|COMMUNITY EYE CARE
|Year : 1997 | Volume
| Issue : 4 | Page : 251-257
Design of a population-based study of visual impairment in India : The Andhra Pradesh eye disease study
R Dandona, L Dandona, TJ Naduvilath, A Nanda, CA McCarty
Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India
Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad
Source of Support: None, Conflict of Interest: None
Reliable population-based epidemiologic data regarding vision and ocular morbidity, as well as those about the perceptions of people regarding visual impairment and eye care, are lacking for the most part in the developing world including India. These data are the basis on which effective eye care services can be developed. To meet this need we designed the Andhra Pradesh Eye Disease Study, a population-based epidemiology study of 10,000 people in the Indian state of Andhra Pradesh. The design of this study is described in this paper. Various options for the sample size, study areas, sampling procedure, and recruitment of subjects were considered. A sample size of 10,000 people, 5,000 each in the < or = 30 and > 30 years age groups, was determined to obtain reasonable confidence in estimating the prevalence of diseases and odds ratios for risk factors of interest. A multistage sampling strategy was chosen for the study which was assumed to give a design effect of 1.5 for the estimates. One urban area, Hyderabad, and three rural areas, West Godavari, Adilabad and Mahbubnagar districts, were selected in Andhra Pradesh. Interview instruments were developed to obtain detailed information about demographic data, diet, ocular and systemic history, risk factors for eye diseases, visual function, quality of life, barriers to eye care, and knowledge about eye diseases. A detailed examination procedure was devised to obtain a broad range of normative and abnormal data related to eyes and vision. A protocol was developed for doing automated visual fields, slitlamp and fundus photography. Computer databases were made in FoxPro for data entry and subsequent analysis with SPSS. Pilot studies were done to test the instruments, procedures, and logistics of the study in urban and rural areas. Information from the Andhra Pradesh Eye Disease Study is expected to help in planning and implementation of effective long-term preventive, curative, and rehabilitative eye care services in Andhra Pradesh.
Keywords: Ocular morbidity, prevalence, risk factors, quality of life, barriers to eye care
|How to cite this article:|
Dandona R, Dandona L, Naduvilath T J, Nanda A, McCarty C A. Design of a population-based study of visual impairment in India : The Andhra Pradesh eye disease study. Indian J Ophthalmol 1997;45:251-7
|How to cite this URL:|
Dandona R, Dandona L, Naduvilath T J, Nanda A, McCarty C A. Design of a population-based study of visual impairment in India : The Andhra Pradesh eye disease study. Indian J Ophthalmol [serial online] 1997 [cited 2019 May 26];45:251-7. Available from: http://www.ijo.in/text.asp?1997/45/4/251/14990
There has been a change in the epidemiological profile of disease in the developing countries with the increase in life expectancy from 46 to 63 years from 1960 to 1990, leading to a larger proportion of old age population. Currently, India with one-sixth of the world's population has one-fifth of the global burden of disease. Though accurate data are not available, it is estimated that India has one-fourth of the global blindness burden. With the expected increase in the number of elderly in India, the upward trend in the burden of blindness may continue as majority of the leading causes of visual impairment are age-related. Due to lack of adequate reliable data on the prevalence of eye diseases and blindness in the developing world, and information about the perception of people regarding visual impairment and eye care, it is difficult to plan effective eye care services. To meet these needs, well-designed population-based studies are required. Such studies should be able to provide sound epidemiologic data by taking into consideration all possible factors that influence the prevalence of eye diseases and visual impairment. These data, along with an assessment of the perception of people regarding the barriers to eye care and the effect of visual impairment on their quality of life, would help to improve upon the present eye care services. In this background, we are conducting the Andhra Pradesh Eye Disease Study (APEDS), a population-based epidemiology study of 10,000 people in the Indian state of Andhra Pradesh. This paper describes the design of APEDS.
| Objectives|| |
The objectives of APEDS are to determine:
- The prevalence rates of eye diseases and the burden of blindness and low vision in a representative sample of Andhra Pradesh.
- The risk factors associated with major eye diseases.
- The effect of blindness and low vision on quality of life.
