A simplified screening test for identifying people with low vision in developing countries.

A simplified screening test for identifying people with low vision in developing countries J.E. Keeffe,1 J.E. Lovie-Kitchin,2 H. Maclean,3 & H.R. Taylor4 Simple but effective tests have been produced for screening subjects with low vision in developing countries. These tests of distance and near vision, based on the E test, were evaluated and validated in trials with people aged 4-90 years, and have been field tested in the health, education and rehabilitation services in 32 developing countries. Their sensitivity and specificity as screening tools for low vision have been calculated: sensitivity of 85% and specificity of 96% for the distance vision test, and sensitivity of 100% and specificity of 84% for the near vision test. The content and format of the tests have been demonstrated to be appropriate for developing countries, and their effectiveness for screening for low vision has been confirmed. Introduction Some 90% of the visually impaired in the world live in developing countries (1). The provision of appro- priate vision tests for use in these areas, where trained eye care staff and resources are scarce, is therefore imperative. For example, in Africa there is, on average, one ophthalmologist per million population (2); despite current efforts focused on training, there is still an enormous shortage of skilled staff for basic eye care and to help prevent blindness. The appropriateness of vision tests is deter- mined by their technological aspects and content, and the training required to administer them. To screen the vision of large numbers of people in a developing country it is often necessary to recruit untrained people within a community. The tests they administer must therefore be simple to learn and to use, with results that are easy to interpret. Test materials should be sturdy and capable of being easily transported. In addition, the test adopted should not be culture-specific or depend on literacy so that it can be used in any part of the world. 1 Research Fellow, Department of Ophthalmology, University of Melbourne, Royal Victorian Eye and Ear Hospital, 32 Gisborne Street, East Melbourne 3002, Australia. Requests for reprints should be sent to this author. 2 Associate Professor, Centre for Eye Research, School of Optometry, Queensland University of Technology, Brisbane, Australia. 3 Associate Professor (Clinical), Department of Ophthalmology, University of Melbourne, East Melbourne, Australia. 4 Professor and Head of Department of Ophthalmology, University of Melbourne, and Director, WHO Collaborating Centre for the Prevention of Blindness, East Melbourne, Australia. Reprint No. 5734 Such screening helps to detect people with im- paired vision who may require referral for treatment, and to identify those with potentially normal or low vision but who have been regarded as blind. For example, many students in programmes and schools for the blind in developing countries have usable vision and are potential print-readers, but because of some visual impairment they were regarded as blind. In East Africa, as many as 80% of students regarded as blind and placed in schools for the blind or in special programmes for the visually impaired had previously unidentified low vision; many of them were taught using nonvisual methods such as Braille (J. Keeffe & G. Nyaga, personal communication). The results of tests of near and distance vision, together with assessments of functional vision, can therefore help to identify suitable methods for rehabilitation and education, including the most appropriate reading format (Braille or print) and whether low vision devices may be useful. Our kit for assessment of low vision in develop- ing countries contains a visual acuity test-card, a pin- hole mask to detect refractive errors, and two manuals with instructions for visual acuity testing, the assessment of functional vision, and background information concerning the effects of low vision (3). The tests are suitable for use with children (above about 5 years of age) and adults. This article de- scribes the development and validation of these tests of distance and near vision. Methods The test card adopted was similar to one previously developed by and available from WHO, which used two sizes of optotypes on a small square plastic card. Bulletin of the World Health Organization, 1996, 74 (5): 525-532 C World Health Organization 1996 525 J.E. Keeffe et al. Fig. 1. The Visual Acuity Test Card. The upper panel shows the optotypes which appear on the outside of the card when it is folded in half. The near vision