologies of a reduced visual acuity in eyes that presented with <6/12 vision were recorded by the examining ophthalmologist (total of 14881 eyes). Refractive error and cataract were recorded as causes in 5463 (36.7%) and 5345 (35.9%) eyes, respectively. The next most common cause was central corneal opacity (912 eyes, 6.1%), uncorrected aphakia (430 eyes, 2.9%) and macular degeneration (418 eyes, 2.8%).
Data were then analysed using the principal cause as shown in table 11.. Overall, an extremely high proportion of all categories of visual loss were due to conditions which could have been treated or prevented. A striking 85.4% of blindness was due to avoidable causes. Unoperated cataract together with uncorrected aphakia and posterior capsule opacification (PCO) accounted for 46.8, 78.1 and 63.7% of MVI, SVI and blindness, respectively. Under/uncorrected refractive errors accounted for 70.2% of visual loss in individuals with <6/12, but only 2.7% of blindness.
Amongst the 47 blind from “other” causes, posterior segment disorders dominated, including retinitis pigmentosa (11 subjects), vitreous haemorrhage (six subjects) and retinal detachment (two subjects).
The main causes of unilateral reduced vision are presented in fig 11.. Overall, more men were unilaterally blind than women (238 men, 187 women, p<0.001).
We have previously reported that the prevalence of blindness increased almost exponentially with increasing age. The prevalence was also associated with female gender, rural dwelling and illiteracy.5
Among blind subjects, cataract was the main cause in all age groups. There were no persons blind as a result of glaucoma or uncorrected aphakia in the 30–39 year age group; however, among those aged 70 years and older, glaucoma and uncorrected aphakia accounted for 9 and 10%, respectively. PCO was not a cause of blindness in the younger age groups (ie, 30–59 years) but in older subjects it was a prominent treatable cause (6.3% in 60–69 year olds). In contrast amblyopia was a more common cause in younger subjects but was not reported as a cause in subjects 60years. The highest proportion of phthsis/absent globe as a cause was found in the youngest age group (7.8%). The distribution in individuals with MVI is shown in fig 22.
The principal causes of MVI stratified by demographic variables are presented in table 22.. In men, the principal cause was cataract (45.4%), whereas in women it was refractive error (45.4%). Provincial differences suggested that refractive error was a more common problem in NWFP (48.5%). Similarly refractive error was more common in urban settings (47.5%) (whereas in rural settings cataract dominated (45.3%)) and in literate subjects (59.3%). The proportion attributed to corneal opacity was highest in Balochistan (9.9%).
The age‐ and gender‐adjusted association analyses of the principal cause in participants with SVI/BL are presented in table 33.. The odds of refractive error as a cause compared with any other cause steadily decreased with age (p=0.025), whereas the odds of cataract and aphakia increased with each decade (p=0.023 and p=0.025, respectively).
If the acuity level of <6/60 is used to denote “operable cataract”, there are 904000 (95% CI 736000 to 1107000) adults requiring surgery. A further 173000 individuals have uncorrected aphakia or PCO. A total of 1390000 adults have a presenting visual acuity of <6/60 in the better eye due to avoidable causes (table 44).). Assuming the prevalence and patterns of causes remain unchanged, the figures for the year 2020 show that a total of 2560000 adults will have avoidable causes of SVI and blindness. The estimate for “operable cataract” is predicted to increase to almost 2 million by the year 2020.
Projections for the year 2020 are shown in table 44.. Regarding adults with MVI (<6/18 to 6/60, better eye), 2140000 would benefit from having their refractive error corrected, this number increasing to 4320000 adults in 2020.
The survey reported in this paper used a diagnostically rigorous methodology, similar to that used in the recent surveys in Bangladesh and India.11,12 The response rate was very high (95.3%), minimising potential non‐response bias. The use of the WHO criteria for coding causes of visual loss allows comparisons with other similarly coded surveys.
The refractive status of every individual was assessed by autorefraction, and, furthermore, subjects with a visual acuity of <6/12 in either eye had trial lens‐corrected visual acuities measured. This allowed an accurate evaluation of refractive error as a cause of visual impairment, these data being important for planning refractive error services (a priority of the global initiative to eliminate avoidable blindness, VISION 2020: The Right to Sight13). The examination process also involved a dilated examination of all eyes with a visual acuity <6/12, allowing the detection of posterior segment disease, which has often been overlooked in the presence of cataract.14
For logistical reasons, fields were not performed on all subjects. Visual fields were only performed on a subset of the sample for normative data collection and on those who had optic disc changes that were suggestive of glaucoma, hence the decision not to use a definition of blindness that involved visual field construction. As fields were not performed on all subjects, constricted fields were not included in our definition of blindness. As the primary cause of a phthisical/absent globe could not always be determined, it is possible that some “unavoidable” cases could have been treatable or preventable (misclassification bias).