- The barriers to eye care services.
| Design|| |
| Study areas|| |
The Indian state of Andhra Pradesh has about 8% of the country's population, 66.3 million according to the 1991 census (approximately 75 million currently). Three-fourths of the population of Andhra Pradesh is rural. The age distribution of the population of Andhra Pradesh is 40% ≤15 years, 30% between 16-30 years, and 30% >30 years of age. Four areas, which together are a reasonable representation of the socioeconomic and urban-rural distribution of the population of the state, have been chosen to obtain the sample population for APEDS. These are Hyderabad (urban), West Godavari district (relatively well-off rural), and Adilabad and Mahbubnagar districts (backward rural).
| Sample size|| |
In order to determine the sample size, the eye diseases of interest with the least assumed prevalence, and the power to detect odds ratios for risk factors were taken into account. The least prevalent eye diseases of interest in the age groups ≤30 and >30 years were assumed to be ocular trauma and age-related macular degeneration, respectively. With the assumption of a prevalence rate of 0.5% for each of these diseases, using Poisson distribution and assuming a design effect of 1.5 due to the sampling strategy, a sample size of 5,000 in each of the two age groups was chosen such that the estimation of these two diseases in their respective age groups would be between 0.3-0.8% at the 95% confidence level. Each of the four study areas would have a sample size of 2,500 to make a total of 10,000 subjects.
The Table shows the ability of this sample size to detect the minimum odds ratio for various risk factors for cataract at the 95% confidence level with 80% power. These calculations are based on prevalence estimates of various variables in those >30 years of age in the pilot study.
Eye diseases are more common in the elderly. However, the demographic distribution of India is pyramidal with about 70% of the population ≤30 years of age. We decided to obtain data on both the larger ≤30 years population and the smaller >30 years population with the higher disease prevalence. The sample was designed to get 5,000 subjects in each of the two age groups. This oversampling of subjects >30 years would be adjusted for when calculating the estimates.
| Pilot study|| |
A pilot study was conducted in Hyderabad before data collection for the urban part of the main study to test the validity and reliability of the instruments and procedures. With the experience of the pilot study, modifications were done for some of the instruments and procedures for a better and more feasible design. A similar pilot study was conducted in West Godavari district before starting the rural data collection.
| Sampling procedure|| |
A multistage sampling procedure has been adopted. In the first stage, stratification was done for urban-rural distribution of the Andhra Pradesh population by taking three-fourths of the sample from rural areas. For urban Hyderabad, the second stage involves stratification by socioeconomic status and religion. Twenty-four blocks (clusters) have been chosen randomly to meet these stratification criteria. Some blocks are randomly assigned to have selection of subjects >30 years of age, and the other blocks subjects of all age groups to enable the overall sample to have about 50% subjects each in the ≤30 and >30 years age groups. In the third stage, the households (a household is defined as all people eating from the same kitchen) in each block are selected with a random systematic procedure. The interval used in the systematic sampling of households is between every third to fifth, depending upon the total number of households in that block. The total number of households in an area are selected with the aim of achieving a recruitment rate of at least 85%.
For the rural areas, the second stage involves stratification for socioeconomic status based on the caste (schedule, backward, and forward) distribution assuming that the different castes roughly reflect the different socioeconomic strata. Stratification by religion is not done as the majority in the selected rural areas are Hindu. In the third stage, 20-25 clusters are chosen randomly in the villages of the designated rural areas within the caste strata. The remaining sampling procedure is the same as for urban Hyderabad.
| Staff|| |
APEDS staff consists of the principal investigator who has overall responsibility for the scientific and administrative aspects of the study; a study coordinator responsible for coordinating all procedures of the study and work of the staff; a statistician for development and management of appropriate databases and analysis system for the study; an ophthalmologist responsible for the anterior and posterior segment examination; two optometrists for visual function assessment, refraction, conducting the external and anterior segment examination, visual fields and photographs; four trained field investigators for subject recruitment and interviews; two data entry operators for entering data into the database; and support staff for transport and examination of subjects at the clinic.
| Recruitment of the study participants|| |
The first step in recruitment is mapping of the selected blocks (clusters) by the field investigators. Community contact is made by contacting the eminent person/leader in each block. The objectives of APEDS are explained and his/her help is sought in contacting the households. Each household in the selected blocks is contacted to record the name of the head of the household (the person who makes the decisions) and the size of the household. Each household is given a number. The pamphlet of information about APEDS is given to all households and they are informed that some households from their block would be selected randomly; and if their household is selected, they would be contacted again.