test types and the instructions, which appear on the inside faces, are shown on the lower panel. mw1l NEAR VISION De_,*sate dmc . tew pal howto do th E ss . show the dIroeet the Es p.t.a. Tedamr ma d videl to faId sh smae E ed perna can see. DISTANCE VISION W a EW E W 3m Either the illiterate, directional E or the Landolt rings were used. The new plastic visual acuity card measures 37cm x 18.5cm; when folded in half, the resulting square has optotypes for testing distance visual acuity on the outside faces. On the inside, the near vision test is printed on one side and the instruc- tions for using both tests on the other (Fig. 1). Screening distance vision The purpose of the distance vision test is to screen for normal or low vision, and not to measure the distance visual acuity accurately. Thus, the test dif- ferences (use of a single letter or a group of four letters) and consequent variation in contour interac- 1. Teutw the4 sm1EMs t6 tre Vision is normal (6f18 or better) if at leass 3 out of 4 small Es can be seen. If able to see smallest Es on near vision test. no more testing neded. If not able to sec as lea.13 of the small Es. tses with the large E 2. Tea wth the lrge E ss 6 metres Turn the card so ss he E I 4 ifferea direetlm If the Es re seen. vision is 6b60. If not ableo see at leasA 3 of the Cheek vla. wth pl_ke - if large Es. tess as mrtres. improved refer for testing for spectacles. if not improved. refer to eye specialist 3. Tes wiSh her E *13 metre Turn she crd tso sa ts E la 4 dIeres d1ttm. If a leas 3 out of 4 Es ae seen. vision If not able to see a leas 3 of the is 3165) Repeat the pinhole procedure lartg Es. tesl with pinhole and refer to for number 2 eye specialis for possible tsatnen. NEAt VISION 4. Let th per hd she r t. tSW card m_dam as huw want& Tess fromn the largest so the small Es. At least 3 out of 4 muss be corrct in each line before testng the nesx. If only the largest iN48s size can be seen. check If manlfiems will help and refer for testing for spectacks and to an eye specialist for possible treatment. The mweim size !J20 is simila to the print in large print books. The smllesit size ENDs is similr to tshe pnnt in books and maazines. If near vision ad is used as 6 nes and the lrw Es ae smn. visio is 6 S1n The nop qe m exphondi Dook I -Sa for aiqe-nd Vim. TeotV _.Aasy tion are not considered drawbacks. Of course, for an accurate measure of visual acuity these factors would need to be controlled (4, 5). It is recommended (see test instruction, Fig. 1) that distance vision be screened with at least four optotypes, irrespective of the level of vision. The criterion adopted for passing any level of vision is 3 out of 4 Es correct. Distance vision is tested by first using the set of four E targets, which are 18m size letters. If 3 of the 4 targets are correctly identified at a test distance of 6 metres, vision is classed as "normal", i.e. 6/18 or better, and no further distance vision testing is required. If 3 of the 4 Es are not correctly identified, the large E (60m size) is presented in four different orientations. If 3 of the 4 presentations are correctly identified, the WHO Bulletin OMS. Vol 74 1996526 Screening test for low vision in developing countries person has low vision of 6/60 or better. If they are not correctly identified, the test should be repeated at a test distance of 3 metres. Irrespective of the finding with the large E, it is recommended that vision be retested with a pinhole. If vision is improved, a significant refractive error may be present and, if possible, the person should be referred for optical correction. If vision does not improve, it is most likely that an ocular disease is present and, where appropriate services exist, refer- ral should be made for investigation and possible treatment of the disease. By rotating the card, the symbols can be orientated differently to avoid memorized responses when testing binocularly, monocularly and with the pinhole. Screening near vision To overcome differences in language or literacy within and between countries, the E symbol was also adopted for the near vision test. This test contains three sizes of optotypes rather than the usual six or more. The point system of designating print size was adopted (6, 7). The first version of the test used two sizes, N8 and N20. Feedback after testing indicated that a larger size was also necessary to identify peo- ple with usable low vision, so N48 was added. The print sizes used are given in Table 1 in points, logMAR and M notations. As the exact heights of letters vary with differ- ent fonts, the heights of letters in numerous tests were measured to establish