Print version ISSN 0042-9686
Bull World Health Organ vol.89 n.10 Genebra Oct. 2011
Acuité visuelle pré-opératoire des patients subissant une opération de la cataracte et état de développement des pays: une étude mondiale
Agudeza visual preoperatoria en los pacientes sometidos a cirugía de cataratas y estado de desarrollo de los países: un estudio global
Shaheen P ShahI,*; Claire E GilbertI; Hessom RazaviI; Elizabeth L TurnerII; Robert J LindfieldI on behalf of the International Eye Research Network
IInternational Centre for Eye Health, London School of Hygiene & Tropical Medicine, London WC1E, England
IIDepartment of Medical Statistics, London School of Hygiene & Tropical Medicine, London, England
OBJECTIVE: To describe the preoperative surgical case mix among patients undergoing cataract extraction and explore associations between case mix, country level of development (as measured by the Human Development Index, HDI) and cataract surgery rates (CSRs).
METHODS: Ophthalmologists in 50 countries were invited to join the newly-established International Eye Research Network and asked to complete a web-based questionnaire about their eye hospitals. Those who complied received a data collection form for recording demographic and clinical data on 100 consecutive patients about to undergo cataract surgery. Countries were ranked into five HDI categories and multivariable regression was used to explore associations.
FINDINGS: Ophthalmologists at 112 eye hospitals (54% of them nongovernmental) in 50 countries provided data on 11 048 cataract procedures over 9 months in 2008. Patients whose visual acuity (VA) before surgery was < 6/60 in the better eye comprised 47% of the total case mix in poorly developed countries and 1% in developed countries (P < 0.001). Overall, 72% of the eyes undergoing surgery had a VA < 6/60. Very low VA before cataract surgery was strongly associated with poor development at the country level and inversely associated with national CSR.
CONCLUSION: The proportion of patients with very poor preoperative VA is a simple indicator that can be easily measured periodically to monitor progress in ophthalmological services. Additionally, the internet can be an effective tool for developing and supporting an ophthalmological research network capable of providing a global snapshot of service activity, particularly in developing countries.
OBJECTIF: Décrire l'éventail des cas chirurgicaux pré-opératoires des patients subissant une opération de la cataracte et analyser les associations qui existent entre l'éventail des cas traités, le niveau de développement des pays (mesuré par l'indice du développement humain - IDH) et le taux d'opération de la cataracte (TOC).
MÉTHODES: Les ophtalmologues de 50 pays ont été invités à rejoindre le tout nouveau Réseau international de Recherche ophtalmologique (International Eye Research Network) et à compléter, sur Internet, un questionnaire relatif à leurs services hospitaliers d'ophtalmologie. Ceux qui ont accepté ont reçu un formulaire de collecte de données permettant de recueillir les données cliniques et démographiques de 100 patients consécutifs sur le point de subir une opération de la cataracte. Les pays ont été classés en cinq catégories IDH et on a utilisé une régression multivariable pour analyser les associations.
RÉSULTATS: Les ophtalmologues de 112 services d'ophtalmologie (dont 54% sont non gouvernementaux) de 50 pays ont fourni des données sur 11048 interventions de la cataracte sur 9 mois en 2008. Les patients dont l'acuité visuelle (AV) avant l'intervention était inférieure à 6/60 pour le meilleur œil représentaient 47% de l'éventail des cas traités dans les pays peu développés et 1% de ceux des pays développés (P < 0,001). Dans l'ensemble, 72% des yeux subissant l'intervention présentaient une AV inférieure à 6/60. Une très faible acuité visuelle avant l'opération de la cataracte a été fortement associée à un développement faible au niveau du pays et inversement associée au TOC national.
CONCLUSION: La proportion des patients présentant une très faible AV pré-opératoire est un indicateur simple qu'il est possible de mesurer périodiquement pour contrôler les progrès réalisés par les services d'ophtalmologie. De plus, Internet peut être un outil efficace dans le de développement et la prise en charge d'un réseau de recherche ophtalmologique capable de fournir un instantané mondial de l'activité des services, en particulier dans les pays en voie de développement.
