SYMPOSIUM E A R L Y V I S U A L D E V E L O P M E N T March 1982, Solna, Sweden

Acta Ophthalrnol (Copenh). Suppl157: 36-45

Dppartment of Experimental Psychology (Heud: N . J. Mackintosh), Universip of Cambridge, England

THE USE OF ISOTROPIC PHOTOREFRACTION FOR VISION SCREENING IN INFANTS

BY

JANETTE ATKINSON and OLIVER BRADDICK

Isotropic photorefraction is a technique which makes possible the rapid, economic, large scale screening of infants and young children for refractive errors. The relative size of blur circles reflected from the fundus in 3 flash photographs with different camera settings allows the size and direction of refractive errors to be estimated. The method has been validated against retinoscopic measures on a large group of infants. Findings of a high incidence of astigmatism in the first year are confirmed. Detection of hypermetropia in early infancy by this screening procedure may allow preventive treatment for accommodative strabismus.

K q words: photorefraction - refractive screening - astigmatism - accommodative strabismus - refraction in infancy.

Over the past few years there has been increasing interest in developing screening programmes to detect visual problems in pre-school children. In Britain, proposals for screening have concentrated on orthoptic checks and acuity testing by means of single optotypes with children aged around 3% years (Cameron & Cameron 1978; MacLellan & Harker 1979).

An alternative approach is to screen children for refractive errors, which is possible at a younger age than reliable acuity testing by conventional methods. Ingram (1977, 1979, 1980) has carried out refractive screening on 6- 12 month infants for some years in the Kettering area. However, the scarcity and expense of skilled retinoscopists who are prepared to spend the time necessary to refract infants on a large scale makes it very unlikely that retinoscopic screening

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programmes like Ingram’s will be widely adopted. The method of isotropic photorefrnction developed in our laboratory (Atkinson et al. 1981) offers a means of refractive screening which can be carried out by paramedical personnel (e.g. orthoptists) after a short training period, with apparatus and procedure that is rapid, reliable, and relatively inexpensive.

The isotropic photorefractor (a development of the earlier instrument of Howland & Howland (1974)) is a simple attachment to the wide-aperture (fi1.2, 50 mm) lens of a standard 35 mm single-lens reflex camera. It consists of a fibre optic light guide which delivers the light of an electronic flash to the subject’s eyes along the optical axis of the camera. The light is reflected from the fundus of the eye, and so the subject’s pupils appear brightly lit in the photograph. If the camera is defocussed, the reflected light appears on the photograph as a blur circle for each eye. The size of this blur circle is a function of the pupil size and of the refractive state of the eye. The optical theory of the device is to be described elsewhere (Howland et al., submitted) and so will not be expounded here.

When we use the method for refractive screening of infant, one drop of 1% cyclopentolate is first administered to each eye and 40 min allowed for the cyclopelgia to take effect. The child is then seated 75 cm from the camera and 3 photographs taken with different focus settings. The speed of the procedure is such that 20 infants per hour can be screened in a well-organized clinic. For each photograph the child’s attention has to be attracted by the operator to avoid off-axis errors, but in other respects the method does not require exact or maintained positioning of the eyes. In the first photograph the camera is focussed on the eyes (see Fig. 1). This photograph allows measurement of pupil size, and inspection of

Fig. 1. Photorefractive image with camera focussed on the subject’s eyes at 75 cm. The size of the transilluminated pupils can be measured, and the symmetry of the corneal reflexes assessed.

This photograph may also be used to include identifying information (not shown here).

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Fig. 2. Photorefractive images obtained with the camera focussed at 150 cm (above) and 50 cm (below). The pattern shown here, with small circles of reflected light from the eyes with the 150 cm focus and larger, more diffuse images with the 50 cm focus is typical of a normal

infant with emmetropic or slightly hypermetropic refraction.