a common or average size at each of the chosen print sizes. The E at each of these sizes is equal to the size of the body of lower case letters, i.e., the height of an "e" or the circular component of "d". The E is constructed using the Snellen principle for the height, width and spacing of gaps (8). Contour interaction is controlled by having the spacing between Es equal to the width of the E for that line, and the between-line spacing equal to the height of the Es in the line above. The smallest optotypes (N8) approximate the size used in regular print books and newspapers. The N20 Es are similar to the print size used in special large-print books. The largest size (N48) approxi- mates the size of print used in headings, labels or Table 1: Equivalent print sizes according to the point system, M units (metric equivalents) (ref. 5), and logMAR at 25cm (ref. 7) Points M units LogMAR N8 1M 0.6 N20 2.5 M 1.0 N48 6 M 1.4 posters. It is also the size of the 6/6 optotype. There- fore it should be noted that if the N48 line is correctly recognized at 6 metres, distance visual acuity is 6/6. The purpose of the test is to give a measure of functional vision for near tasks by estimating thresh- old print size and not measuring near visual acuity per se, hence no standard test distance is required. The test card can be held at whatever distance gives optimum near vision for each person, and the test distance should be recorded. It may need to be em- phasized that children can hold material very close to their eyes to obtain natural or relative distance mag- nification. The person being tested is asked to indi- cate the direction of the Es, starting from the largest symbols. Again, the pass criterion is 3 out of 4 cor- rect. If the smallest (N8) symbols are correctly iden- tified, then near vision is regarded as functionally normal. If the medium size (N20) is correctly identi- fied, these persons should be able to make use of their low vision but should be referred for optical and/or medical investigation. It is recommended that if only the largest (N48) size can be recognized, mag- nification devices may help and there should be re- ferral for investigation. Field testing in developing countries Four separate series of field tests were conducted over 3 years by health workers, educationists and community-based rehabilitation workers in 32 coun- tries in the Pacific region, Asia, Africa and eastern Europe to assess whether the materials performed satisfactorily and met the needs of the different set- tings. The test-card, accompanying booklets, and evaluation forms were sent by mail to investigators in developing countries who needed to test the vision of people in current programmes or to conduct vision screening. The administration of the test-card by previously untrained users was observed (by the first author) to ascertain whether correct procedures were being used. This author used the test in eye care programmes, and in community-based rehabilitation and educational centres in the Philippines, Fiji, India, Kenya, and Uganda. The final version of the test-card was used in the assessment of 127 students aged 5-20 years in schools for the blind in Kenya and Uganda. These students were part of the same sam- ple assessed by Silver et al. (9), except for the exclu- sion of those found to have no perception of light. The results of the screenings were compared with the comprehensive assessment results from Silver et al. (9), using x2 tests. Evaluation forms were completed after each series of field tests. Information collected included whether the instructions were understood and could be followed, and which optotype (the Landolt ring or WHO Bulletin OMS. Vol 74 1996 527 J.E. Keeffe et al. Table 2: WHO categories of visual impairment and the equivalent visual acuities in the new test of distance vision and on the Snellen test chart WHO category New test Snellen chart "Normal" vision 6/6, 6/18 6/6, 6/9, 6/12, 6/18 Low vision 6/60, 3/60 6/24, 6/36, 6/60, 3/60 Blind <3/60 <3/60 the E) was preferred. Appropriate changes were made between each series of field testing. Validation The sensitivity and specificity of the low-vision screening test was assessed and compared with con- ventional vision tests. The sensitivity of a test (i.e. the percentage of correct referrals) indicates the prob- ability that the test will correctly identify people who have low vision. The specificity (i.e. the percentage of correct non-referrals) is the probability that the test will correctly identify those people who have normal vision. Owing to the lack of facilities for accurate as- sessment of distance and near acuities during