Childhood vision screening in Canada: public health evidence and practice.

Mema, Silvina C. ; McIntyre, Lynn ; Musto, Richard 等

Screening is an important public health strategy for disease prevention. It enables the early identification of disease in asymptomatic individuals and allows them to benefit from direct preventive action. (1) Preschool vision screening has been used for the early detection of amblyopia or lazy eye, which is the leading cause of monocular blindness in the 20 to 70 year age group in higher-income countries. (2) Amblyopia is defined as reduced visual acuity (VA) with no obvious structural or pathologic causes that cannot be improved by corrective lenses. (3) The condition develops within the first six years of life, (4) and once established, persists.

Many countries have well-established amblyopia screening programs. (5) In Sweden, for example, children beginning at age 4 are assessed for VA by public health nurses in a program that has an uptake of over 99%. (6) Historically, Canadian public health efforts routinely targeted early detection of amblyopia with trained public health nurses, (7) but a 2007 report (8) showed wide variation in amblyopia screening practices among Canadian provinces.

The decline of amblyopia screening seems to be attributed to reservations about the level of evidence that exists to justify such programming. (9) Evidence-Based Medicine (EBM), a concept embraced in clinical practice, privileges evidence derived from randomized controlled trials. (10) In the case of amblyopia screening, a Cochrane systematic review of the literature stated that "despite the large amount of literature available regarding vision screening no trials designed to compare the prevalence of amblyopia in screened versus unscreened populations were found" (ref. 11, pg. 1). This review (11) and a second systematic review (5) both concluded that there was insufficient evidence on the impact of vision screening.

The lack of evidence from randomized controlled trials about amblyopia screening has been interpreted as lack of evidence in support of screening, with the result that data from cohort studies and natural experiments, which have shown benefits from screening, have been ignored. In addition, little consideration has been given to whether amblyopia fulfills the World Health Organization's principles for screening (12) (Table 1).

We suggest that an Evidence-Based Public Health (EBPH) approach might better bridge evidence and practice in the case of amblyopia screening. (10) EBPH offers a decision-making framework based on research evidence as well as the population characteristics and resources available, and directs policy-makers to accept the 'best available' as opposed to the 'best possible' evidence. (10) In this paper, we review the evidence underlying amblyopia screening using an EBPH approach, and consider implications for Public Health provision of universal screening programs for amblyopia in Canadian jurisdictions in light of present practices.

METHODS

Our review of the evidence for amblyopia screening began with a search of the literature to address each major screening criterion (disease, test, treatment, program requirements). (12) Sources examined included original studies, literature reviews and the grey literature which included Canadian and international practice guidelines. We then searched for the best available evidence necessary to assess Public Health-led amblyopia screening. For this step, we reappraised papers cited in the previously mentioned systematic reviews, from which five original studies had been identified. (5,11) We also searched for new amblyopia studies and found none. The five studies had (at least initially) met the inclusion criteria of the systematic reviews and therefore were considered the "best available evidence"; in addition, they represented population characteristics and used screening tests that were applicable to Canada. (5,11) We then reappraised these studies according to the EBPH framework.

To assess the current involvement of Public Health in amblyopia screening, a short questionnaire was e-mailed to the provincial/territorial Chief Medical Officers of Health (CMOH) asking them to provide information on their jurisdiction's current practice. In some cases, a regional medical officer of health responded to the survey; in others, the CMOH directed the person responsible for the program to reply. Where screening was in place, we asked for details on the tests, the ages of the target group of children, and the qualifications of the personnel administering the tests--factors that are extensively discussed in the literature. (8) Open comments often accompanied the replies. We then compiled all replies and matched the answers with the optometric coverage for each jurisdiction, which was available through the Canadian Association of Optometrists.

RESULTS

Principles for screening and amblyopia

Two detailed reviews provide evidence that amblyopia fulfills the criteria for screening. (13,14) Table 2 summarizes the key characteristics of amblyopia and how the condition meets major criteria for screening.

EBPH evidentiary review of amblyopia screening studies

In Table 3, we summarize the design and key findings of the five public health intervention studies initially considered in two systematic reviews (5,11) that found no studies met inclusion criteria for effectiveness analysis. Despite study design flaws, particularly the lack of randomization, an EBPH approach would suggest that the best available evidence is in favour of preschool screening for amblyopia.

Public Health provision of vision screening in Canadian provinces and territories

Through our survey, we ascertained the status of preschool vision screening programs for the 13 Canadian jurisdictions (Table 4). We found that 7 jurisdictions in Canada have a public health vision screening program and 6 do not. Provinces that reported not having screening were Quebec, Ontario, Saskatchewan, Nunavut and Alberta. Manitoba reported that vision screening was a voluntary program for school divisions and was therefore classified as not having a provincial program. We did not ascertain if other subprovincial jurisdictions offered screening if the program was not provincially mandated.