We found 14881 eyes with visual acuity <6/12. When all pathological findings in each eye were analysed together, refractive error caused as much visual loss as cataract (36.7 and 35.6%, respectively).
Almost 75% of individuals who were blind had treatable causes, and >90% of subjects had treatable causes of visual impairment. The two most important treatable causes of blindness were unoperated cataract (or uncorrected aphakia, 8.6%; and PCO, 3.6%) and glaucoma (7.1%), others being refractive error (2.7%) and diabetic retinopathy (0.2%). In the 1990 study,4 cataract accounted for 66.7% of blindness, whereas the current survey found unoperated cataract accounting for 51.5%. It is not possible to compare data for cataract between these studies directly, as different age groups were used, but the observed reduction almost certainly represents a real reduction, given the large‐scale increase in cataract surgical service delivery in Pakistan. However, the finding that despite this increase nearly 1 in 10 adults in Pakistan were visually impaired (<6/12) due to unoperated cataract highlights the importance of continued support of the NCPB for extending cataract surgical services in Pakistan.
In this survey, >12% of blindness was due to the sequelae of cataract surgery (ie, uncorrected aphakia 8.6% and PCO 3.6%). A survey conducted in rural NWFP identified uncorrected aphakia as the second most common cause of blindness.15 A hospital‐based study in Lahore, Pakistan showed that only 50% of eyes among individuals returning for follow‐up after cataract surgery had had intraocular lens (IOL) implantation.16 As low‐cost, high‐quality IOLs are now readily available, IOL surgery should be routine. Visual loss from PCO is certain to increase as cataract surgical rates increase, and YAG (yttrium–aluminium–garnet) lasers need to be made available for hospitals delivering high‐volume cataract surgery, with training in their use, upkeep and repair made a priority.
After cataract and the sequelae of cataract surgery, glaucoma was the next most important cause of treatable blindness, accounting for 7.1%. This is lower than the 11% quoted for the WHO Eastern Mediterranean region, subregion D, which includes Pakistan,18 but much higher than in a similarly designed survey in Bangladesh (1.2%). A survey in India of adults aged 50 years, which used a blindness definition of <6/60, showed that 5.8% of blindness was due to glaucoma.19 The earlier study in Pakistan estimated the number of people blind from glaucoma to be 80000,4 which is similar to the estimate from the current survey (89000). As cataract is becoming increasingly controlled in developing countries, strategies for the detection and treatment of glaucoma will need to increase in profile. In this survey, glaucoma blindness was highest in Balochistan, the most deprived province (89% of the rural population live in high deprivation districts)20 with limited access to eye care services.
Diabetic retinopathy accounted for 0.2% of blindness. However, this is likely to underestimate the true burden of retinopathy in the population as diabetic patients are more likely to have cataract, which would be preferentially recorded as the cause of their visual loss, and vitreous haemorrhage (possibly from diabetic neovascularisation) was classified in the ‘other' category. It is predicted that with rapid urbanisation, Pakistan will be among the five countries with the highest prevalence of diabetes by the year 2025.21 This is likely to alter the existing pattern of blindness and, in order to prevent a public health problem, preventive strategies need to be established.
In the 1990 Pakistan study, refractive error (including uncorrected aphakia) accounted for 11.4% of blindness,4 which is identical to the figure in the current survey, indicating that there has been minimal progress in addressing this highly treatable condition. Refractive error was the leading cause of MVI, particularly among the economically active working age group. Targeting this group, particularly with ready‐made spectacles, should be made a priority as this would prove to be extremely cost effective. It must be recognised, however, that barriers exist—for example, the social implications of spectacle wear. In addition, many individuals have mild myopia and, as they have adequate near vision, they may not feel impaired.