the symmetry of the corneal reflexes to check for manifest strabismus. Two further photographs are taken with the camera focussed at 150cm and 50cm, i.e. 0.67 dioptres in front of and behind the infant. If the blur circles in the image with 150 cm focus are smaller than those with 50 cm focus the refraction is hyper- metropic with respect to the working distance of 75 cm, which is the normal case for a child who is emmetropic or slightly hypermetropic under cycloplegia (Fig. 2). If the blur circles are larger in the image with 150 cm focus than in the image with the 50 cm focus, a myopic error is indicated (Fig. 3). In either case, the larger refractive error, the larger and more diffuse are the blur circles. Fig. 4 shows the photographs obtained for a child with a large hypermetropic error (+4-5D). Differences in refraction between the 2 eyes (anisometropia) are readily visible, as in Fig. 5 which shows an infant with one normal and one markedly hypermetropic eye. Anisometropia is an important condition to detect in screening because if

uncorrected it is highly likely the result in amblyopia. Astigmatic errors are also easy to detect, since they lead to the blur images being elongated in the axis of greater refractive error (see Fig. 6).

The relationship between the size of the blur image and the refractive error may be calculated theoretically by computer ray tracing (Howland et a]., submitted). The method may also be empirically calibrated by photographing adult individuals of known refraction under cycloplegia, wearing a range of spectacle corrections, and this calibration (Braddick et al., in preparation) is more directly relevant to practical interpretation of the photographs. We have constructed an empirical table relating the dimension of the blur image to pupil size and refraction. It should be emphasised that the smaller blur images can be measured more accurately than large and diffuse ones. Thus the method is well adapted for identifying that an individual has a refractive error greater than some criterion value, say +4D, which is the requirement for a screening instrument. The measurement of the exact value

Fig. 3. As Fig. 2, but for a myopic infant. In this case the 150 cm camera focus (above) shows larger image than the 50 cm focus (below). The ‘ringed’ appearance of the images with 150 cm focus

is also characteristic of moderately myopic refractions.

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Fig. 4. As Fig. 2, but for infant with marked hypermetropia (+4.0 dioptres). As in Fig. 2, the images are larger and more diffuse with 50cm focus (below) than with 150cm focus (above), indicating hypermetropia relative to the camera distance, but both sets of images are much

more diffuse than those of Fig. 2, indicating a larger refractive error.

of such a large error, needed for proper correction of a case identified in screening, cannot be done accurately from the photographs and requires a careful retino- scopic refraction.

We have been able to compare photorefractive and retinoscopic screening by photorefracting, under cycloplegia, infants who were also taking part in Ingram’s refractive screening programme in the Kettering area. Fig. 7 shows a comparison between the 2 types of measurement for 383 infants (each contributing 4 points - 2 axes of each eye - to the analysis). Dioptric values have been obtained from the photorefractive images according to the empirical calibration. There is a good overall correlation of the 2 measures (r = 0.77) with the line of best fit very close to equality. The mean refraction was f 1.4D by retinoscopy and + 1.3D by photore- fraction.

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A feature of interest in refractive screening data is the incidence of astigmatism. It has been found (Howland et al. 1978; Mohindra et al. 1978) that at least 50% of children are astigmatic in the first year of life, but the possibility has been raised (Fulton et al. 1980) that this is a consequence of non-cycloplegic refractive methods. Data from the Kettering screening and a smaller population in Cambridge (Fig. 8) show that, on the contrary, approximately 50% of each of these populations of infants around 6 months show astigmatism of ID or more under cycloplegia. It is often stated that in infancy ‘against-the-rule’ astigmatism (i.e. vertical axis more hypermetropic) predominates. Our data show that in the Kettering population the reverse is true, while in Cambridge with- and against-the-rule astigmatisms are about equally common. Thus there are curious population differences in this respect, although the overall high incidence of infant astigmatism appears to be general.

Fig. 5. As Fig. 2, but for an anisometropic infant. The left eye is about +6D, and shows similar images to those of Figure 4 but more diffuse, while the right eye is +2D and thus shows

images like those of Fig. 2 but rather larger.

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Fig. 6. As Fig. 2, but for an astigmatic infant. The images are overall larger with 50 cm focus (below) than with 150 cm focus (above) indicating that both axes are hypermetropic with respect to the camera distance. The elongation of the images in a near-vertical direction indicates the axis with the more hypermetropic refraction. Astigmatism is commonplace in infants under 1

year of age, but many infants lose their astigmatism by the time they are 2 years of age.