The selected households are contacted after systematic random sampling. The purpose of the study is explained in detail to the head/lady of the household. After the consent for participation in the study is given, the names and ages of all members in the household are documented and subject identification cards are issued. The convenient date and time for interview is sought and accordingly the household is revisited. The interviews are done (described later) and an appointment for examination at the clinic is given. The subjects are transported to and from the clinic for examination in the APEDS transport. In case of refusal of the selected subjects to participate in the study, they are contacted again and attempts made to convince them to participate. No monetary incentive is offered for participation. If unsuccessful after five attempts, basic information about the members and their eye health is collected if possible.
| Eligible subjects|| |
All members of the selected household in half the blocks (clusters), and members of selected households who are>30 years of age in the other half of blocks.
| Ineligible subjects|| |
Individuals who are visiting the household temporarily for a short period of time during the study; students staying together but not cooking in the house; those who die after the initial contact but before the examination; those who are not available either because of hospitalisation or tour/vacation during the study period in their block.
| Ethics approval|| |
This study has been approved by the Ethics Committee of the L.V. Prasad Eye Institute, Hyderabad, India.
| Interview|| |
APEDS has nine instruments to collect data from the subjects. Information on subjects≤15 years is collected from the mother/father of the subject. Instruments VI A, VI B, VII, VIII, and IX are not applicable to subjects≤15 years of age.
| Instrument I A (Family demographic data)|| |
This instrument is designed to assess the socioeconomic status of the family and is administered to the head of the household.
| Instrument I B (Individual demographic data)|| |
This instrument is used to record subject's age, sex, marital status, educational status, occupation, and health insurance. Subject's participation in the study is also recorded in this instrument. If an individual refuses to participate in the study, the refusal is documented along with basic information on the ocular status of that individual.
| Instrument II (Diet)|| |
This instrument is employed to elicit detailed dietary intake of each member of the family. This is administered to that member of the household who cooks regularly. This information would help in assessing the dietary risk factors for various eye diseases. For analysis, information regarding the constituents present in various food items and their proportion has been obtained from the National Institute of Nutrition, Hyderabad.
| Instrument III (Ocular history)|| |
This instrument is designed to record the status of health of the eyes until the day of the interview along with the treatment for any eye problem. If treatment is not sought for some eye problem, the reasons for this are elicited. It is also used to document family history of blindness and the following eye diseases: cataract, glaucoma, squint, macular degeneration, high myopia, night blindness, and retinitis pigmentosa.
| Instrument IV (Systemic history)|| |
This instrument is used to record details of systemic diseases: diabetes mellitus, hypertension, tuberculosis, and leprosy. Information is also elicited on intake of drugs, reproductive history in case of female subjects, and post-natal history of subjects ≤15 years of age. It also includes specific history of severe dehydration as some studies have suggested that dehydration is a major risk factor for cataract whereas others have not found any such association.
| Instrument V (Risk factors)|| |
This instrument is utilised to record details of some risk factors for eye diseases, such as smoking (both active and passive), cooking fuel, chewing tobacco/supari, drinking alcohol, and exposure to sunlight (not applicable to subjects ≤15 years of age).
| Instrument VI A (Visual function)|| |
This instrument is designed to record the levels of difficulty as perceived by the subject in various aspects of visual function such as distance and near vision, vision in bright and dim light, adaptation to light and dark surroundings, color vision, visual field, and depth perception.
| Instrument VI B (Test of visual function)|| |
It is administered to those subjects with presenting near visual acuity worse than logMAR 0.48 (6/18) in both eyes. A practical assessment of the level of difficulty faced by the subject in daily life is done using coins of different shapes and sizes. The use of coins was decided because this activity was thought to be common in the daily life of the majority in our culture.
| Instrument VII (Quality of life)|| |
This instrument is used to assess the subject's quality of life by probing his/her own view of well-being. This instrument is an adaptation of the World Health Organization Quality of Life Assessment, and will be described in another publication. The aspects dealt with are physical, psychological, level of independence, social relations, environment, and religion. Questions are framed to assess the level of difficulty and satisfaction in the above mentioned aspects. The contribution of eye-related problems to decrease in quality of life is assessed. This instrument is also used to assess the attitude of subjects towards blindness and the blind /visually impaired.
| Instrument VIII A (Barriers to eye care)|| |
This is designed to assess barriers to eye care services as perceived by the subjects, and also documents dissatisfaction with eye care professionals.
| Instrument VIII B (Barriers to eye care after referral)|| |
This is designed to document the reason(s) for which the subject did not report to the hospital for further management on being given an appointment for the same after the APEDS examination.
| Instrument IX (Knowledge about eye diseases)|| |
This instrument is employed to assess the subject's knowledge about various eye diseases and their treatment (prevention). These diseases are cataract, diabetic retinopathy, glaucoma, trachoma, night blindness (prevention), and squint. It is also used to record knowledge about eye donation.
