Community photoscreening of six to nine month old infants for amblyopiogenic risk factors

Original Articles

Community photoscreening of six to nine month old infants for amblyopiogenic risk factors Carolyn Hope, FRACO* Joanne Roulston, DBOT Caroline Hoey, DBOt Andrew Wong, MB CHBS Gillian Clover, PhD, FRACO, FCOphth"

Abstract Photorefraction (PR) is gaining acceptance as potentially the most effective objective screening technique for amblyopia risk factors in the preverbal child. This study determined the validity and feasibility of using the Auckland eccentric photorefractor in the detection of amblyopiogenic factors in six to nine month old infants in an established community-based vision screening program. Photographs were analysed and compared to results of clinical examination including cycloplegic refraction.

Amblyopia risk factors were present in 7.2% of the infants clinically examined. Analysis only of readable photographs in children who were also clinically examined, gave sensitivities ranging from 71% to 79%, and specificities ranging from 81% to 86%. Inclusion in the analysis of photofailures lowered sensitivity figures to 56% to 61 %, and specificity to 63% to 70%. Photofailures were predominantly due to poor operator technique. Calculation of kappa scores indicated fair observer reliability.

In conclusion, PR could provide a feasible and sufficiently reliable screening technique in the

infant, but requires adequate training and auditing of screening personnel performance for optimum effectiveness.

Key words: Amblyopia risk factors, community, photorefraction, vision screening.

School entrant vision screening programs detect children with previously undiagnosed amblyopia and amblyopia risk factors. Treatment resulting from these presentations is less effective than treatment begun at an earlier age. Evidence exists that treatment at an earlier age results in a better final visual result with shorter treatment times, more rapid improvement in visual acuity and better overall compliance with treatment regi- mens compared with the school-age child.'"

Emerging evidence from the work of Atkinson and others, suggests that accurate correction of hyperopic refractive errors in infancy not only reduces the incidence of accommodative esotropia, but also improves the final visual

However, refractive correction may inhibit emmetropisation and the timing for refractive correction has yet to be e~tablished.'.~

*Ophthalmic Section, Department of Surgely, University of Auckland Medical School, Auckland, New Zealand.

torthoptics Unit, Auckland Healthcare, Auckland.

$Department of Community Medicine, University of Auckland Medical School, Auckland.

Reprints: Dr C Hope, Ophthalmic Section, Department of Surgery, School of Medicine, University of Auckland, Private

Bag 92019, Auckland, New Zealand.

Photorefraction in infants 193

Ingram's data suggests that the amount of meridional hypermetropia, that is the maximum amount of hypermetropia in the most hypermetropic meridian of a pair of eyes, is a good predictive indicator of later development of squint and

His prospective study determined that amblyopia was highly likely (48%) if a child had +3.50 or more dioptres of hyperopia at age one year; 45% of children with this refraction also had a squint."

Photorefraction provides an effective means of screening for conditions known to be associated with the later development of squint and amblyopia, in particular high ametropia in one or both eyes. Its major advantages are that it is an objective test and thus not age dependant, requires minimal cooperation, is non-invasive, and can be administered by lay personnel. Additionally, the Auckland model (Optocam) is ideally suited for use in the community. A pilot study screening predominantly older children referred to Auckland Hospital Ophthalmology Department with suspected vision defects found the camera to be potentially more effective in identifying amblyopia risk factors, and to be at least as effective as conventional screening methods in the older child.'*

This study aimed to determine the potential effectiveness of the Auckland photorefractor as a community vision screening device for amblyopia risk factors and to establish the prevalence of vision defects in this population. Screening was carried out by non-medical personnel in a population of six to nine month old infants. This cohort was selected since it is the youngest age at which the fixation required by this technique is reliable. Also, infants of this age are still in frequent contact with primary health care providers (there being a 50% fall-off in attendance at the 12 and 18 month routine developmental screenings), who acted as a referral source for this study, as well as a potential contact point if PR screening should become established in the community.

Subjects and methods

Details of the basic camera design and photographic technique have been described previously.'2 The film material used was 200 ASA Kodak Ektachrome with lens aperture set to 5.6, and to four for pigmented fundi.