OBJETIVO: Describir la variedad de casos quirúrgicos preoperatorios en pacientes sometidos a extracción de cataratas y examinar las asociaciones existentes entre la variedad de casos, el nivel de desarrollo del país (de conformidad con la medición del Índice de desarrollo humano [IDH]) y las tasas de cirugía de catarata (TCC).
MÉTODOS: Se invitó a oftalmólogos de 50 países a que se unieran a la recién creada Red Internacional de Investigación Ocular y se les solicitó que completaran un cuestionario basado en web sobre sus hospitales oftalmológicos. Aquellos profesionales que cumplieron lo solicitado recibieron un formulario de recopilación de datos para el registro de datos demográficos y clínicos de 100 pacientes consecutivos que se iban a someter a una cirugía de cataratas. Se clasificó a los países en cinco categorías de IDH y se utilizó una regresión multivariable para evaluar las asociaciones.
RESULTADOS: En el año 2008, oftalmólogos de 112 hospitales oftalmológicos (el 54% de ellos no gubernamentales) de 50 países proporcionaron datos sobre 11.048 cirugías de catarata durante 9 meses. Los pacientes cuya agudeza visual (AV) antes de la intervención era < 6/60 en el mejor ojo constituyeron el 47% de la variedad de casos total en los países poco desarrollados, cifra que fue del 1% en los países desarrollados (P <0,001). En términos generales, el 72% de los ojos sometidos a cirugía presentó una AV <6/60. Una AV muy baja antes de la cirugía de cataratas estaba fuertemente relacionada con un bajo nivel de desarrollo a nivel nacional e inversamente relacionado con la TCC del país.
CONCLUSIÓN: La proporción de pacientes con una AV preoperatoria muy deficiente es un indicador simple que se puede medir fácilmente con cierta regularidad con el fin de supervisar los avances de los servicios oftalmológicos.Asimismo, Internet puede convertirse en una herramienta eficaz para el desarrollo y apoyo de una red de investigación oftalmológica capaz de ofrecer una visión global de la actividad de los servicios, especialmente en los países en desarrollo.
Blindness is both a cause and an outcome of poverty. Cataract, defined by the World Health Organization (WHO) as a visual acuity (VA) of less than 3/60 in the better eye, is the leading cause of blindness in the world. It affects approximately 18 million people, 90% of them in low- and middle-income countries.2 A recent study in three countries has demonstrated that successful cataract surgery in previously blind individuals can improve a household's economic status. This suggests that cataract surgery should be an integral part of strategies designed to reduce poverty.3
Thanks to improvements in surgical techniques for cataract extraction (e.g. sutureless microsurgery and use of intraocular lenses with a wide range of powers), surgery can be performed as an outpatient procedure under local anaesthesia and can restore normal VA almost immediately. It can also be offered much earlier, so that blindness can be prevented even in developing countries, where these techniques are now the norm. Clearly cataract surgery is practised under very different circumstances in different parts of the world and although its cost varies enormously, it is one of the most cost-effective of all health interventions.4 Thus, cataract surgery is a priority for VISION2020: the Right to Sight initiative (http://www.vision2020.org), a partnership between the World Health Organization (WHO) and other agencies working in eye care, including the International Agency for the Prevention of Blindness (IAPB).5,6
Over the last two decades, increased demand has led to a marked increase in the number of cataract operations in most countries, and surgery is being performed earlier on average because patients' needs and the results that they expect to obtain from surgery have changed. From a public health standpoint, this change in the indications for surgery dramatically affects the number of people eligible for surgery. In Australia, for example, reducing the visual impairment threshold from 6/60 to 6/12 would increase the number of people eligible for surgery by nearly fivefold.7 Concerns have been raised that in some high-income countries too many cataract operations are being performed,8 often to correct refractive error rather than to reverse visual impairment.