Interviewer's observations on the subject's mobility and disabilities are also recorded to correlate these with the subject's assessment of his/her own disability.
| Examination procedure|| |
The examination is attempted for all eligible participants at a clinic especially set up for APEDS. Examination commences after the subject gives informed consent. The procedures that are routinely done on all subjects are described below.
| Measurement of visual acuity|| |
Distance visual acuity is measured with the logMAR (logarithm of minimum angle of resolution) chart, obtained from Australian Vision Charts, Forest Hill, Australia. LogMAR "E" chart is used for the illiterate subjects in order for the visual acuity measured for them to be comparable to the alphabetical logMAR chart. LogMAR chart is being used for its various advantages described elsewhere. The visual acuity is tested with the subject's current refractive correction, if used, and with illumination of at least 200 lux. Near visual acuity is measured using the logMAR near vision chart with the current refractive correction, if any. An additional 100 watts bulb is used to assess near vision. For subjects uncooperative for measurement of visual acuity by logMAR chart, fixation pattern, steadiness of fixation, and maintenance of the same are assessed using a torch light.
| Refraction|| |
Refraction is performed for all subjects who have presenting distance or near visual acuity less than logMAR 0.0 (6/6). Cycloplegic refraction is performed in all subjects ≤15 years of age and in clinical conditions wherein the optometrist feels the need to do so.
| Colour vision and stereopsis|| |
Colour vision is tested using Ishihara pseudo-isochromatic colour plates. It is not done for eyes with presenting near vision less than logMAR 1.0 (6/60). Stereopsis is tested by the Titmus fly/Random Dot stereo test. Both the tests are done with the refractive correction being used by the subject.
| Examination of orbit and adnexa|| |
Extraocular movements are assessed by broad H test. Alignment of the visual axes is assessed by Hirschberg test using a pen torch light with the refractive correction being used by the subject. Examination of lacrimal sac for regurgitation and eyelids for ptosis is also done.
| Examination of anterior segment|| |
A detailed clinical examination of the anterior segment is done using slitlamp biomicroscope. The abnormalities detected are graded based on severity as mild, moderate, or severe. Abnormalities of the tear meniscus, eyelids, conjunctiva, cornea, and inflammation in anterior chamber are graded in this manner. Any corneal pathology detected is further graded based on the extent to which it covers the visual axis.
Gonioscopy is attempted on all subjects. A combination of Scheie's and Shaffer's classifications is used to grade the angle. Iris and pupil are assessed for any abnormality, including relative afferent pupillary defect. Depth of the anterior chamber is assessed by van Herick technique if gonioscopy is not possible in uncooperative subjects.
Intraocular pressure is attempted using Goldmann applanation tonometer in all subjects. Perkins applanation tonometer or Tonopen is used for those who cannot sit at the slitlamp. Intraocular pressure is estimated digitally for those who are uncooperative with tonometry or have active infection.
| Dilatation|| |
All subjects are dilated for lens and posterior segment examination unless contraindicated due to risk of angle closure. Tropicamide 1% and phenylephrine 2.5% are used for subjects >15 years of age. In subjects ≤15 years of age, tropicamide 1% and cyclogyl 1% are used. Phenylephrine is not used if contraindicated. Attempt is made to obtain a pupillary diameter of at least 8 mm for lens and posterior segment examination.
| Dilated examination|| |
Size of the pupil and intraocular pressure are recorded again after dilatation. Lens is examined using slitlamp. For crystalline lens, nuclear cataract is graded according to the Lens Opacities Classification System (LOCS) III. The Wilmer classification[18-20] is used to assess cortical and posterior subcapsular cataracts. The possible etiology of cataract is also documented. The intraocular lens, if present, is assessed for its position. Status of posterior capsule is recorded for both aphakic and pseudophakic eyes.