Between August 1991 and July 1992, six to nine month old infants living in the South Auckland suburb of Manurewa were recruited by Plunket Nurses when they attended for routine developmental screening checks at one of five community health clinics in the area. No parent declined to have their child photographed. Statistics from the Royal New Zealand Plunket Society show that 95% of infants routinely attend Plunket Nurses in South Auckland at this age. No attempt was made to recruit infants from other sources, such as public health nurses or general practitioners. Manurewa was selected because of its ethnic diversity and population of 1100 infants.

Informed consent was obtained from the care giver for photography, subsequent ophthalmic examination and use of information in the study. Infants were photorefracted by the nurse as part of routine vision screen. This includes a parental questionnaire on visual behaviour, and an examination for squint, using the Hirschberg corneal reflex test. Infants were identified by means of a code on a self-adhesive label placed on the fore- head. The code consisted of a letter (assigned to one of five Plunket Centres), and a number (corresponding to the chronological order in which the photograph was taken). The only facility necessary was a room at least two metres long, which could be sufficiently darkened for the photography of dilated pupils. Photographs were repeated if there was doubt as to whether the infant was fixating the camera. Before the study began, Plunket nurses received training in the technique, and a trial period to ensure good quality photographs were being obtained. As rolls of film were completed they were sent to the Ophthalmology Department for development, and analysis by an orthoptist (JR) and by an ophthalmologist (CH).

Ophthalmic examination

All infants enrolled in the study were sent appointments to attend the ophthalmology clinic for a complete eye examination, including cycloplegic refraction and funduscopy. Examinations were performed in a masked fashion by the same ophthalmologist, within three months of photography. Retinoscopy was performed at two to three metres, 40 minutes after instillation of a combination of one drop I % cyclopentolate and one drop 2.5% phenylephrine. After allowing for

Australian and New Zealand Journal of Ophthalmology 1994; 22(3) 194

working distance, cycloplegic refraction and the maximum meridional refraction (MMR) was recorded for a pair of eyes, that is, the maximum plus in either meridian for either eye in hypermetropic refractions, and the maximum minus for myopic refractions. 0.75 dioptres was additionally deducted from all hypermetropic refractions, which is conventionally allowed for the effects of cycloplegia.

Our criteria for significant ophthalmic abnormality were:

manifest squint; hypermetropia (MMR of +3.50 D or more in

anisometropia ( 2 D or more between corre-

pathology (including external eye disease and

either eye);

sponding axes; and

media opacity).

Photo analysis

Unmounted colour photographs were projected using a Slidex projector (Slidex Corp, Tokyo, Japan), which gives a magnification which approximates actual corneal size and thus allows direct measurement of different parameters. Measurements, using a millimetre rule, of pupil diameter (PD) and maximum bright crescent width (Cw) were recorded as a ratio CW:PD for each eye. In the absence of cycloplegia - which results in both variations in individual pupil size and amount of accommodation taking place - predictions about expected crescent width for different degrees of refractive error and pupil size based upon theoretical formulae becomes more complex. Thus we used this simple ratio, C W P D for screening purposes, based on a pilot study which showed that infants with hypermetropia greater than 3.00 D had crescents predominantly involving at least one-third of the pupil diameter, and with crescent width increasing with increasing defocus.12

The pupil diameter under scotopic conditions in our experience is usually greater than 6 mm (range 5 to 8.5 mm).

Our criteria for photographic abnormality were as follows.

Squint Squints were recognised by a measured displacement of the corneal reflex andlor a generalised brightening of the red reflex in the squinting eye.

Hypermetropia As detailed previously, (CW:PD >0.33) was used as the criterion for abnormality with respect to hypermetropia.

Anisometropia Any measured difference in ratio CW:PD between eyes was identified as possible anisometropia.

Pathology We looked for the presence of ocular pathology, since photorefraction allows examination of the clarity of the ocular media, and for presence of external ocular disease.

Other refractive errors considered were as follows.

Myopia Analysis of photographs included identification of myopic crescents. However, myopia was not considered a risk factor for amblyopia for the purposes of this study and thus not included in the statistical analysis of photorefraction validation, but was included in statistics on the prevalence of visual defects.