Some of the jurisdictions where screening is not offered reported that their local associations of optometrists, in partnership with schools, ran a program aimed at creating awareness among parents and advocating for a visual assessment for children of kindergarten age. For at least one jurisdiction, the local Public Health screening program was discontinued as a consequence of this campaign, as parents were encouraged to take advantage of the annual free optometry exam offered in the province. We then determined the relationship between Public Health screening and optometric coverage in each province and territory (Table 4).

In jurisdictions where preschool vision screening is offered, VA testing is preferred and focuses on children between 3 and 5 years of age. Other screening tests mentioned were stereopsis and eye alignment for strabismus. Personnel administering the test are mainly public health nurses but other trained personnel were also deployed. Programs appear to operate within best practice guidelines. (8) Further details are presented in Table 5.

DISCUSSION

Amblyopia screening has been deemed lacking in evidence of effectiveness using EBM methodology because trials have not been conducted that demonstrate a beneficial effect. Due to ethical concerns, it is unlikely that randomized trials will ever be conducted. (5) Therefore, a decision not to screen--as is the case in six Canadian jurisdictions, including the two most populous provinces --raises concerns about missed amblyopia detection and subsequent visual disability. The ground may be set for a natural experiment evaluation; however, there is no surveillance system for the monitoring of visual outcomes in childhood.

Here we reaffirm that amblyopia fulfills the screening criteria. The burden of disease is not limited to a decreased VA, which has been described as the 'tip of the iceberg'. (15) Children are usually asymptomatic and the degree of vision loss increases the longer the condition remains undiagnosed. (16) Most importantly, early detection and treatment of amblyopia are essential because the success of the treatment mainly depends on the age at which treatment is initiated. (17)

An EBPH approach would suggest that the best available evidence favours universal screening for amblyopia. Guidelines from pediatric and ophthalmologic societies agree that visual acuity should be tested in children 3 to 5 years of age (18-20) and Canadian vision care providers support and have interest in the development of screening programs. (21) However, there remains the issue of the lack of rigorous evidence from randomized controlled trials.

The precautionary principle has been used to assist public health decision-making where there is uncertainty. (22) The principle is against delaying a potentially useful intervention while waiting for evidence to support such an intervention, especially when potential adverse health outcomes might result. A precautionary approach to amblyopia screening would therefore take into consideration whether its goal--prevention of a lifelong visual impairment --is sufficiently important to accept the degree of uncertainty with regard to the 'risks of doing nothing' or the risks of other alternatives. (23)

In Canada, there is a Public Health divide with respect to amblyopia prevention, with some jurisdictions offering screening and others not. Screening is offered in all four provinces where children's optometric exams are not covered, and in some with funded optometric coverage. Amblyopia screening and the optometric exam are not mutually exclusive; in fact, children who screen positive need referral to an optometrist or ophthalmologist.

It is important to distinguish among three related but very different approaches to amblyopia prevention: opportunistic vision screening by physicians; universal screening; and the comprehensive eye exam by an eye professional. Preschool vision testing by physicians at well-child visits is a discretionary practice and the literature shows lack of compliance with vision testing by doctors. (24) In addition, physicians must be familiar with vision testing requirements for preliterate children. This practice also relies on parents to bring their children for a check-up at the age that is appropriate for amblyopia screening. Universal screening, on the other hand, is administered by Public Health and implies a systematic population-based approach where all children are reached. Once identified, individuals suspected of having VA difficulties are referred for confirmation by an eye care professional (usually an ophthalmologist or optometrist); the latter will then perform the comprehensive exam, which is the ultimate diagnostic or 'gold standard' test. The purpose of universal screening is to identify those children who are more likely to have amblyopia and refer them for further testing to confirm or rule out the condition. The more expensive, time-consuming diagnostic examination is therefore spared for those who screen positive.

Several concerns arise with regard to Public Health defaulting responsibility of amblyopia screening to optometrists through a comprehensive eye exam system. After intensive pilot implementation campaigns, the optometry exam showed an uptake in Alberta of only 45%, (25) and in Saskatchewan (26) of 63.7%, among whom 51% had received an eye exam previously. In addition, 70% of children examined in the Saskatchewan pilot were diagnosed with vision problems; as this would far exceed amblyopia prevalence, the implication is that the examination either has high false positive rates for amblyopia or is looking for conditions such as regular refractive problems that would not meet public health criteria for screening. Concerns about over-diagnosis leading to unnecessary treatment have been raised in the literature, (27) and the Canadian Pediatric Society states that "routine comprehensive professional eye examinations of healthy children with no risk factors have no proven benefit". (18) Moreover, evidence fails to support the comprehensive eye exam for every child. (27)

The suboptimal uptake of the comprehensive eye exam programs has the potential to increase health disparities. Risk factors for amblyopia are closely linked with socio-economic status (Table 2); this has important public health implications in terms of equity and justifies a universal rather than a voluntary approach for screening.