Vitreous clarity is documented, and graded using the previously mentioned scheme. Fundus examination is done using 78 D lens with the slitlamp and 20 D lens with indirect ophthalmoscope. It involves examination of disc, macula, and retina. Abnormalities of the disc that are looked for include thinning of neuroretinal rim, atrophy, peripapillary chorioretinal atrophy, haemorrhage, and neovascularisation. The International Classification and Grading System for age-related maculopathy and age-related macular degeneration is used in which the features that are looked for are hard and soft drusen, retinal pigment epithelium changes, geographic atrophy, choroidal neovascular membrane, and disciform scar. The presence of microaneurysms, hard and soft exudates, intra-retinal haemorrhages, venous beading, intraretinal microvascular abnormalities, macular edema, and neovascularisation is looked for diagnosis of diabetic retinopathy which is classified according to a standard classification. Any other abnormality detected is also documented.
| Anthropometric measurements|| |
Blood pressure, height, and weight of all subjects are measured and documented, except that blood pressure is not recorded for subjects ≤15 years of age.
| Examination at home|| |
Examination is performed at home for the physically debilitated subjects who are unable to visit the clinic. The examination is performed in a manner similar to the examination at the clinic except that gonioscopy and measurement of height and weight are not done. Portable slitlamp is used for examination. Intraocular pressure is measured using Perkins applanation tonometer or Tonopen. Fundus examination is done with indirect and direct ophthalmoscopy.
| Diagnostic procedures|| |
| Visual fields|| |
Humphrey 24-2 central threshold automated visual field testing is done for subjects who are suspected to have glaucoma or any other optic nerve pathology, and also those with significant macular or retinal pathology such as retinitis pigmentosa. If the visual field shows abnormalities or is unreliable, it is repeated on another day. The test is performed as described in the Humphrey Visual Field Analyser User's Guide. Confrontation test is performed if the subject is unable to cooperate for visual field testing with the Humphrey Visual Field Analyser.
| Photography|| |
Any corneal, disc, macular or other retinal pathology is photographed. Stereo-photographs of the disc and macula are taken with Zeiss fundus camera. Corneal photographs are taken with Nikon photo-slitlamp.
| Test for blood glucose|| |
Random blood glucose test is done with fingerstick and glucometer on subjects who are detected to have evidence of diabetic retinopathy but are not known diabetics. The World Health Organization criteria for the diagnosis of diabetes are used. If the random blood glucose is found >120 mg/dl, test for fasting blood glucose is done on another day. Diagnosis of diabetes mellitus is confirmed if fasting blood glucose is found >120 mg/dl. Confirmed diabetics are referred to a physician for management.
| Diagnosis|| |
Diagnosis of the ocular condition is made based on the clinical findings and diagnostic procedures. The visual fields and photographs are reviewed by experts for confirmation of final diagnosis. The major cause of visual impairment is documented.
| Referral|| |
Subjects needing further management in Hyderabad study area are referred to the L.V. Prasad Eye Institute, and those in the other three study areas are referred to the base hospitals associated with the L.V. Prasad Eye Institute. In case of referrals for iridectomy/ iridotomy for narrow angles and cataract extraction, a dilated examination form is completed at the referral hospital after the procedure.
| Data entry and analysis|| |
| Data collection|| |
Data on each subject are recorded on APEDS data collection forms, each of which has self coded options as answers for each question. APEDS data collection forms are filled by the field investigators, and APEDS examination form is filled by the clinicians at the clinic.
| Data entry|| |
Data entry is done at the study headquarters, L.V. Prasad Eye Institute. Data are entered on daily basis for Hyderabad and on weekly basis for the rural areas. Data are entered by trained data entry operators on separate logins in FoxPro databases, and are stored in a common database placed in a network server.
| Data check|| |
Data entered in APEDS databases are compared on a daily basis using another FoxPro database containing basic information on all subjects seen at the clinic. This is done by the study coordinator to look for any discrepancies. Data entered by one data entry operator are checked by the other data entry operator. To determine and verify outliers, monthly range and consistency checks are done using the SPSS software.
| Data analysis|| |
Age and sex specific prevalence rates for blindness, low vision, and all eye diseases would be calculated by socio-demographic co-variates. Odds ratios for risk factors associated with visual impairment and the various eye diseases would be determined using mulitvariate analyses. Analysis of the perception of people regarding visual impairment and its effect on their quality of life, as well as their perception about barriers to eye care, would be done. The statistical tools used for analysis would include prevalence rates with their confidence intervals; chi-square and t-tests for univariate analyses; and logistic and polychotomous regression for multivariate analyses. SPSS would be used for statistical analysis.
| Quality control|| |
Validity of the methods was tested over a series of focus group discussions and trial runs. The focus group consisted of eye care professionals, public health experts, and field investigators. Methods considered appropriate after the group discussions were tested on a group of patients in a pre-pilot study, and the input of patients noted for improving the methods. Pilot studies were done in both urban and rural areas to check the validity and reliability of the methods developed for APEDS. Modifications were made wherever necessary until found satisfactory.
