Journal of Audiovisual Media in Medicine 1990,13.

Photorefraction: two methods and their clinical applications

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J. DEUTSCH, T. J. SMELLIE and J. TOVEY Photorefraction is an increasingly used research screening tool and numerous studies have demonstrated the simple and effective techniques available. This article describes the eccentric (off axis) and isotropic methods in detail, including the costs and difficulties involved. It is likely that photorefractive screening will become more widespread in its use as a screening tool in detecting the early factors responsible and associated with delayed visual maturation in the very young and that hospital departments of medical illustration may have to become conversant with the use of this new tool. We screened 400 patients in a hospital out-patient clinic setting. The patients were children attending for routine orthoptic review. This article describes our methods, the problems and costs encountered. We describe how photographs can be evaluated and what uses this screening technique can be put to.

Visual developmentfrom birth Delayed maturation of vision is a serious ophthalmic problem. An eye that has failed to develop normal vision, but appears structurally normal, is said to be amblyopic (lazy eye) and affects about 5 % of the population. In a large percentage of these patients this goes undetected and untreated (Hoyt, 1987). The need for early detection and treatment of these disorders has been emphasized by both clinicians and scientists interested in the plasticity of the developing visual systems. Visual acuity is a measure of visual function and is defined as the capacity for discrimination of fine visual detail (Hof-van Duin; 1989). This is measured directly or indirectly as a visual angle subtended by the unit of J. Deutsch, FRCS. FCOphrh is Ophthalmic Registrar at the Birmingham and Midland Eye Hospital, Birmingham 8 3 2NS. T . J . Smellie D E O ~ D )is Senior Orthoptist and J . Tovey is Senior Medical Photographer, both at the Selly Oak Hospital, Selly Oak, Birmingham B29 6JD, U K. @ 1990 Butterworth-HeinemannLtd.

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detail which makes up the visual target. The target may be a grating, various letters of a standard type or specific objects. The minimum angle of resolution is the threshold of each acuity and is of the order of 40 sec arc in the adult when tested with acuity card gratings. Snellen type acuity (recognition testing) is measured in units of 1 min arc and this represents an acuity of 616 or 20120 vision. Absolute measures of the minimum angle of resolution depend on the method of testing and in the newborn - 1 month age group it is of the order of 30-60 min arc using grating card acuity methods (very roughly equivalent to 61180 Snellens, although these acuity measures are not strictly comparable). This then rapidly improves over the first years of life to 8 min arc at 6 months, 2 min arc at 2 years and adult acuity by 4-5 years. Accommodation (focusing for near objects by the eye) can be demonstrated and measured in the infant by photorefraction (Howland and Sayles, 1987). Accommodative accuracy increases in the first years of life but even the newborn demonstrates accommodation to an appropriate target. For this process of visual maturation to occur the retinal image must be equal in both eyes and sharply focused (any corneal distortion or opacity, lens or vitreous abnormality will degrade this image). If one or both eyes are disadvantaged amblyopia will set in, and if not treated will be permanent. The development of amblyopia can be halted, and even reversed up to

6-9 years of age while the visual system is still developing and retains its plasticity. Disorders of visual alignment (squint) may predate any development of amblyopia and be the cause of amblyopia in the non-fixing eye. Conversely, amblyopia or any other cause of reduced vision in one eye, compared with the other, may result in squinting, as the central neural mechanisms for binocular vision may not be able to maintain the visual axes. Furthermore, recent photorefractive studies (Howland and Sayles, 1987; Atkinson et al., 1984) revealed significant refractive errors in the normal infant. Five per cent were hypermetropic (long sighted) by 3.5 dioptres (D) or more in one or more axes, 4.5% were myopic (short sighted) and 1.4% were anisometropic (differing refraction in each eye) by 1 D or more. Astigmatism is common in infancy and there appears to be a diminishing of these refractive errors or ‘emmetropisation’ in the first years of life. Infants with hypermetropia have a 20 times greater risk of developing squint and amblyopia. Results from longitudinal photorefractive studies of infants screened will assess the value of early detection and the effect of treatment.

Principlesof photorefraction Photorefraction is a term which applies to a family of techniques using an adapted photographic system with

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a flash source close to the photographic axis of the camera. This captures the reflected light from the retina of the photographed eye, this having passed twice through the optics of the eye, and, therefore, containing some information on the refractive state of the eye. There are three main methods: orthogonal, isotropic, and eccentric photorefraction. The arrangement of the optical components of the cameras vary and we will describe two of these. Photorefraction is a technique particularly suited to the examination of children: The child’s attention need only be attracted to the camera and sustained for the time necessary to take a photograph. The instrument is at some distance from the child’s face. Both eyes are recorded together allowing comparison and confirmation of the alignment of the visual axes on a durable record. Precise alignment of the eyes onto the camera is not critical and there is some degree of tolerance. However, photorefraction is limited by the quality of reflex obtained and measurements are not precise enough to allow accurate refraction of the eye. Large errors are readily seen, as are differences between the eyes. The range of refractive error which can be most accurately assessed is also limited and varies with the type of system used.