Even when Public Health delegates the task of screening for amblyopia, it remains responsible for ensuring that whoever is accountable for screening is actually doing it. Surveillance systems are required for the collection of epidemiological data on the incidence and prevalence of diseases that would have important implications for Public Health.

In summary, we have revisited the evidence on amblyopia screening with an EBPH perspective in order to set the grounds for a discussion on whether Public Health's involvement in screening in Canada is justified. The characteristics of amblyopia make it a suitable case for screening despite the absence of randomized controlled trial evidence. There is a Public Health divide in amblyopia screening practice in Canada; while some provinces maintain organized programs, others have chosen to delegate the task to other well-meaning professionals, without a concurrent surveillance function. We conclude that amblyopia deserves attention from Public Health: efforts should be made to maintain existing programs; and provinces without organized screening programs should reconsider their role in the prevention of amblyopia. We acknowledge the need for additional population-based research in order to establish the utility of preschool vision screening in general, (5) and particularly in the Canadian context. (8)

Acknowledgement: We thank Dr. Andre Corriveau, Chief Medical Officer of Health for Alberta, for facilitating the survey.

Conflict of Interest: None to declare.

REFERENCES

(1.) Raffle AE, Gray JAM. Screening: Evidence and Practice. New York, NY: Oxford University Press, 2007.

(2.) Ederer F, Krueger D. Report on the National Eye Institute's Visual Acuity Impairment Survey Pilot Study. Bethesda, MD: Office of Biometry and Epidemiology. NEI, NIH, PHS, DHHS 1984 (NTIS order no. PB84-156173);81-84.

(3.) Hillis A, Flynn JT, Hawkins BS. The evolving concept of amblyopia: A challenge to epidemiologists. Am J Epidemiol 1983;118(2):192-205.

(4.) Keech RV, Kutschke PJ. Upper age limit for the development of amblyopia. JPediatr Ophthalmol Strabismus 1995;32(2):89-93.

(5.) Schmucker C, Grosselfinger R, Riemsma R, Antes G, Lange S, Lagreze W, et al. Effectiveness of screening preschool children for amblyopia: A systematic review. BMC Ophthalmol 2009;9:3.

(6.) Kvarnstrom G, Jakobsson P, Lennerstrand G. Screening for visual and ocular disorders in children, evaluation of the system in Sweden. Acta Paediatr 1998;87(11):1173-79.

(7.) Ross E, Murray AL, Stead S. Prevalence of ambylopia in grade 1 schoolchildren in Saskatoon. Can J Public Health 1977;68(6):491-93.

(8.) Dunfield L, Keating T. Preschool Vision Screening. Health Technology Report. Canadian Agency for Drugs and Technology in Health, February 2007.

(9.) Bray LC, Clarke MP, Jarvis SN, Francis PM, Colver A. Preschool vision screening: A prospective comparative evaluation. Eye (Lond) 1996;10 (Pt 6):714-18.

(10.) Brownson RC, Fielding JE, Maylahn CM. Evidence-based public health: A fundamental concept for public health practice. Annu Rev Public Health 2009;30:175-201.

(11.) Powell C, Hatt SR. Vision screening for amblyopia in childhood. Cochrane Database Syst Rev 2009;(3):CD005020.

(12.) Wilson JM, Jungner YG. Principles and practice of mass screening for disease. Bol Oficina SanitPanam 1968;65(4):281-393.

(13.) Carlton J, Karnon J, Czoski-Murray C, Smith KJ, Marr J. The clinical effectiveness and cost-effectiveness of screening programmes for amblyopia and strabismus in children up to the age of 4-5 years: A systematic review and economic evaluation. Health Technol Assess 2008;12(25):iii, xi-194.

(14.) The Value of Screening for Amblyopia Revisited. 2010. Available at: http://www.springerlink.com/content/pp4m7x7450617362/ (Accessed July 6, 2011).

(15.) von Noorden GK. Amblyopia: A multidisciplinary approach. Proctor lecture. Invest Ophthalmol Vis Sci 1985;26(12):1704-16.

(16.) Donahue SP. Relationship between anisometropia, patient age, and the development of amblyopia. Am J Ophthalmol 2006;142(1):132-40.