The cataract surgery rate (CSR), which represents the number of cataract extractions performed per million population per year in a given location, is a key indicator for monitoring eye care services. To reduce visual impairment from cataract the CSR must be greater than the incidence rate of cataract.9
According to WHO, whose estimates of country-level CSR are the best available,10 in 2004 (the most recent published data) high-income countries had a CSR ranging from 4000 to 6000, whereas in most of Africa, China and the poorer countries of Asia the CSR was often less than 500. CSR has increased substantially since then, but to understand what currently constitutes an operable cataract it is best to examine the local surgical case mix (i.e. the range of patients' visual acuities before cataract surgery and the number of first-eye surgeries performed).
This is the first study undertaken by a newly formed research network of ophthalmologists whose members were asked to collect information about the cataract surgery case mix in their hospitals. The purpose of the study was to assess variations in case mix by place and demographic characteristics, and to describe the relationship between preoperative VA, country development level and national CSRs.
The International Eye Research Network (IERN), whose members are listed in Appendix A (available at: http://www.iceh.org.uk/download/attachments/15827118/iern-participants.pdf), is a newly established international collaboration of ophthalmologists. Recruitment was as follows: First, alumni of the Masters in Community Eye Health programme at the London School of Hygiene & Tropical Medicine (LSHTM) were approached by e-mail. Second, regional representatives of the IAPB, directors of international nongovernmental organizations involved in eye care and staff at the International Centre for Eye Health were asked for contacts. Third, an advertisement was placed in the Community Eye Health Journal (http://www.cehjournal.org) a quarterly publication that circulates among more than 34 000 eye care specialists in at least 180 countries.
Potential network members were sent an invitation to participate in the study, along with information on the study's aims and objectives. Interested respondents were sent a more detailed study protocol and a link to the web site for membership registration. Using a web-based questionnaire, network members then recorded the following information about the hospitals where they worked: type of hospital (government, private, other, etc.); volume of outpatient practice and cataract surgery in 2008; number of cataract surgeons; and status of the hospital as a provider of ophthalmology training. Information on the availability of ophthalmic ultrasound (used to assess the intraocular lens power needed) and on facilities for paediatric and vitreoretinal surgery was also collected to assess the level of specialization of each hospital, since equipment of this type is usually only available in highly specialized eye care units.
Members with complete online registration were e-mailed a questionnaire with which to record information on the next 100 consecutive patients undergoing cataract surgery in their hospital. Supporting documents describing how to collect and enter the data were also supplied. Individuals were also asked to provide data on age, sex, literacy and patients' preoperative VA, as well as on the date when each cataract extraction was performed. Anyone who could read, understand and sign the surgical consent form was deemed literate. All patient information was provided anonymously, without collecting any identifiers, and study documents were available in Chinese and Spanish. Members returned completed questionnaires either electronically or by regular mail.
The study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of the LSHTM. IERN members were requested to obtain ethical approval from their institutions before collecting any patient information, and they had to indicate that this had been done. Patients gave verbal or written consent for cataract surgery as required by each hospital's policy.
Data management and statistical analysis
Hospital and patient data were entered into Microsoft Excel (Microsoft, Redmond, United States of America) by a designated data officer at the International Centre for Eye Health. Hospital information and individual patient data were merged for analysis. Random cross-checking of 10% of the entries yielded an error rate of less than 1%.
The VA of each patient in the better eye determined their placement into one of the following categories: blindness or severe visual impairment if VA < 6/60 (very poor visual acuity); moderate visual impairment if VA > 6/60 but < 6/18; nearly normal vision if VA > 6/18 but < 6/12; and normal vision if VA > 6/12.11 Hospital surgical output, expressed as the number of surgeries per year, was categorized as low (< 1000), medium (10002999) or high (> 3000). A hospital specialization score was calculated as the sum of four binary values (i.e. yes= 1; no =0) that comprised training, vitreoretinal services, paediatric services and B-scan ultrasonography. Hospitals were then categorized into five levels of specialization ranging from 0 to 4. The United Nations Human Development Index (HDI), a composite index that measures average achievement in health, education and standard of living, was used to assess each country's socioeconomic development. The following categories of development were used: very high (HDI > 0.9), middle-high (> 0.6 but < 0.9), middle-low (> 0.5 but < 0.6) and low (< 0.5).12 Country-specific CSR data for 2004 were obtained from WHO.10
Data were imported into STATA 10.0 (StataCorp LP, College Station, USA) for analysis. Data were summarized at the eye, individual (person), hospital and WHO region levels and by HDI. Associations at the hospital level were assessed using the χ2 test. Estimates of effect size at the individual level were determined using logistic regression models that accounted for the clustered nature of the data (at the eye hospital) using the svy command in STATA. The multivariable model was built following a manual forward stepwise method with a cut-off point at P > 0.2.