Astigmatism Astigmatism is determined by a rotation of the position of the bright crescent depending upon the axis. We did not identify the presence of astigmatism as a separate clinical entity in the photoanalysis, but astigmatism was clinically identified in statistics on the prevalence of visual defects.

A more detailed description of how photographs were analysed is made in an earlier publi- cation.I2

Three separate analyses of each photograph were carried out. This allowed US to examine the interobserver reliability of photoanalysis. Initially, an orthoptist who was trained in photoanalysis read all the films in a masked fashion during the course of the study (Reader 1). At study completion the ophthalmologist (CH) read all the films (Reader 2). The h a 1 reading was carried out by the same orthoptist (Reader 3), but after study completion.

Photographs of insufficient quality for analysis were deemed photofailures (Pn. Despite efforts to recall PFs for repeat photographs, this did not prove to be feasible. Allowances have been made in statistical analysis for PFs and DNAs.

Results

A total of 368 infants were photographed, 194 male and 174 female, comprising the following racial mix: Europeans 254 (69%); Maoris 71 (19%); Pacific Islanders 25 (7%); and Other (Chinese, Indian) 18 (5%). Two hundred and


seventy eight infants presented for ophthalmic examination (90 DNAs). Photographs were analysed by each reader and compared to clinical examination/ refraction.

Infants identified with risk factors for amblyopia

Two hundred and seventy eight infants were clinically examined, including cycloplegic refraction. Details of the infants identified with amblyopia risk factors are in Table 1.

Squint Three infants were identified with manifest squint (Cases 9, 10, 11). No cases of squint were missed. These squints were readily detected by a displacement of the corneal reflex

Table 1. Infants clinically examined with risk factors for amblyopia (n=278)

No. Race Risk factor Refraction MMR R1 R2 R3 (max cw:pd)

1. 0

2. E

3. M

4. E

5. E

6. E

7. E

8. E

9. E

10. M

11. E

12. E

13. E

14. E

15. E

16. E

17. E

18. P

H

H

H+A

H

H

H+A

H

H

H+S (ET) S (ET) S (XT) P

P

A

H

H

H

H

+4.75/-1.00~180 +4.75 0.42 0.42 0.41 +4.25/-1.00~180 +4.75/-2.00~180 +4.75 0.67 0.67 0.58 +4.25/-1.50~180 +3.25/-0.50~180 +6.75 0.44 0.50 0.40

t3.75 +4.75 0.67 0.56 0.67 +6.75/-1.50~180

t4.751-0.50~180 t3.751-1.50~180 t3.75 0.15 0.07 0.14 +3.25/-1.00~180 +6.75/-1.50~180 t6.75 0.16 0.42 0.38 +3.75/-0.50~180 +3.25 +3.75 0.57 0.70 0.14 +2.75 +3.75/-1.00~180 +3.75 0.00 0.00 0.00 +3.75/-1.00~180 +5.75 +6.75 0.67 0.67 0.67 +6.75 ET ET ET +0.75 +0.75 ET ET ET

+0.50/-1.00~180 XT XT XT +0.75/-2.00~180 +0.75/-2.00~180 P P P +0.25 +0.25 P P P + 1.25/-1.00~180 0.00 0.00 0.00 +3.25/-1.50~180 +3.75/-3.00~180 +3.75 photo failure

+4.75/-2.00~180 +5.25 photo failure +5.25/-2.00~180 +3.75/-1.00~180 +3.75 photo failure +3.75/-1.00~180 +5.25/-0.50~180 +5.25 photo failure +4.75

+0.50/-1.00~180

+3.75/-3.00~180

H=hypermeuopia; A=anisomeuopia; %squint; P=pathology; ET=esouopia; XT=exotropia; O=Chinese; E=European; M=Maori; P=Polynesian.

associated with a significant generalised brightening of the red reflex in the squinting eye.

False positive: Reader 2 and 3 identified four infants, and reader 1, seven infants with a lightening of the red reflex in one eye. There was no associated measured displacement of the red reflex. This slight lightening of the reflex in one eye could not be explained easily upon difference in fundus pigmentation (in only one eye), or in difference in pupil size in the presence of equally measured pupil sizes. A sufficiently large refractive error may produce a total brightening of the pupil.