(17.) Wiesel TN, Hubel DH. Single-cell responses in striate cortex of kittens deprived of vision in one eye. J Neurophysiol 1963;26:1003-17.

(18.) Vision screening in infants, children and youth. 2010. Available at: http://www.cps.ca/english/statements/cp/cp09-02.htm (Accessed August 10, 2010).

(19.) A Joint Statement of the American Association for Pediatric Ophthalmology and Strabismus and the American Academy of Ophthalmology. Vision Screening for Infants and Children, 2007.

(20.) Rourke Baby Record. 2010. Available at: http://www.rourkebabyrecord.ca/ (Accessed August 18, 2010).

(21.) Reed MJ, Kraft SP. Vision health care providers' attitudes and experiences with preschool vision screening in Ontario. Optom Vis Sci 2004;81(7):548-53.

(22.) Richter ED, Laster R. The Precautionary Principle, epidemiology and the ethics of delay. Int J Occup Med Environ Health 2004;17(1):9-16.

(23.) Nuffield Council on Bioethics. Public Health: Ethical Issues. 2007.

(24.) Marsh-Tootle WL, Wall TC, Tootle JS, Person SD, Kristofco RE. Quantitative pediatric vision screening in primary care settings in Alabama. Optom Vis Sci 2008;85(9):849-56.

(25.) Muzychka M. Environmental Scan of Vision Health and Vision Loss in the Provinces and Territories of Canada. For the National Coalition for Vision Health, 2009.

(26.) The Saskatchewan Association of Optometrists. Available at: http://www.optometrists.sk.ca/childrens_vision.php (Accessed January 11, 2011).

(27.) Wallace DK. Mandating comprehensive eye examinations for children: Where is the evidence? Ophthalmology 2008;115(8):1271-72.

(28.) Webber AL, Wood J. Amblyopia: Prevalence, natural history, functional effects and treatment. Clin Exp Optom 2005;88(6):365-75.

(29.) Barry JC, Konig HH. Test characteristics of orthoptic screening examination in 3 year old kindergarten children. Br J Ophthalmol 2003;87(7):909-16.

(30.) Preslan MW, Novak A. Baltimore Vision Screening Project. Phase 2. Ophthalmology 1998;105(1):150-53.

(31.) Drover JR, Kean PG, Courage ML, Adams RJ. Prevalence of amblyopia and other vision disorders in young Newfoundland and Labrador children. Can J Ophthalmol 2008;43(1):89-94.

(32.) Webber AL, Wood JM, Gole GA, Brown B. The effect of amblyopia on fine motor skills in children. Invest Ophthalmol Vis Sci 2008;49(2):594-603.

(33.) Hrisos S, Clarke MP, Kelly T, Henderson J, Wright CM. Unilateral visual impairment and neurodevelopmental performance in preschool children. Br J Ophthalmol 2006;90(7):836-38.

(34.) Stifter E, Burggasser G, Hirmann E, Thaler A, Radner W. Monocular and binocular reading performance in children with microstrabismic amblyopia. Br J Ophthalmol 2005;89(10):1324-29.

(35.) Chua B, Mitchell P. Consequences of amblyopia on education, occupation, and long term vision loss. Br J Ophthalmol 2004;88(9):1119-21.

(36.) Packwood EA, Cruz OA, Rychwalski PJ, Keech RV. The psychosocial effects of amblyopia study. J AAPOS 1999;3(1):15-17.

(37.) Rahi J, Logan S, Timms C, Russell-Eggitt I, Taylor D. Risk, causes, and outcomes of visual impairment after loss of vision in the non-amblyopic eye: A population-based study. Lancet 2002;360(9333):597-602.

(38.) Thompson JR, Woodruff G, Hiscox FA, Strong N, Minshull C. The incidence and prevalence of amblyopia detected in childhood. Public Health 1991;105(6):455-62.

(39.) Attebo K, Mitchell P, Cumming R, Smith W, Jolly N, Sparkes R. Prevalence and causes of amblyopia in an adult population. Ophthalmology 1998;105(1):154-59.

(40.) Rantakallio P, Krause U, Krause K. The use of the ophthalmological services during the preschool age, ocular findings and family background. J Pediatr Ophthalmol Strabismus 1978;15(4):253-58.

(41.) Miller M, Israel J, Cuttone J. Fetal alcohol syndrome. J Pediatr Ophthalmol Strabismus 1981;18(4):6-15.

(42.) Dubowy SM. Vision screening of young children: Taking a second look. JAAPA 2005;18(8):41-43, 47-49.