Hospital and patient data were provided by 112 eye hospitals from 100 cities and towns in 50 countries from all six WHO regions. The African Region comprised the largest number of countries (19), followed by the Region of the Americas and the European Region (8 each). Nigeria provided the most data (from 15 hospitals), followed by Pakistan (14 hospitals) and India (11 hospitals).
The following sectors participated: non-government, 54%; government, 38%; and other (e.g. publicprivate partnerships), 7.3%. In 2008 a median of 1700 cataract procedures were performed per hospital (interquartile range, IQR: 8004000). Each eye hospital had a median of 2 surgeons (IQR: 38) and over one third of all surgeons (38%) had performed more than 750 cataract procedures in 2008.
Approximately one third of the units (34%) admitted all patients overnight after cataract surgery, especially in less developed countries (odds ratio, OR: 2.5 per one-level decrease in HDI ranking; 95% confidence interval, CI: 1.34.9). Hospital facilities varied in terms of level of specialization: 60% had a working B-scan ultrasound machine; 40% provided vitreoretinal services; 67% offered paediatric services, and 64% provided ophthalmology training. Lower hospital specialization scores were associated with lower country development: no hospitals in highly developed countries had a very low specialization score compared with 21% of hospitals in poorly-developed countries (P < 0.001). Although hospital specialization score was also positively correlated with the number of cataract surgeries performed (73% of highly specialized hospitals performed > 3000 surgeries per year compared with 18% hospitals with low specialization [P < 0.001]), 19% of hospitals that performed > 3000 cataract procedures a year did not have a functioning B-scan ultrasound machine.
We recruited into the study 11 048 patients who underwent cataract extraction in 2008. Complete covariate data was available for 10 802 (98%) of these patients. No significant demographic differences were found between individuals with and without missing data. Nearly two thirds (61%) of the data originated from countries in middle-low or low state of development. Patients had a median age of 65 years (IQR: 58 73). Over one third (37%) of them were less than 50 years of age (Table 1). The oldest patients lived in the European Region (median age: 72 years), whereas the youngest lived in the Eastern Mediterranean Region (median age: 60 years). More women than men underwent cataract surgery everywhere except in the regions of South-East Asia (48.5%) and the Eastern Mediterranean (47%). The largest sex difference was observed in the European Region, where 57% of patients were women. Almost half of all patients (46.4%) were literate, but literacy ranged from 29% in the Eastern Mediterranean Region to 83% in the European Region. Male patients were more literate than females on average (53% versus 40% literacy, respectively; P < 0.001). The largest sex difference was noted in the Eastern Mediterranean Region (40% males versus 17% females).
Overall, 3127 (29%) patients were blind or had severe visual impairment before surgery (Table 1). Preoperative VA varied depending on the availability of functioning equipment and the hospital's level of specialization. In hospitals without a B-scan ultrasound machine, 36% of patients were blind or had severe visual impairment before surgery, compared with 24% in hospitals that did have a B-scan machine in working order (P < 0.001). Hospitals that were not highly specialized were significantly more likely to conduct surgery on eyes with more severe visual impairment (Table 1, P < 0.001).