Hypermetropia All infants with refractions greater than +4.00 D were identified with CW:PD ratios greater than 0.4 (Cases 1-4, 6, 7, 9).

False negatives: Case 5 had an insignificant bright crescent and Case 8 no visible crescent on the photograph. These refractions were at the lower limit of abnormality and the absence of significant crescent reflects the variable accommodation occurring in the absence of cycloplegia. There were two misreadings, Cases 6 and 7, by readers 1 and 3 respectively.

False positives: significant crescents were identified in one or both eyes of 19 infants (Reader l), 16 infants (Reader 2), and 15 infants (Reader 3) in the absence of significant refractive error on clinical examination. Cycloplegic refractions ranged from plano to +2.75 D.

Anisometropia There were only two infants with clinically significant anisometropia. Case 3 was identified by all readers as having significant hypermetropia with anisometropia.

False negatives: Case 14 demonstrated no visible crescent in either eye.

False positives: small differences in hypermetropic CW:PD ratios were not infrequent, but none was associated with any refractive differences on clinical examination. They accounted for 12 of the total false positives (Readers 1 and 3) , and 10 (Reader 2).

Pathology Case 12, with significant ptosis associated with blepharophimosis, and Case 13, with left ptosis, were readily identified on the photographs and had not previously been clinically diagnosed. No other ocular pathology, including fundus abnormality, was clinically detected in this group of infants.

196 Australian and New Zealand Journal of Ophthalmology 1994; 22(3)

Photofailures

We grouped and analysed the cause of photofailures for each reader (Table 2). Operator technique, and non-fixation were the main reasons for photofailure. Of all the photographs (n= 368), the readers classified between 19.8% and 24.1% to be photofailures. Some readers were able to analyse photographs that others could not.

Photorefraction result

Table 2. Photofailures

Positive: infant True positive False positive appears to have risk

Negative: infant False negative True negative appears not to have ( 4 (d) c+d risk factors

a+b (b) factors (a)

Did not attenders

Twenty five per cent (90 of 368) of the initial population screened were DNAs. Photo readings of the DNAs for the three readers respectively were as follows. Photofailures 25.6%, 24.4%, 27.8%, abnormal criteria 15.6%, 14.4%0, 14.4%, no abnormality 58.8%, 61.2%, 57.8%. These figures may be compared with the photoreadings of the infants who attended for clinical examination: Photofailures 22%, 19.1%, 23%, abnormal criteria 17.6%, 14.7%, 14.8%, no abnormality

These figures are comparable, and show that 60.4%0, 66.2%, 62.2%.

both groups are of a similar clinical spectrum.

Comparing clinical examination to photorefraction

The current gold standard for diagnosis of visual defects in children is a complete eye examination including cycloplegic refraction. We sought to compare the results gained from PR to those

Reader 1 Reader 2 Reader 3

1. Operator technique Poor focus 24 9 24 No film in the camera 7 7 7 Film exposed on removal 5 5 4 Total 36 21 35

Non-fixing on camera 32 39 43 Non-cooperation 4 3 1 Total 36 42 44

Flash failure 7 I I Overexposed 3 3 2 Dark photo 1 0 1 Total 11 10 10

Total 83 (22.5%) 73 (19.8%) 89 (24.1%)

2. Child

3. Camera fault

gained by the gold standard. T o do so we drew up ‘two by two’ tables, as in Figure 1. This allowed for the calculation of sensitivity, specificity, positive and negative predictive values and the prevalence for each reader. It is possible to create many scenarios in analysing the above data. We have chosen only two which are outlined below.

Scenario 1 Here we analysed only those photographs that could be read and only for those children who attended for clinical examination. All DNAs and PFs were ignored (Table 3).

factors risk factors

I a+c I b+d I a+b+c+d I Sensitivity - proportion of infants with risk factors for amblyopia in the screened population that are identified as such by PR=(a/a+c) Specificity - proportion of infants with no risk factors for amblyopia in the screened population that are identified as such by

Positive predictive value - the probability that an infant identified by PR as having amblyopia risk factors does have them =( a/a+ b) . Negative predictive value - the probability that an infant identified by PR as not having amblyopia risk factors does not have them =(d/c+d). Prevalence - the number of infants in the screened population at that time with amblyopia risk factors =(a+c/ a+b+c+d).