(43.) Schmucker C, Grosselfinger R, Riemsma R, Antes G, Lange S, Lagreze W, et al. Diagnostic accuracy of vision screening tests for the detection of amblyopia and its risk factors: A systematic review. Graefes Arch Clin Exp Ophthalmol 2009;247(11):1441-54.

(44.) Kvarnstrom G, Jakobsson P. Is vision screening in 3-year-old children feasible? Comparison between the Lea Symbol chart and the HVOT (LM) chart. Acta Ophthalmol Scand 2005;83(1):76-80.

(45.) Agence nationale d'accreditation et d'evaluation en sante (ANAES) (2002). Early vision screening in children to prevent amblyopia. 2002. Available at: http://www.has-sante.fr/portail/upload/ (Accessed September 15, 2010).

(46.) Stewart CE, Fielder AR, Stephens DA, Moseley MJ. Treatment of unilateral amblyopia: Factors influencing visual outcome. Invest Ophthalmol Vis Sci 2005;46(9):3152-60.

(47.) Hussein MA, Coats DK, Muthialu A, Cohen E, Paysse EA. Risk factors for treatment failure of anisometropic amblyopia. J AAPOS 2004;8(5):429-34.

(48.) Loudon SE, Fronius M, Looman CW, Awan M, Simonsz B, van der Maas PJ, et al. Predictors and a remedy for noncompliance with amblyopia therapy in children measured with the occlusion dose monitor. Invest Ophthalmol Vis Sci 2006;47(10):4393-400.

(49.) Hudak DT, Magoon EH. Poverty predicts amblyopia treatment failure. J AAPOS 1997;1(4):214-15.

(50.) Loudon SE, Passchier J, Chaker L, de Vos S, Fronius M, Harrad RA, et al. Psychological causes of non-compliance with electronically monitored occlusion therapy for amblyopia. Br J Ophthalmol 2009;93(11):1499-503.

(51.) Horwood J, Waylen A, Herrick D, Williams C, Wolke D. Common visual defects and peer victimization in children. Invest Ophthalmol Vis Sci 2005;46(4):1177-81.

(52.) Schmucker C, Kleijnen J, Grosselfinger R, Riemsma R, Antes G, Lange S, et al. Effectiveness of early in comparison to late(r) treatment in children with amblyopia or its risk factors: A systematic review. Ophthalmic Epidemiol 2010;17(1):7-17.

(53.) Neubauer AS, Neubauer S. Cost-effectiveness of screening for amblyopia. Klin Monbl Augenheilkd 2005;222(2):110-16.

(54.) Eibschitz-Tsimhoni M, Friedman T, Naor J, Eibschitz N, Friedman Z. Early screening for amblyogenic risk factors lowers the prevalence and severity of amblyopia. J AAPOS 2000;4(4):194-99.

(55.) Williams C, Northstone K, Harrad RA, Sparrow JM, Harvey I, ALSPAC Study Team. Amblyopia treatment outcomes after screening before or at age 3 years: Follow up from randomised trial. BMJ 2002;324(7353):1549.

(56.) Williams C, Northstone K, Harrad RA, Sparrow JM, Harvey I, ALSPAC Study Team. Amblyopia treatment outcomes after preschool screening v school entry screening: Observational data from a prospective cohort study. Br J Ophthalmol 2003;87(8):988-93.

(57.) Rasmussen F, Thoren K, Caines E, Andersson J, Tynelius P. Suitability of the Lang II random dot stereotest for detecting manifest strabismus in 3-year-old children at child health centres in Sweden. Acta Paediatr 2000;89(7):824-29.

Received: March 31, 2011

Accepted: July 20, 2011

Silvina C. Mema, MD, MSc, [1] Lynn McIntyre, MD, MHSc, FRCPC, [2] Richard Musto, MD, FRCPC [3]

Author Affiliations

Department of Community Health Sciences, Faculty of Medicine, University of Calgary, Calgary, AB

[1.] Public Health and Preventive Medicine Resident

[2.] Professor and CIHR Chair in Gender and Health

[3.] Clinical Associate Professor

Correspondence: Dr. Silvina Mema, Department of Community Health Sciences, Faculty of Medicine, University of Calgary, 3rd Floor, Teaching Research & Wellness Building, 3280 Hospital Drive NW, Calgary, AB T2N 4Z6, Tel: 403-210-7034, Fax: 403-270-7307, E-mail: scmema@ucalgary.ca

Table 1. Principles of Screening (12)

1. The condition should be an important health problem.

2. There should be an accepted treatment for patients with
recognized disease.

3. Facilities for diagnosis and treatment should be available.

4. There should be a recognizable latent or early symptomatic
stage.

5. There should be a suitable test or examination.

6. The test should be acceptable to the population.

7. The natural history of the condition, including development from
latent to declared disease, should be adequately understood.