Fig. 7 shows that the commonest refractive error in infancy is marked hypermetropia. In previous studies Ingram has found (Ingram 8c Walker 1979; Ingram et al. 1979) that children with a large degree of hypermetropia at 1 year are at much greater risk than others of developing strabismus and/or amblyopia later. Hypermetropic children are readily identified in our refractive screening, although no strabimus is usually manifest at that stage. However, the identification of such children is only worthwhile if some preventative measures are possible. If the constant accommodation accompanying hypermetropia is in fact a cause of strabismus, early spectacle correction of infant hypermetropes might be an effective

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means of prevention. Controlled trials of this treatment are under way, but the outcome cannot be known for some years.

As well as hypermetropes, photorefractive screening identifies the smaller number of severely myopic and anisometropic infants for whom early correction is desirable. It is our hope that infant vision screening programmes including photorefraction will allow a range of visual defects to be detected promptly and the number of pre-school children with untreated eye problems to be greatly reduced in the future.

I

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Fig. 7. Relationship between photorefractive measurements and retinoscopic measurements of refraction on the same infants' eyes. Each point represents one axis for one eye, i.e. each infant contributes four points. The dashed line represents equality of measurements by the

two methods, while the solid line is the line of best fit, whose equation is given at top left.

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201

151

101

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more hyperrnetroplc axis:

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50 c 5 3 O / n > 1D

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0 - - w v I 1 1 1 A 1 1 1 1 A

m 0 , o v , o L " O o - - N 0 - - w

Astigmatism - dioptres Kettering screening Cambridge volunteers N = 804 eyes

Fig. 8. Incidence of different extents and axes of astigmatism in two populations of 6-month infants. Left: infants participating in Dr. R. M. Ingram's screening programme in the Kettering area. Right: infants attending as volunteers in our research unit in Cambridge. The data plotted are from retinoscopic measurements (photorefractive measurements on the same infants

reveal similar incidences of astigmatism).

N = 184 eyes

Acknowledgments

This work was supportd by the Medical Research Council of Great Britain. Howard Howland collaborated in the development of the isotropic photorefractor. We are very grateful for the co-operation of Dr. R. M. Ingram and Ms. Kim Durden who carried out the comparative retinoscopic examinations and the assistance in photography and analysis of data by Elizabeth Pimm-Smith, Carol Evens, Lesley Ayling and Jackie Day.

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References

Atkinson J, Braddick 0 J, Ayling L, Pimm-Smith E, Howland H C & Ingram R M (1981): Isotropic photorefraction: a new method for refractive testing of infants. Doc Ophthalmol ProcSer 30: 217-223.

Braddick 0, Atkinson J, Durden K, Ayling L, Pimm-Smith E & Ingram R M: Photorefractive and retinoscopic measures of refraction in infants, (in preparation).

Cameron J H & Cameron M (1978): Visual screening of pre-school children. Br Med J 2:

Fulton A B, Dobson V, Salem D, Mar C, Petersen R A & Hansen R M (1980): Cycloplegic

Howland H C, Atkinson J , Braddick 0 & French J (1978): Infant astigmatism measured by

Howland H, Braddick 0, Atkinson J & Howland B (submitted) Isotropic photorefraction: a

Ingram R M (1977): Refraction as a hasis for screening children for squint and amblyopia. Br

Ingram R M (1979): Refraction of 1-year-old children after atropine cycloplegia. Br J

Ingram R M (1980): Vision screening of children. Lancet 2: 367. Ingram R M & Walker C (1979): Refraction as a means of predicting squint or amblyopia in

pre-school siblings of children known to have these defects. Br J Ophthalmol63: 238-242. Ingram R M, Traynar M J. Walker C & Wilson J M (1979): Screening for refractive errors at

age 1 year: a pilot study. Br J Ophthalmol63: 243-250. MacLellan A V & Harker P (1979): Mobile orthoptic service for primary screening of visual

disorder in young children. Br Med J 1 : 994-995. Mohindra I, Held R, Gwiazda J & Brill S (1978): Astigmatism in infants. Science 202:

1693- 1694.

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Author's add7es.i:

Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge, CB2 3EB, England.

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