the field trials, which is why these screening tests are needed, the validation of the tests was conducted at the Royal Victorian Eye and Ear Hospital in Mel- bourne. Free and informed consent was obtained from all participants. Tests of both distance and near vision were administered by a masked examiner to 125 consecutive patients aged 4-90 years attending clinics at the hospital. The sample included people with albinism, age-related macular changes, cata- racts, aphakia or pseudophakia, and refractive er- rors. Many had no significant ocular abnormalities. Best-corrected distance and near vision were tested binocularly following the instructions for the vision tests in order to replicate procedures used in the field. Tests used for comparison were the traditional Snellen test charts using letters or Es for distance visual acuity, and the Sheridan Gardiner (SG) letter- matching or, for sizes above N18, the letter-matching booklet of the Near Vision Test for Children (NVTC)a for near visual acuity. Visual acuities meas- ured with these conventional tests were gathered as part of routine clinical testing which was carried out on the same day by a different examiner. Direct one-to-one correspondence between visual acuities from the conventional charts and the screening tests is not possible, and is not required considering the purpose of the test. Measures of a Published by Options Australia, Canterbury, Victoria, Australia. Table 3: Three measures on the new test of near vision and their SG (Sheridan Gardiner) and NVTC (Near Vision Test for Children) equivalents New test SG or NVTC N8 (normal) N5, N6, N8 N20 (impaired) N10, N12, N16, N18, N20 N48 (poor) N24, N32, N40, N48 visual acuity from the Snellen test and the SG and NVTC were grouped to list equivalent measures with the new tests (Tables 2 and 3). The WHO categories are given for each group of distance visual acuities. Results The new tests of distance and near vision were suc- cessful in both developing countries and hospital clinics in Australia, where they included children as young as 4 years and people who did not speak the same language as the tester. The lack of a common language was not a serious obstacle because the test could be demonstrated. Of the 127 students attending schools for the blind in Kenya and Uganda, over 60% were cat- egorized as low vision on the basis of the distance- vision test results (Table 4), even though the majority of the students were being educated using nonsighted methods such as Braille. The numbers of students in each vision category were not signifi- cantly different from those obtained in a similar population by Silver et al. (9) (X2 test = 3.1; P = 0.2). Near-vision testing revealed that 61 (49%) of the students had near vision within the normal range and an additional 43 (34%) had the potential to read large print or regular print with magnification (Table 4). Validation. The two subjects (out of 125) who could not complete the validation testing in the Royal Vic- Table 4: Results of distance and near vision testing in schools for the blind in Kenya and Uganda (n = 127) No. of students WHO category: "Normal" vision (-'6/18) 14 Low vision (<6/18 to 3/60) 78 Blind (<3/60) 35 Near vision: N8 61 N20 43 N48 9 No response (>N48) 14 528 WHO Bulletin OMS. Vol 74 1996 Screening test for low vision in developing countries Fig. 2. Scatter plot of distance visual acuity results by the E test and the Snellen test. The numbers on each coordinate indicate the number of people and the results obtained on the respective tests. For example, the point near the lower left corner shows that 20 people achieved visual acuity of 6/6 on both tests. <3/60 3/60 6-- a 6/60 w 6/18 6/6 6/6 6/9-6/18 6/24-6/60 3/60 Snellen test <3/60 torian Eye and Ear Hospital were young children with intellectual disabilities in addition to impaired vision. Data from the screening tests and conven- tional visual acuity tests were therefore available from 123 subjects; of these, 104 (85%) showed the same result from both tests of distance visual acuity (Fig. 2). In all, 115 (93%) were determined to be in the same visual acuity category (i.e., normal, low vision, or blind) (Table 5). Four subjects (3%), who would not have been detected or correctly referred by using the screening Table 5: Comparison of results in the simplified dis- tance vision test with the Snellen test at the Royal Victorian Eye and Ear Hospital. The figures in the hori- zontal rows were obtained with the new E test, i.e. 4 and 19 were found to have low vision. The figures in the vertical rows were obtained with the Snellen test. Three were in the blind category in both tests