8. There should be an agreed policy on who to treat as patients.

9. The cost of case-finding (including diagnosis and treatment of
patients diagnosed) should be economically balanced in relation to
possible expenditure on medical care as a whole.

10. Case-finding should be a continuing process and not a "once and
for all" project.

Table 2. Principles of Screening for Amblyopia

The Condition

* Prevalence of amblyopia among preschool children in Western
countries is between 2% and 5%. (28-30)

* In Canada, studies have reported prevalences of 8.3 per thousand
(7) and 4.7%, (31) respectively.

* Disability associated with amblyopia is not limited to reduced
VA. Other functions affected are: normal development of stereopsis,
essential for 3D vision; motor skills; (32,33) and reading speed
ability. (34)

* Patients with amblyopia achieve lower levels of educational
attainment (35) and suffer from poorer emotional well-being. (36)

* There is a substantial risk of blindness in amblyopic patients
from injury or disease to the healthy eye. (37)

* Amblyopia is clinically defined as having one or more lines
difference in VA between eyes, (38) can present with varying levels
of severity and usually affects one eye only.

* Predisposing conditions for amblyopia are: a difference in
refraction between eyes, or anisometropia (50%); strabismus (19%);
a combination of both (27%); and more rarely, media opacification
(4%). (39)

* Amblyopia develops in infants and very young children, beginning
only during the first few years of life; (4) once established, it
typically persists for life.

* Risk factors for amblyopia are: prematurity; small for
gestational age; having a first-degree relative with amblyopia; and
neurodevelopmental delay. Maternal smoking and maternal use of
drugs or alcohol during pregnancy are also associated with
increased risk of amblyopia in the child. (40,41)

* Amblyopia produces few symptoms because usually only one eye is
affected and the patient has normal acuity; preschool children can
function using only one eye even if vision is severely reduced in
the other eye. (42)

* The earlier in childhood the predisposing condition and the
longer the duration, the more profound the level of amblyopia. (28)
In one study, the prevalence of anisometropic amblyopia was found
to rise rapidly after two years of age. The authors concluded that
by age 3, amblyopia has already occurred in most children in whom
it will develop, and that although after this age the prevalence of
amblyopia increases only slightly, the degree of amblyopia becomes
more profound. (16)

The Screening Test

* Sensitivity and specificity in visual acuity tests ranged between
9 and 100% and 8 and 100%, respectively; in auto and
photorefractors, between 46 and 95% and 53 and 100%, respectively;
in stereoacuity tests, between 14 and 100% and 76 and 99%,
respectively. (43)

* The overall sensitivity of screening tests improved with the age
of the child (8, 12, 18, 25 and 31 months vs. 37 months) while
specificity remained unchanged.43 Three-year-old children can be
reliably examined with VA tests, however the testability rate was
approximately 10% better in 4-year-old children. (44)

* Nurse screening showed a sensitivity of 83% and a specificity of
95%; (43) personnel with training are more accurate than lay
persons (i.e., parents) in detecting deficits of VA. (13)

* The gold standard for diagnosis is a comprehensive eye
examination including cycloplegic retinoscopy. (45)

The Treatment

* Treatment involves correction of the underlying predisposing
factor, refractive correction, and/or patching or pharmacological
blurring of vision in the good eye in order to stimulate the 'lazy
eye'. (14)

* The success of the treatment for amblyopia is measured by the
extent of vision restoration. (14)

* Treatment for amblyopia is more effective prior to the age of 7
years; despite some evidence to suggest that successful treatment
of amblyopia in the older child is possible, earlier intervention
is more advantageous. (14)

* Additional factors that impact visual outcome are the depth of
vision loss at the start of the treatment (46) and compliance with
the treatment. (47)

* The threshold applied for failing vision screening varies
depending on local or national practice patterns, the test used and
the age at screening. (11)

* 'The clinical management of amblyopia is determined following
careful consideration on a case-per-case basis, taking into account
a number of factors including the type of amblyopia present, the
patient's age, and the level of VA in the amblyopic eye' (ref. 14,
pg. 103).

* Compliance with treatment is essential for success of the
therapy; often, the amount of occlusion the child receives is less
than that prescribed by the clinician. (14)

* Predictors of low compliance are: poor parental fluency in the
national language, a low level of education, and poor acuity at the
start of treatment; (48) poverty has been associated with poorer
outcomes in amblyopia treatment. (49) Psychological or other causes of
non-compliance (50) are: poor parental knowledge about amblyopia,
perceived distress in the child when patched, and lower self esteem
in the child when patched.