Reliability check is done at various levels. Reliability was tested amongst the field investigators for administration of the APEDS instruments, especially those on visual function, quality of life, and barriers, as their subjective nature is well understood. Reliability assessment was also done between the principal investigator and the clinicians at the APEDS clinic for gonioscopic grading of the angle, slitlamp grading of cataract with LOCS III and Wilmer classifications, assessment of age-related macular degeneration and diabetic retinopathy, and evaluation of optic disc.
Data entered in the FoxPro databases are checked for accuracy using the method described previously. All the above mentioned quality control measures are an on-going process to maintain the quality of the data obtained and entered.
| Conclusion|| |
Data collection for the Andhra Pradesh Eye Disease Study is expected to take two and a half years. These data would include a broad range of normative and pathological information about vision and eyes, as well as information about how the population perceives visual impairment and eye care services. This information would enable planning and implementation of an effective and sustainable long-term program for preventive, curative, and rehabilitative eye care services in Andhra Pradesh. Reasonable extrapolations from this study could also be used to improve eye care and reduce the burden of blindness in other parts of India.
| References|| |
United Nations Development Programme. Human Development Report 1992.
New York: Oxford University Press; 1992.p 134-35.
The World Bank. World Development Report 1993.
New York: Oxford University Press; 1993.
Thylefors B, Negrel AD, Pararajasegaram R, Dadzie KY. Global data on blindness. Bull WHO
Tielsch JM, Sommer A, Witt K, Katz J, Royall RM. Blindness and visual impairment in an American urban population: the Baltimore Eye Survey. Arch Ophthalmol
Klein R, Klein BEK, Linton KLP, DeMets DL. The Beaver Dam Eye Study: visual acuity. Ophthalmology
Registrar General and Census Commissioner, India. Census of India 1991: Final Population Totals.
New Delhi: Government of India; 1992.
Chief Planning Officer, Hyderabad. Handbook of Statistics Hyderabad District 1993-94.
Hyderabad: Chief Planning Officer; 1994.
Rosner B. Fundamentals of Biostatistics.
2nd ed. Boston: PWS Publisher; 1986. p 84-92.
Lwanga SK, Lemeshow S. Sample Size Determination in Health Studies: A Practical Manual.
Geneva: World Health Organization; 1991.
Gopalan C, Sastry BVR, Balasubramanian SC, Rao BSN, Deosthale YG, Pant KC. Nutritive Value of Indian Foods.
Hyderabad: National Institute of Nutrition; 1996.
Minassian DC, Mehra V, Verrey JD. Dehydrational crises: a major risk factor in blinding cataract. Br J Ophthalmol
Bhatnagar R, West KP, Vitale S, Sommer A, Joshi S, Venkataswamy G. Risk of cataract and history of severe diarrheal disease in southern India. Arch Ophthalmol
The WHOQOL Group. The World Health Organization Qualify of Life Assessment (WHOQOL): position paper from the World Health Organization. Soc Sci Med
Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt
Ferris FL, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol
Lovie-Kitchin JE. Validity and reliability of visual acuity measurements. Ophthalmic Physiol Opt
Chylack LT, Wolfe JK, Singer DM, Leske MC, Bullimore MA, Bailey IL, et al. The Lens Opacities Classification System III. Arch Ophthalmol
Taylor HR, West SK. A simple guide for the clinical grading of lens opacities. Lens Res
Taylor HR, West SK. The clinical grading of lens opacities. Aust N Z J Ophthalmol
Taylor HR, Lee JA, Wang LF, Munoz B. A comparison of two photographic systems for grading cataract. Invest Ophthalmol Vis Sci
The International ARM Epidemiological Study Group. An international classification and grading system for age-related maculopathy and age-related macular degeneration. Surv Ophthalmol
Olk RJ, Lee CM. Diabetic Retinopathy: Practical Management.
Philadelphia: J.B. Lippincott Company; 1993. p 3-20.
|23.|Humphrey Field Analyzer II User's Guide.
San Leandro: Humphrey Instruments Inc; 1994.
|24.|WHO Expert Committee On Diabetes Mellitus. Second Report. Technical Report Series 646.
Geneva: World Health Organization; 1980.p 8-14.