Snellen test: "Normal" vision Low vision Blind Simplified test: "Normal" vision 93 4 0 Low vision 4 19 0 Blind 0 0 3 Sensitivity Specificity 85% 96% Table 6: Comparison of results in the new near vision test with the conventional (SG and NVTC) at the Royal Victorian Eye and Ear Hospital. The figures in the hori- zontal rows were obtained with the new E test, i.e. 27 and 8 were found to have low vision. The figures in the vertical rows were obtained with the SG or the NVTC Conventional tests: "Normal" vision Low vision Simplified test: Low vision 8 27 "Normal" vision 42 0 Sensitivity 100% Specificity 84% test, were found to have low vision on Snellen visual acuity testing (false negatives). A further four (3%) were detected by the new test but had visual acuities of 6/18 or better (false positives). Thus, there were 8 (6%) incorrect referrals. The sensitivity (percentage of correct referrals as low vision or blind) was 85% and the specificity (percentage correctly identi- fied as normal vision and not requiring referral) was 96%. Of the 77 whose near vision was tested, 69 (90%) achieved comparable results with the new screening test and the conventional clinical tests (Table 6). All 42 subjects who could see N8 on the new test could name or match letters from N8 to N5 on the SG or NVTC. Eight of the 35 who read N20 but not N8 on the new test could recognize smaller letters between N20 and N8 on the tests used for comparison. Of the total number, 10% would have been referred with "low vision" which they did not have. The sensitivity and specificity of the near test were 100% and 84%, respectively (Table 6). Discussion Size of optotypes. The use of two sizes of opto- types for distance visual acuity is adequate for screening purposes and to establish basic infor- mation for an assessment of functional vision. By testing at 6 and 3 metres when needed, these sizes permit classification of acuity according to the broad WHO categories of "normal" vision, low vision, or blindness (10). The near vision test is used to determine whether there is useful vision for near tasks, the approximate print size for reading, and whether re- ferral is warranted for correcting refractive errors or prescribing low vision devices. The near vision of those who can recognize the smallest Es could be WHO Bulletin OMS. Vol 74 1996 . 3 l 2 m4 .16 *1 .6 .65 .4 m20 *2 II 529 J.E. Keeffe et al. described as within normal limits; recognition of only the larger sizes indicates useful near vision but with reduced ability to discriminate detail. If the largest size cannot be seen even at a very close distance, nonvisual methods for accessing print and other in- formation should be considered. Test symbols. Other tests for screening vision, which have been developed for nonreaders because of their age or illiteracy and for people with multiple disabilities, are the Lighthouse cards (11), Kay pictures (12), the Allen cards (13) and the LH symbols.b The pictures in these tests have been se- lected because of their familiarity and their ease of recognition (12). The pictures are often of objects that are common in industrialized countries, but not in developing countries where there are problems due to unfamiliarity with the pictures across cultures. There is less of a problem in the LH testsb where the more stylized symbols do not represent unique objects. Symbols for illiterate persons (the E chart, the Landolt ring and Sjogren's Hand) have been used extensively and recommended in screening pro- grammes (13, 14-18). Use of the E chart or Landolt ring may be problematic in young pre-school chil- dren because of the directional, left-right orienta- tion of the symbols (13), but they have been used and are recommended from the age of 5 years (11, 18).b The effectiveness of the E test will depend on the age of the person being screened and the purpose of screening. Weale has drawn up a screening protocol for use by teachers to detect children with defective vision who need referral for eye care (18); the screening instrument is an E chart with three sizes of E for testing distance vision. The test procedure for the E chart and the Landolt ring is simple to explain, and language barri- ers can easily be overcome. There is also the advan- tage that one person is able to administer the test alone. In our project, both the E chart and Landolt ring were used for distance and near testing in devel- oping countries. Feedback from a majority of testers in different countries indicated a preference for the E chart. The field testing was predominantly with, but not restricted to, school-age children. The E chart, which does not require reading