As shown in both parts of Fig. 1, a nonlinear inverse relationship between preoperative VA and CSR was observed, along with a linear inverse relationship between preoperative VA and HDI (Pearson's r = 0.79; t = 8.8; P < 0.0001). In countries with a low HDI, nearly half (47%) of the patients were blind or had severe visual impairment at the time they underwent cataract surgery; in countries ranked as having middle-low and middlehigh development, such patients were only 32% and 23%, respectively, and in very highly developed countries, they were only 1% (Table 2). This association remained highly significant in the multivariable analysis (Table 3). Illiterate patients had almost three times the odds of having very poor preoperative VA as literate patients (Table 3). Older patients also had higher odds of having very poor preoperative VA than younger patients (P < 0.001).
Three quarters of the patients in the study received surgery to their first eye and most eyes (75%) had a VA of < 6/60. The proportion of patients who underwent surgery in the first eye decreased as country HDI increased (81% in countries with a low HDI compared with 66% in those with a very high HDI; P < 0.001). Illiterate patients were significantly more likely to undergo surgery in the first eye (OR: 1.4; 95% CI: 1.171.65; P < 0.001). The more specialized the hospital, the less likely it was to be operating on first eyes (OR: 0.72 per category increase; 95% CI: 0.550.93; P = 0.015). No difference in sex (P = 0.93) was found. Of the patients who underwent surgery to their second eye, 10% were blind or had severe visual impairment in both eyes preoperatively.
Overall, 72% of the eyes undergoing surgery were blind or had severe visual impairment. This proportion varied in accordance with a country's HDI (15% for countries with a very high HDI; 62% for those with a middle-high HDI; 80% for those with a middle-low HDI, and 90% for those with a low HDI; P < 0.001). In Australia, 75% of the eyes had a preoperative VA level of 6/12 (the VA cut-off used in many countries to determine eligibility for driving), whereas in Malawi this figure was 7%. No differences were observed by sex (75.3% of men versus 74.9% of women; P = 0.79).
The current study provides a snapshot of the visual acuity thresholds at which patients are undergoing cataract surgery throughout the world. It detected a clear association between better preoperative VA and higher HDI score. In countries with a high HDI the visual acuity threshold for cataract surgery has changed over time. For example, in the United Kingdom of Great Britain and Northern Ireland, 15% of cataract patients who underwent surgery had a preoperative VA of < 6/60 in 1997, but this fraction had declined to 1.6% by 2000 and to 0% in the current study. This decrease paralleled a dramatic increase in the CSR over the same period.13 Highly developed countries have very little cataract-related blindness despite an aging population, which suggests that the volume of surgery is at least equal to, if not greater than, the incidence of visual loss from cataract. On the other hand, inadequate provision of cataract surgical services is likely to be reflected in a greater proportion of people with very poor preoperative VA. Indeed, countries with low CSRs tended to have higher levels of poor preoperative VA. It follows, therefore, that the proportion of people who undergo cataract surgery with a very poor preoperative VA can serve as a simple measure that can easily be obtained periodically to monitor progress towards the elimination of cataract-related blindness.
The annual number of cataract operations performed globally has increased from approximately 5 million in 1988 to around 15 million in 2008.2 Ideally any patient who has symptoms of cataract should be offered surgery, but service providers in low-income settings inevitably have to carefully manage their limited resources. In keeping with the objectives of VISION2020, people who are blind from unoperated cataract stand to benefit the most from surgery and should, therefore, be prioritized. However, many other patient-related factors age, co-morbidity, occupation, social support, mental state, etc. must also be considered when recommending surgery. In the United States, the method of surgeon remuneration has been shown to influence surgery rates.14 We found that in countries with a low HDI, most surgeries were performed on people who were not blind or having severe visual impairment, even in non-specialized hospitals. By contrast, one decade ago most people in India and Kenya had a VA < 3/60 before undergoing cataract surgery.15,16 It may be that the relatively recent and dramatic increase in the number of cataract operations reported in India has not yet had an impact on the number of people who are blind from cataract (still estimated at 7.75 million).17 This may explain India's position as an outlier in Fig. 1.