PR=(d/b+d).

Figure 1 Form of analysis of data.


Scenario 2 Again DNAs are excluded. The worst possible outcome is then assumed for all photofailures. T o do this we examined the results of refraction of all the photofailures. Children either did or did not have risk factors for amblyopia (by our abnormal clinical criteria). Those who had risk factors were called false negatives, and those that did not have risk factors were called false positives (Table 4).

Interobserver reliability

Each reader has been compared to the gold standard by calculating sensitivities and specificities.

Table 3. Scenario 1: analysis of successful photographs in infants clinically examined

Reader 1

Clinical examination Positive Negative Total

Positive Positive 10 39 49 predictive

photo value 20% Negative Negative

4 164 168 predictive photo value 98% Total 14 203 217 Sensitivity 71%; specificity 81%; prevalence 6.5%.

Reader 2





Reader 3



photo value 24%

Negative Negative 4 169 173 predictive

photo value 98%

Total 14 200 214

Sensitivity 71%; specificity 85%; prevalence 6.5%.

T o compare the readers to each other, we calcu- lated the Kappa scores" for each reader. This has given an idea of the interobserver reliability. Kappa scores were 0.24, 0.34 and 0.29 for each of the three readers respectively (for all readers the figures from Scenario 1 are used). Scores of this magnitude indicate fair agreement between readers."

Refractive status

The prevalence of refractive errors on clinical examination was as follows (Table 5) .

Table 4. Scenario 2: analysis of photographs (including photofailures) in infants cl i n ica I I y examined

Reader 1





Reader 2


~ ~~



predictive 7 181 188 photo value 96% Total 18 260 278 Sensitivity 61%; specificity 70%; prevalence 6.5%.

Reader 3




8 169 177 predictive photo value 96% Total 18 260 278

Sensitivity 56%; specificity 65%; prevalence 6.5%.

Australian and New Zealand Journal of Ophthalmology 1994; 22(3) 198

Hypermetropia 239 infants (85.6%) were hypermetropic, range 0.25 to 3.25 D, and 13 (4.7%) with a range of 3.50 to 6.75 D.

Myopia Five infants (1.8%) were clinically identified with MMR range -1.00 to -2.50 D. Two photographs showed small myopic crescents, but there were no visible crescents on the remaining three photographs. There were no highly myopic refractions in our series (greater than -4.00 D).

Astigmatism 23% (63) infants in this series were astigmatic and have been grouped as less than or more than 3.00 D (Table 6).

Forty of 43 infants with hypermetropic astigmatism were 'with the rule' (+ x 90"). The photograph of the single case in this series with hypermetropic astigmatism (>3.00 D) was a photofailure (non-fixation). Of the four children with mixed astigmatism (>3.00 D), one child was identified with a large myopic crescent in both eyes (refraction R +0.75/-3.00 x 75"L +0.75/- 3.00 x 135"). One photograph was poorly focused and thus could not be analysed. The two remaining photographs had insignificant crescents (refractions +2.75/-3.00 x 180" OU and +0.50/-3.00 x 180" OU).

Prevalence of risk factors

Of the 278 infants clinically examined and refracted under cycloplegia, 7.2% infants were identified with amblypiogenic potential including squint (three), hypermetropia (1 3), anisometropia (two) and pathology (two) (Table 7).

Table 5. Refractive status

Status MMR No. Per cent

Hypermetropia* <+3.50 239 86 Hypermetropia* >+3.50 13 4.7 Myopia* <-2.50 5 1.8 Mixed astigmatism 21 7.5 Total 278 100

*Includes compound astigmatism.

Table 6. Infant astigmatism

Status <3.00 D >3.00 D

Hypermetropic 42 1 Myopic 4 0 Mixed 17 4 Total 63 5

Discussion

Photorefraction is gaining increasing acceptance worldwide as potentially the most useful and cost effective objective screening technique for vision defects or potential vision defects, especially in the preverbal child. Evidence in the scientific literature on the value of early correction of refractive errors both for normal development of vision, and because of the potential for prevention of squint associated with hypermetropia is now Early correction requires early detection and photorefraction is one possible way of achieving this.