* Educational programs primarily aimed at the child have been
proven to improve compliance. (48)

* Concerns about bullying of children, especially with eye
patching, at school leading to poor compliance (51) further support
earlier treatment of amblyopia, i.e., prior to school entry. (52)

The Screening Program

* Screening for amblyopia is cost-effective;53 universal eye
examination for preschool children has a low cost per QALY. (8)

* The location of the screening program is likely to affect uptake.
(13)

Table 3. Design and Critique of Articles Cited in Recent Systematic
Reviews (5,11) on the Effectiveness of Vision Screening in Reducing
the Prevalence of Amblyopia

Eibschitz-Tsimhoni et       * Description: Retrospective cohort study
  al., 2000 (54) *          from Israel that presents the results of
                            a natural experiment. The authors
                            compared the prevalence of amblyopia in
                            two populations of 8 year olds from two
                            different cities, one of which offers
                            systematic screening for amblyopia or its
                            risk factors for children between the
                            ages of 1 and 2.5 years, the other of
                            which does not offer screening.

                            * Findings: Statistically significant
                            difference in the prevalence of amblyopia
                            was found between groups: 1% in the
                            screened vs. 2.6% in the control
                            population. In addition, the prevalence
                            of severe amblyopia (defined by the
                            authors as a VA [less than or equal
                            to]20-60 in the amblyopic eye) was 17
                            times higher in the non-screened
                            population.

                            * Amblyopia definition: Corrected VA
                            [less than or equal to]20/40 or >1 line
                            difference in corrected VA between both
                            eyes.

                            * Critique: Of the 988 children invited
                            for screening at between 1 and 2.5 years
                            of age, 808 complied and were examined,
                            and all 808 were later re-examined at 8
                            years of age. Schmucker et al. (5)
                            criticized the exclusion of the 180 non-
                            screened children (almost 20%) from the
                            statistical analysis.

Williams et al., 2002       * Description: This trial assessed the
  (55) * ([dagger])         effectiveness of early treatment for
                            amblyopia in children from a larger
                            cohort study. Intervention was intensive
                            orthoptic screening at 8, 12, 18, 25, 31
                            and 37 months (intensive group). Children
                            in the control group were offered similar
                            testing at 37 months only. Children were
                            'pseudo-randomized' into either group
                            according to the last digit in the day of
                            the mother's date of birth. The study
                            included 3,490 children in total; any
                            child in either group who failed a test
                            was referred to the hospital eye service.
                            The outcome assessment consisted of a VA
                            exam of the children at 7.5 years of age.

                            * Findings: Amblyopia in children at 7.5
                            years of age was statistically less
                            prevalent in the intensive group than in
                            the control group (0.6% vs. 1.8%).
                            Additionally, children with amblyopia in
                            the intensive screening protocol had
                            better acuity than those screened only at
                            37 months.

                            * Amblyopia definition: Better VA (with
                            glasses or pinhole) of 0.2 LogMAR (or
                            worse), or a difference of best acuity
                            between the two eyes of 0.2 or more.

                            * Critique: Attrition; only 54% of
                            children in the intervention group and
                            55% in the control group attended the
                            final examination. The authors
                            acknowledged selection bias; children who
                            attended the final examination were more
                            likely to have more educated mothers and
                            less likely to have been born to a
                            teenage mother or to have weighed less
                            than 2500 g at birth.

Williams et al., 2003       * Description: This report extends the
  (56) * ([dagger])         study by Williams et al., 2002 and uses
                            data from all children who attended
                            examination at age 7.5. The intervention
                            group received vision screening at 37
                            months while children in the control
                            group came from districts where screening
                            is not offered. Outcome was prevalence of
                            amblyopia at age 7.5 in each group.

                            * Findings: The prevalence of amblyopia
                            was approximately 45% lower in the group
                            who received preschool screening than in
                            the control group (adjusted ORs 0.63 to
                            0.72 for different amblyopia
                            definitions). However, because only 67%
                            of the invited children attended
                            screening, the analysis by 'intention to
                            screen' reduced and eliminated the
                            benefit attributable to early screening.

                            * Amblyopia definition: Same as in
                            Williams et al., 2002.

                            * Critique: Attrition; of the
                            approximately 14,000 children recruited
                            in the original study, around 85% were
                            eligible to participate. Of those, only
                            67% attended visual examination at age 7.
                            Powell et al. (11) contacted the authors
                            directly, but data were not available for
                            children who had missed screening in this
                            series of two studies.