Our study also found that people who are blind from cataract and who present for surgery are more likely to be illiterate. Improving literacy, a specific objective under the second Millennium Development Goal, should therefore contribute to reducing the barriers to the uptake of cataract surgery. The fact that in two WHO regions (South-East Asia and the Eastern Mediterranean) more men than women had surgery reflects gender-based biases that are well known to exist. Women bear approximately two thirds of the burden of blindness in the world.18 Hence, access to care will be equal for women and men only when women comprise 6070% of the individuals undergoing cataract surgery.18,19
In this study 10% of people who had surgery in their second eye were blind or had severe visual impairment in both eyes. This proportion, which resembles the proportion that existed in India over a decade ago,20 suggests that the surgery to the first eye failed to restore vision, either because the patient had co-morbidity or surgical complications. To avoid patient dissatisfaction, the care provider should clearly discuss visual prognosis with the patient. Efforts to improve surgical outcomes should be based on honest auditing of surgical complications, which should be prioritized as highly as improving the volume of surgery. We found that hospitals with low specialization scores were significantly more likely to operate on patients having worse preoperative vision, perhaps because hospitals with less equipment tend to be located in more remote areas and serve marginalized, poor populations. Surgery on dense cataracts is more complex and prone to complications and results in poorer postoperative results. In addition, hospitals that lack specialized equipment, such as B-ultrasound for biometry, tend to obtain less satisfactory postoperative results, and this may influence a surgeon's decision to operate. For these reasons, hospitals that are poorly specialized should be prioritized when allocating new resources, especially if they perform high-volume surgery.
The internet has been used in ophthalmology as a tool for reporting adverse events.21 However, to our knowledge this is the first study in which the focus has been primarily on ophthalmic surgery in less developed countries. With the internet now readily accessible in these settings, it can be used to develop and support a research network in a costeffective manner.
We imposed no exclusion criteria and all hospitals volunteered to take part in this study. Thus, the data may not be representative of all eye care providers nationally or internationally and may be subject to selection bias. For example, eye units in government facilities and in the private sector may have been underrepresented, and patients accessing these services probably differ from those accessing providers in nongovernmental organizations. Whether any hospitals enforced a VA threshold criterion for surgery was not investigated, but informal feedback from IERN members suggests that they did not. In addition, all data were selfreported and no validation studies were performed. We assessed literacy through a simple method easily applicable in all centres. Although the HDI is widely used to measure a country's level of development, its accuracy has been questioned because of wide within-country variation in the indicators used. Similarly, we could not address within-country variations in the CSR because WHO only provides data at the national level. The findings of our study may therefore be subject to an ecological inference fallacy and need to be interpreted with caution.
Individual results were provided to each participating hospital. Subsequent feedback from the IERN suggests that the information proved useful for planning surgical resources at the local level. In summary, the findings of this study suggest that the proportion of preoperative patients who are blind or have severe visual impairment before cataract surgery could be a very simple indicator, easy to measure periodically, for monitoring local progress towards the elimination of visual impairment from cataract.
Competing interests: None declared.
Funding: This study was funded by the British United Provident Association (BUPA) foundation. The study sponsors had no role in the study design, analysis or drafting of this paper.