of numbers or letters, was adopted for the test of near vision. This test indicates the amount of detail that can be discriminated; it is not a reading task and only assesses the threshold print size. Reading is not b Hyvarinen L. Measurement of visual acuity in visually impaired children. Transactions of the Vth International Orthoptic Congress, Lyon, France, 1984: 91-95. the sole or, for many people, a relevant near-vision task. An advantage of the E chart or the Landolt ring is that the symbols can be repeatedly shown; with different orientations they remain an intel- ligible test object, because the direction of the E or gap in the ring can be changed with each ex- posure of the letters. With pictures, numbers or other symbols, this rotation is not possible so that many optotypes at each acuity are needed or two test cards are required. Large or multiple test cards are neither feasible nor desirable in field settings. Relevance to developing countries. Qualitative feed- back from users of the test card in developing coun- tries has indicated that the format and content employed are suited to local needs. The acuity test card is appropriate, easy to learn, and simple to use in a variety of settings. The test also performed well in the validation study. None of those with low vision was missed with tests for near vision and only 3% for distance vision. Given the simplicity of the tests, these are highly acceptable figures. In a review of the accuracy of vision screening techniques used in developed coun- tries, Schmidt found considerable variation between equipment and procedures used for screening (19). The total proportion of incorrect referrals varied from 0% to 58%, but the criteria for referral were stricter than those recommended for developing countries. Only when the vision screening in- cluded an eye examination did the proportion of over- and under-referrals fall below that found in this study. The difference in performance between our near vision test and the SG and NVTC tests may be due to the types of charts used. The near vision test used three symbols with uniform spacing, in equal logarithmic steps of sizes according to the principles described by Bailey & Lovie (4) (Fig. 1). Discrimina- tion and recognition of optotypes in this format are more difficult than the well-spaced or single letters used in the SG and NVTC tests. The field testing of these low vision screening tests during their development has ensured that they are appropriate for use in developing countries. The tests satisfactorily categorize people as having "nor- mal" vision, low vision, or blindness. The results can be used to make decisions concerning referrals for treatment or correction of refraction and, in con- junction with findings from the assessment of func- tional vision, for assessment of low vision or for rehabilitation. This study has confirmed these low vision tests as valid instruments in the situations described. 530 WHO Bulletin OMS. Vol 74 1996 Screening test for low vision in developing countries Resume Test simplifie pour le depistage de la perte de vision dans les pays en developpement En raison du manque de personnel qualifie et de ressources dans le domaine des soins ophtalmo- logiques, il est imp6ratif de disposer de tests utilisables dans les pays en d6veloppement pour identifier les personnes ayant un d6faut de vision. La qualit6 des tests de vision est d6terminee par leur technologie, leur contenu et la formation n6cessaire pour leur execution. Les tests utilis6s dans ce contexte doivent etre simples a apprendre et a ex6cuter, et donner des resultats faciles a interpreter. Ils doivent etre independants de la cul- ture locale et du degr6 d'alphab6tisation, de fa9on a pouvoir etre utilises partout dans le monde. Le d6pistage aide a identifier les personnes presentant un defaut de vision et susceptibles d'etre orientees sur un service specialis6 pour y recevoir un traitement, et a identifier les personnes dont la vue est potentiellement normale ou faible mais qui ont et6 classees comme aveugles. L'ar- ticle decrit la mise au point et la validation de tests de vision lointaine et rapprochee, imprim6s sur une fiche unique pliee en deux (37 cm x 18,5 cm). Quatre series d'essais pratiques ont ete r6alisees par des agents de sante, des educateurs et des agents de r6adaptation dans 32 pays de la r6gion du Pacifique, d'Asie, d'Afrique et d'Europe orientale pour s'assurer que le mat6riel donne des r6sultats satisfaisants et qu'il repond aux besoins des populations concern6es. La version d6finitive de la fiche de test a 6te utilisee pour evaluer la vision de 127 eleves ag6s de 5 a 20 ans frequentant