Initial measurement validity was determined only for those children who attended the clinical examination and whose photographs could be read. All PFs and DNAs were excluded. Photo- refraction was found to be sufficiently sensitive (range from 71% to 79%) and specific (81% to 86%) for three readers in the detection of amblyopia risk factors (manifest squint, significant hypermetropia of >4.00 D, anisometropia and pathology) (Table 3). Scenario 2 included photofailures in the analysis of those children who underwent refraction, and assigned them as false negative where significant clinical defects were found, and false positives in the absence of any clinical abnormality. This reduced the sensitivity (range from 56% to 61%) for three readers. This was primarily because of four false negatives (Table 4). Similarly, the specificity fell (range of 63% to 70%) for three readers.

We feel this to be an overly pessimistic way of analysing the data. In reality, photofailures would have resulted in repeat photographs. Unfor- tunately, this was not feasible in this study owing both to manpower and time constraints (on the part of the nurses) which resulted in reluctance to recall infants, and also because delays in the receiving and processing of rolls of film meant that the infant's age would then have fallen outside the specified range for the purpose of this study.

Table 7. Prevalence of risk factors in infants clinically examined ( e 2 7 8 )

Status Per cent

Squint 1.1 Hypermetropia 4.1 hisometropia 0.7 Pathology 0.7 Total 7.2


We believe the photofailure rate can be significantly reduced. Most photofailures were traced to one Plunket centre, and more specifically to one camera operator. Camera fault was only found to be a problem early in the study. Thus, paying more attention to training and auditing of screening personnel (who did not have uniform capability with the photographic technique) would also reduce the number of photofailures. Infants of this age group do not reliably, or will only briefly, fixate a target. However, if cases of non-fixation are considered to be partly a result of the operator’s failing to encourage infants to attend the target, the failure rate can be seen to be predominantly one of poor technique.

The statistical analysis we undertook looks only at those cases that attended all parts of the trial. DNAs are excluded. When photoanalysis of DNAs is compared with photoanalysis of infants who attended for clinical examination, the clinical spectrum is comparable. Non-attendance for clinic appointments would not seem to be a fault of PR per se, but rather a fault of the system. Our analysis has sought to discriminate camera failure from system failure.

A valid screening test demands the presence of precision (i.e. the same test applied to the same patient will give the same result). The Kappa score gives an idea of how reproducible the test results will be between different readers. Our calculation of Kappa scores (actual agreement beyond chance or potential agreement beyond chance) indicates that we can expect a fair degree of reproducibility in test results, even among different readers (assuming readers have the required skills to analyse photographs). All scores were of a similar degree (0.24, 0.29, 0.34). The lack of additional skilled personnel to analyse the photographs necessitated the reading of the photos twice by the same orthoptist, and once by the ophthalmologist who also performed the clinical examination.

The advantage of immediate results, that can be obtained by Polaroid photographs, is too expensive for mass screening programs at present. Videorefractors such as the Fortune videorefractor (Fortune Optical VRB-100) allows determination of accurate fixation and provides an instant reading. It thus has enormous advantages over photographic processing and avoids the need to recall patients where photographs are of poor quality. However, at present their lack of portability and cost makes their use less acceptable for community screening, particularly in Auckland where the

population is widely separated and the cost of attending a central location for screening is unacceptable.

In the absence of cycloplegia, the underestima- tion of symmetrical spherical hypermetropia owing to variable accommodation, must be accepted (Cases 5 , 8). However, lowering of the ratio to <0.330 would result in an unacceptable overreferral rate for hypermetropia. Some reading errors occurred which resulted in a number of false positives. Pitfalls included a slightly off- axis fixation being interpreted as a large crescent on the same side of the pupil as the eyes were looking. For example, if the eyes are slightly elevated a large, apparently “hypermetropic’, crescent appears. Also, if the eyes are slightly deviated in the horizontal plane this may be interpreted as a squint. Methods to improve accuracy of fixation are currently being investi- gated.