Rasmussen et al., 2000      * Description: A Swedish randomized
  (57) * ([dagger])         controlled clinical trial aimed at
                            examining the suitability of adding a
                            test for stereopsis (the Lang Stereotest)
                            at three years of age as a supplementary
                            method to the already existing vision
                            screening program aimed at 4 year olds in
                            Sweden. Children were randomized to
                            either the intervention or control group.
                            The outcome was prevalence of strabismus
                            and amblyopia at 6.5 years of age.

                            * Findings: At 6.5 years of age, there
                            was no statistical difference in the
                            prevalence of amblyopia between groups.

                            * Amblyopia definition: Not specified.

                            * Critique: This study is 'the only
                            randomized controlled clinical trial'
                            among papers studying the effect of
                            screening for amblyopia, and it 'did not
                            find a difference in the prevalence rate
                            of amblyopia between the groups'. (8)
                            This interpretation has to be read
                            carefully, however, because the study was
                            aimed at examining whether adding a
                            supplementary test would change
                            prevalence. One must also take into
                            account that Sweden provides universal
                            visual screening of children at age four.

Bray et al., 1996 (9) *     * Description: Comparison of three
                            cohorts of children residing in three
                            different areas with different preschool
                            vision screening programs. Only one
                            cohort received orthoptic screening; in
                            the other two groups, screening was
                            conducted by other personnel. Outcome was
                            presence of vision defects at age 7
                            years.

                            * Findings: Although many more cases of
                            amblyopia were detected in the cohort
                            screened by an orthoptist, the overall
                            amblyopia prevalence at age 7 years was
                            similar in each cohort.

                            * Amblyopia definition: 6/9 or worse on
                            Snellen chart.

                            * Critique: Ecologic study. Lack of

                            effects on the prevalence rate of
                            amblyopia using an 'intention to screen'
                            approach.

* Cited in systematic review by Schmucker et al. (5)

([dagger]) Initially considered and then excluded from systematic
review by Powell et al. (11)

Table 4. Public Health/operated Preschool Vision Screening and
Optometry Coverage in Canadian Provinces/Territories

                                              Optometric Coverage
Province                     Screening           for Children *

Alberta                          No       Yes
British Columbia                Yes       Yes
Nova Scotia                     Yes       Yes
Northwest Territories           Yes       Yes (Note: No optometrists
                                          practice in this territory)
Prince Edward Island            Yes        No
Yukon                           Yes       Yes
Newfoundland and Labrador       Yes        No
New Brunswick                   Yes        No
Ontario                          No       Yes
Manitoba                         No       Yes
Quebec                           No       Yes
Saskatchewan                     No       Yes
Nunavut                          No       Yes (Note: No optometrists
                                          practice in this territory)

* Source: Canadian Association of Optometrists.

Table 5. Characteristics of Programs in Provinces Where Preschool
Vision Screening is Offered

Province            Details

British Columbia    * Test: Automated vision screening, stereopsis. A
                      few health units use VA

                    * Age: In 2008, began piloting screening with 3
                      year olds. Intend to transition to a universal
                      3-year-old screening and discontinue
                      kindergarten screening once universal 3-year-
                      old coverage is met

                    * Personnel: Public health nurses, health unit
                      aides, trained screeners, trained First Nations
                      community health staff

Nova Scotia         * Test: VA, stereopsis

                    * Age: Preschool children, approximately 4.5 to 5
                      years of age (a few areas offer screening to
                      slightly younger children, but the youngest
                      would be 3.5 years)

                    * Personnel: Public health practitioners (LPNs
                      and nurses)

Northwest           * Test: VA
Territories

                    * Age: Preschool children

                    * Personnel: Public health nurses

Prince Edward       * Test: Comprehensive Vision Health History--
Island                Parent/completed questionnaire (when child is 4
                      months old, or any age if this has not been
                      done before); Brief Vision Health History *;
                      external inspection (external eye and
                      surrounding structures) *; pupillary
                      examination/light response; observation for
                      alignment (to identify constant strabismus) *;
                      corneal light reflex; Cover Uncover Test (near
                      and distant); distance visual acuity *;
                      stereopsis

                    * Age: 4 years and *Grade Four in our School
                      Health Program

                    * Personnel: Public health nurses

Yukon               * Test: Eye motility, VA and stereopsis

                    * Age: Pre-Kindergarten

                    * Personnel: Public health nurses and primary
                      health care nurses (RNs working in an expanded
                      role) depending on the community

Newfoundland and    * Test: Eye alignment and motility, VA
Labrador

                    * Age: Between 3 years 9 months and 4 years 4
                      months

                    * Personnel: Public health nurses

New Brunswick       * Test: Visual inspection of the eye, VA and
                      stereopsis

                    * Age: 3.5 year olds

                    * Personnel: Public health nurses