des ecoles pour aveugles au Kenya et en Ouganda. La sensibilit6 et la sp6cificit6 du test de d6pistage de la perte de vision on ete 6valu6es et comparees a celles de tests classiques pratiqu6s sur 125 pa- tients ages de 4 a 90 ans dans un h6pital australien. Les nouveaux tests de vision lointaine et rapprochee ont donne de bons resultats aussi bien dans les pays en d6veloppement que dans des h6pitaux australiens, ou ils ont 6te ex6cutes sur des enfants de 4 ans et sur des personnes ne parlant pas la meme langue que l'examinateur. La sen- sibilite et la sp6cificite etaient respectivement de 85% et 96% pour les tests de vision lointaine et de 100% et 84% pour les tests de vision rapprochee. L'utilisation de deux tailles d'optotypes pour I'acuit6 visuelle a distance convient aux fins de depistage et pour obtenir des informations de base en vue de 1'6valuation de la vision fonctionnelle. Pr6sent6s a 6 metres et a 3 metres, ces optotypes permettent de classer l'acuit6 visuelle du patient selon les grandes cat6gories de l'OMS, a savoir vision normale, perte de vision ou cecit6. Le test de vision rapproch6e sert a determiner si le patient a une vision utile pour ex6cuter des travaux de pres, quelle est la taille de caracteres n6cessaire pour la lecture, et s'il est justifi6 d'orienter le patient en vue d'un traitement ou d'une correction des vices de refraction. L'information qualitative recueillie aupres des utilisateurs des fiches dans les pays en d6veloppe- ment montre que la pr6sentation et le contenu de ces tests sont adapt6s aux besoins locaux. Les tests ont permis de classer correctement les sujets examin6s comme ayant une vision normale, une perte de vision ou une c6cit6. Ces resultats peuvent etre utilises pour decider d'orienter le patient en vue d'un traitement ou d'une correction des vices de r6fraction et, s'ils sont joints aux resultats de l'6valuation de la vision fonctionnelle, de l'orienter en vue d'une 6valuation de la perte de vision ou d'une r6adaptation. Cette etude a confirme la validite de ces tests dans les contextes examin6s. References 1. Brohier WG. Setting the scene. In: Johnston AW, Lawrence M, eds. Low vision ahead 11. Melbourne, Association for the Blind, 1990: 9-13. 2. Foster A, Johnson G. Blindness in the developing world. British journal of ophthalmology, 1993, 77: 398-399. 3. Keeffe JE et al. Low vision assessment and training materials for use in developing countries. In: Kooijman AC et al., eds. Low vision: research and new develop- ments in rehabilitation. Amsterdam, IOS Press, 1994: 47-50. 4. Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Americanjoumal of optometry and physiological optics, 1976, 53: 740-745. 5. Lovie-Kitchin JE. Validity and reliability of visual acuity measurements. Ophthalmic and physiological optics, 1988, 8: 363-370. 6. Law FW. Reading types. British joumal of ophthalmol- ogy, 1952, 36: 689-690. 7. Johnston AW. Making sense of the M, N, and logMAR systems of specifying visual acuity. Problems in optometry, 1991, 3: 394-407. 8. Sloan LL, Brown DJ. Reading cards for the selection of optical aids for the partially sighted. American jour- nal of ophthalmology, 1963, 55: 1187-1199. 9. Silver J et al. Low vision in East African blind school students: need for optical low vision services. British journal of ophthalmology, 1995, 79: 814-820. 10. International classification of impairments, disabilities, and handicaps. Geneva, World Health Organization, 1980. WHO Bulletin OMS. Vol 74 1996 531 J.E. Keeffe et al. 11. Faye EE. A new visual acuity test for partially-sighted non-readers. Journal of pediatric ophthalmology, 1968, 5: 210-212. 12. Kay H. New method of assessing visual acuity with pictures. British journal of ophthalmology, 1983, 67: 131-133. 13. Fonda GE. Management of low vision. New York, Thieme-Stratton, 1981. 14. Duke-Eider S, ed. System of ophthalmology. Vol. 7. The foundations of ophthalmology. London, Kimpton, 1962. 15. Oliver N, Nawratzki I. Screening of pre-school chil- dren for ocular anomalies. British joumal of ophthal- mology, 1971, 55: 462-466. 16. Thylefors B. Vision screening of illiterate populations. Bulletin of the World Health Organization, 1977, 55: 115-119. 17. Taylor HR. Applying new design principles to the construction of an illiterate E chart. American journal of optometry and physiological optics, 1978, 55: 348- 351. 18. Weale RA. Screening children's eyesight: guide for schoolteachers. World health forum, 1983, 4: 121- 124. 19. Schmidt PP. Vision screening. In: Rosenbloom AA, ed. Principles and practice of pediatric optometry. Philadelphia, PA, Lippincott, 1990: 467-485. 532 WHO Bulletin OMS. Vol 74 1996