It is also possible that some emmetropisation may have occurred in the two to three months before refraction with a less-than-expected degree of hypermetropia on refraction relative to the crescent size. On-going clinical assessment of the at-risk infants detected in this study, together with previous studies on emmetropisation during the first 18 months, suggests the targeting of a slightly older age group - between 12 and 24 months - would maximise detection of infants likely to remain hyperopic and thus at greatest

There was also a high rate of false positives for anisometropia. This reflects the difficulty in accuracy of measurements of small crescents, especialy when there is often no exact edge to the bright crescent. Small measured differences also account for the variation between some readings. Further studies are required to determine what should be considered a significant measured difference and whether these small measured differences between eyes can be safely ignored.

In our pilot study, detection of myopia was less reliable if less than 2.00 D, with 75% detection rate for myopia <1.75 D, and 26% if ~ 0 . 7 5 D.’? The incidence of myopia is recognised as being low in this age g r ~ u p . ’ ~ ” ~ Non-detection of low myopia is of less significance in the infant with respect to risk for amblyopia, and we did not include myopia as a risk factor in this study. Since there were no significantly myopic infants in this study (greater than -4.00 D), we were unable to determine the success of its detection in this study group.

risk. 8,9J 8


The incidence of astigmatism in infancy is known to be high, with both the incidence and degree declining between one and two years of age,"-Ib We did not specifically look for astigmatism in this study, since early astigmatism appears to leave no long-term In Atkinson's study, meridional amblyopia was found to be strongly associated with uncorrected hypermetropic astigmatism but only if this persisted after two years of age and was considered to have probably contributed to the generalised delay in visual development of infants with uncorrected hyperopia who were assessed at four years of age.4

The prevalence of refractive errors in this age group was similar to that found in other studies, with a similar prevalence of risk factors for amblyopia (Table 5 ) , which is of importance for planning of screening p r o g r a m ~ . ~ . ~ J l ~ ~ ~ J ~ Appli- cation of these prevalence rates to other groups of the population must be done with care. We have dealt with a small group of children from a specific area with unique demographics. It was not possible to compare the prevalence of amblyopiogenic factors in the different ethnic groups because of the small numbers in this study. However, it was interesting to note that propor- tionately there were no differences between groups (Table 1). As far as we are aware, there have been no studies in New Zealand to determine the prevalence of risk factors for amblyopia or prevalence rates in the different ethnic groups. Further research in this area is required.

We chose six to nine months of age for this study because this is the youngest age at which we consider PR to be possible, recognising that there are limitations due to unreliable fixation; and also because this age group is easily targeted through the Plunket service. As discussed above, we are exploring the feasibility of accessing a slightly older age group (between 18 and 30 months) for future studies, when refractive changes have stabilised and fixation is more reliable.

Infants with significant hypermetropia are being studied longitudinally for changes in their refractive status, and for development of squint and/or amblyopia. It is also our intention to clinically examine all study infants, including visual acuity measurement and cycloplegic refraction, at the age of 3.5 years so that we can determine the most appropriate timing of photorefraction to maximise detection of amblyopiogenic factors.

Further pilot studies are planned, targeting slightly older infants, aged between 18 and 30

months, which will help to establish the most appropriate age group for screening (to maximise detection of risk factors), to determine the most appropriate screening personnel, ways to access targeted populations, and ultimately to assess in our community, the suggested effectiveness of early corrective treatment of hyperopia in the prevention of squint and amblyopia.

Conclusion

We believe photographic screening by this simple technique, which uses a mobile camera within a community setting, has sufficient potential to detect pathology, squint and significant hypermetropia (not compensated for by accommodation), and allows early identification of infants wirh amblyopiogenic risk factors not detected by conventional screening methods. The high photofailure rate, predominantly due to poor technique by one operator, affected overall validity. Infants could not be rephotographed. This stresses the need to pay particular attention to training and auditing of screening personnel for effective and efficient use of photoscreening in the community.

Photorefraction at six to nine months is the lowest age at which photorefraction can be undertaken, but this study indicates it is too young for maximum effectiveness.

Acknowledgements We wish to thank the National Child Health Research Foundation and the Royal New Zealand Foundation for the Blind, who gener- ously funded this study, the South Auckland Plunket Nurses for their vital work in the field, Mr Alec Fraser, Photographic Department, Auckland Healthcare, for technical assistance, and Mr Greg Gamble, Department of Medicine, Auckland Healthcare, for statistical advice.

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Community photoscreening of six to nine month old infants for amblyopiogenic risk factors