Corneal pathology, the main preventable cause, was the second most common cause of blindness overall (11.8%), again similar to that found in the 1990 survey.4 There are many causes of corneal scarring, trachoma being one. Trachoma is still endemic in parts of the country, and a recent rapid assessment found that of 233 villages surveyed, 151 (64.8%) had individuals requiring trichiasis surgery.22 Pakistan has set up a dedicated national task force for trachoma control and is part of the GET 2020 alliance.23
Phthisis/absent globe was the most important cause of unavoidable blindness and the third most important cause of unilateral SVI/BL. This highlights the importance of ocular trauma in Pakistan. A previous study on ocular trauma in NWFP found that 57.7% of cases had a perforating injury and that men outnumbered women by 5:1.24 A similar gender difference in unilateral blindness was noted in our survey.
A comparison of the findings of this survey with others in the region shows that Pakistan has the lowest proportion of SVI/BL (57%) due to cataract (India 62.4%,20 Bangladesh 82%12). Refractive error (including uncorrected aphakia) was lowest in Bangladesh (7.5%), followed by Pakistan (12%) and India (19.7%). Corneal opacity, responsible for nearly 1 in 10 cases of SVI/BL, was particularly prevalent in Pakistan but accounted for <1% of SVI/BL in India20 and Bangladesh.12 The reason for this high disparity is unclear but warrants further investigation. In subjects with <6/12, cataract and refractive error were of similar importance in Pakistan; however, this contrasts with Bangladesh where the main cause was cataract (73.4%), with refractive error only accounting for 18.9%.12
Our findings are markedly different from the findings in high income countries where the primary causes of blindness are age‐related macular degeneration, diabetic retinopathy and myopic degeneration, one survey in the USA indicating that these causes accounted for 63% of blindness.25
Based on the findings from this survey and future population dynamics, eye care service delivery needs to continue to expand in Pakistan, focusing principally on high‐quality cataract surgery and aftercare, and on increased capacity for the correction of refractive errors. This recommendation is consistent with the prioritised areas of action for the region as outlined by the WHO South‐East Asia policy VISION 2020: The Right to Sight. The more challenging conditions to control, namely glaucoma, macular degeneration and diabetic retinopathy, are also emerging as priorities.26,27
The authors are grateful for the contribution of the “Pakistan National Eye Survey Study Group” which consisted of the following individuals: Professor Shad Mohammed, Professor Zia Uddin Sheik, Professor Asad Aslam, Professor Nasim Panazai, Dr Shabbir Mir Dr Niaz Ali. Mr Pak Sang Lee (Technical Coordinator, International Centre for Eye Health, London), Ikram Ullah Khan (Biomedical Engineer, Pakistan Institute of Community Ophthalmology), Dr Haroon (Sight Savers International), Dr Rubina Gillani (Fred Hollows Foundation), Dr Babar Qureshi (Christoffel Blindenmission), Dr Mohammed Shabbir and Dr Falak Naz (Clinical and Community Ophthalmologists, respectively, North West Frontier Province team), Dr Abdul Ghafoor and Dr Kiramatullah (Survey Ophthalmologists, Punjab and Baluchistan Teams), and Dr Waheed Shaikh and Dr Amjad Shaikh (Survey Ophthalmologists, Sindh Team). The survey was financially supported by the “International Blindness Prevention Collaborative Group” which consisted of: The Government of Pakistan, the World Health Organization East Mediterranean Regional Office and Pakistan Office, Sight Savers International, Christoffel Blinden Mission, and the Fred Hollows Foundation. The authors also wish to thank Mr Tauqeer Abbas and Mr Fakhre‐e Alam for data entry, Dr Mahwash Akhtar‐Khan, Yelena Alexander and Rahul Shah for assisting in data cleaning, and Mr Fazl‐Subhan for assisting with financial management. Heidelberg Engineering (Heidelberg, Germany) kindly lent two HRT‐II instruments. Lateef Brothers, Lahore and S. Haji Ameerdin and Sons, both based in Lahore, Pakistan, were generous in their instrument support. Ophthalmic medications were generously donated by the NWFP divisions of the companies Remington and Kobec.
This study was supported financially by the “International Blindness Prevention Collaberative Group”:
The Government of Pakistan, Sight Savers International, Christoffel Blindenmission, Fred Hollows Foundation, WHO Pakistan Office, The International Centre for Eye Health, Pakistan Institute of Community Ophthalmology.
BL - blindness
IOL - intraocular lens
logMAR - logarithm of minimum angle of acuity
MVI - moderate visual impairment
NCPB - National Committee for the Prevention of Blindness
NWFP - North West Frontier Province
OR - odds ratio
PCO - posterior capsule opacification
SVI - severe visual impairment
WHO - World Health Organization
YAG - yttrium–aluminium–garnet
Competing interests: None declared.