1. Gilbert CE, Shah SP, Jadoon MZ, Bourne R, Dineen B, Khan MA et al.; Pakistan National Eye Survey Study Group. Poverty and blindness in Pakistan: results from the Pakistan national blindness and visual impairment survey. BMJ 2008;336:2932. doi:10.1136/bmj.39395.500046.AE PMID:18087076
2. Foster A, Gilbert C, Johnson G. Changing patterns in global blindness: 19882008. Community Eye Health 2008;21:379. PMID:19030126 [ Links ]
3. Polack S, Eusebio C, Mathenge W, Wadud Z, Rashid M, Foster A et al. The impact of cataract surgery on activities and time-use: results from a longitudinal study in Kenya, Bangladesh and the Philippines. PLoS ONE 2010;5:e10913. doi:10.1371/journal.pone.0010913 PMID:20531957 [ Links ]
4. Lansingh VC, Carter MJ, Martens M. Global cost-effectiveness of cataract surgery. Ophthalmology 2007;114:16708. doi:10.1016/j.ophtha.2006.12.013 PMID:17383730 [ Links ]
5. Resnikoff S, Pascolini D, Etya'ale D, Kocur I, Pararajasegaram R, Pokharel GP et al. Global data on visual impairment in the year 2002. Bull World Health Organ 2004;82:84451. PMID:15640920 [ Links ]
6. Resnikoff S. Integrating blindness with non-communicable diseases. Keynote address delivered at the: 8th General Assembly of the International Agency for the Prevention of Blindness, 2528 August 2008, Buenos Aires, Argentina. [ Links ]
7. Taylor HR, Vu HT, Keeffe JE. Visual acuity thresholds for cataract surgery and the changing Australian population. Arch Ophthalmol 2006;124:17503. doi:10.1001/archopht.124.12.1750 PMID:17159035 [ Links ]
8. Dua HS, Said DG, Otri AM. Are we doing too many cataract operations? Cataract surgery: a global perspective. Br J Ophthalmol 2009;93:12. doi:10.1136/bjo.2008.143685 PMID:19098039 [ Links ]
9. Foster A. Vision 2020: the cataract challenge. Community Eye Health 2000;13:179. PMID:17491949 [ Links ]
10. Global cataract surgical rates [Internet]. Geneva: World Health Organization; 2004. Available from: http://www.who.int/blindness/data_maps/cataract_surgery_rate/en/index.html [accessed 26 July 2011] [ Links ].
11. ICD-10 International statistical classification of diseases and related health problems, 10th revision. In: Johnson GJ, Weale R, editors. The epidemiology of eye disease. London: Chapman & Hall Medical; 1998. pp. 8-30. [ Links ]
13. Sparrow JM. Cataract surgical rates: is there overprovision in certain areas? Br J Ophthalmol 2007;91:8523. doi:10.1136/bjo.2006.111211 PMID:17576705 [ Links ]
14. Shrank W, Ettner SL, Slavin PH, Kaplan HJ. Effect of physician reimbursement methodology on the rate and cost of cataract surgery. Arch Ophthalmol 2005;123:17338. doi:10.1001/archopht.123.12.1733 PMID:16344447 [ Links ]
15. Kapoor H, Chatterjee A, Daniel R, Foster A. Evaluation of visual outcome of cataract surgery in an Indian eye camp. Br J Ophthalmol 1999;83:3436. doi:10.1136/bjo.86.5.543 PMID:11973251 [ Links ]
16. Yorston D, Gichuhi S, Wood M, Foster A. Does prospective monitoring improve cataract surgery outcomes in Africa? Br J Ophthalmol 2002;86:5437. doi:10.1136/bjo.83.3.343 PMID:10365045 [ Links ]
17. Murthy G, Gupta SK, John N, Vashist P. Current status of cataract blindness and Vision 2020: the right to sight initiative in India. Indian J Ophthalmol 2008;56:48994. doi:10.4103/0301-4738.42774 PMID:18974520 [ Links ]
18. Abou-Gareeb I, Lewallen S, Bassett K, Courtright P. Gender and blindness: a meta-analysis of population-based prevalence surveys. Ophthalmic Epidemiol 2001;8:3956. doi:10.1076/opep.18.104.22.1680 PMID:11262681 [ Links ]
19. Lewallen S, Courtright P. Gender and use of cataract surgical services in developing countries. Bull World Health Organ 2002;80:3003. PMID:12075366 [ Links ]
20. Limburg H, Kumar R, Bachani D. Monitoring and evaluating cataract intervention in India. Br J Ophthalmol 1996;80:9515. doi:10.1136/bjo.80.11.951 PMID:8976720 [ Links ]
21. Fung AE, Rosenfeld PJ, Reichel E. The International Intravitreal Bevacizumab Safety Survey: using the internet to assess drug safety worldwide. Br J Ophthalmol 2006;90:13449. doi:10.1136/bjo.2006.099598 PMID:16854824 [ Links ]
(Submitted: 18 February 2011 Revised version received: 10 July 2011 Accepted: 14 July 2011 Published online: 6 September 2011)