Eccentric photorefraction

Eccentric or off axis photorefraction system requires the subject to be placed at a distance approaching optical infinity (in reality 4.5 m). To obtain workable images of the eyes and reflexes a 500 mm catadioptric lens is used. The light source is placed at the edge of the camera lens and flash photographs are taken with the flash light source at various positions on the camera lens perimeter (Day and Norcia, 1986). As distinct from the isotropic system, the axis of any astigmatism is not easily assessed with this method. The position of the flash determines the axis of photorefraction. Thus, we used two positions, one at the vertical with the flash placed above the lens and the other at the horizontal, on the photographers left of the lens. To obtain good quality photographs and bright reflexes, the pupils are usually dilated with a short acting mydriatic such as guttae cyclopentolate hydrochloride 1YO. Undilated photorefraction is performed in a darkened room with illuminated objects used to attract the child’s attention. This brings accommodation into play and good pupil dilation is difficult to achieve. Interpretation of the photographic result will, thus, be more difficult but may be sufficient to provide a screening result and may even be superior to dilated photorefraction (Kennedy and Sheps, 1989).

focused for 1.5 m, and for the third it was focused in front of the patient, i.e. at 50 cm.

Eccentric photorefractor (Figure 2)

In this case two photographs were taken of each patient and the patient was seated 4.5 m from the camera. The camera was focused on the patient in both instances, and the flash held initially above the camera lens and then beside the lens. In both methods the patient was fixing centrally on the camera lens. Professional colour negative film rat, d at 125 ASA was used in all instances. The patients were all dilated using cyclopentolate hydrochloride 1YO or atropine 1 % and following photorefraction underwent a full cycloplegic refraction by the hospital optician.

Materials and methods

lsotropic photorefraction

This requires a central light source in front of a standard camera lens. This is usually a fibreoptic bundle connected to a flash light couch. Three images are obtained with the patient 75 cm from the camera. A reference photograph focused at the plane of the eyes gives the pupil size. Corneal reflexes give an indication of the alignment of the visual axes, and any eyelid, corneal or other media opacities will be identified. Two further photographs, one focused at 50cm (0.67 D in front of the eyes) and another at 150 cm (0.67 D behind the eyes) give the refractive measurements (Atkinson et al., 1981).

We screened 400 patients in the outpatient clinic attending routine orthoptic appointments. These were children with and without orthoptic abnormalities. Verbal consent was obtained and ethical committee approval was given. In addition to their photorefraction, full ophthalmic, orthoptic and refractive assessments were made.

Figure 1. /sotropic photorefractor

lsotropic photorefractor (Figure 1)

Three photographs were taken of each patient. For all three the patient was seated 75 cm from the camera. For the first photograph the camera was focused on the patient, for the second it was focused behind the patient, i.e.

The Journal of Audiovisual Mediu in Medicine (1990) Vol. 13/No. 4

Figure 2. Eccentric photorefractor.

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Calibration (Figure 3 ) Calibration photographs were taken from each system. An emmetropic subject (with no refractive error) was chosen, pupils dilated with guttae cyclopentolate hydrochloride 1%, and serial photographs taken using a range of corrective lenses held in front of one eye (powers used were -4.0, - 2.0,0, + 1.5, +2.5, +3.5, +5.0 and +8.0). These were then used as reference photographs, with pupil and reflex size measured and plotted in graphical form (Figures 4a-c). Reflex size for the photographs using higher plus lens powers required a small correction for the magnifying effect of the lens. This graph was then used to give an approximate refractive power. It should be noted that the calibration graph only relates to reflexes obtained with our technique and a pupil size of approximately 4 mm.

Results The photographs can now be assessed and the screening result given.

Isotropic photorefraction (Figure 5 ) Three photographs were taken per patient. The first reveals the quality of the red reflex. Are there any opacities in the media, are the eye lid positions normal and symmetrical? Is the corneal reflex in the visual axis (usually just nasal to the geometric centre of the cornea)? The second is taken at 0.5 m. This photograph is less informative than the next but is useful in detecting myopic errors. Look for reflex size, quality and symmetry. The third is taken at 1.5 m and provides the main measurements. Measure the direction of the long axis of any ovaling of the reflex. Measure any spherical reflexes and check symmetry. The examples show some of the abnormalities found in our series. In Figure 5 the left reflex is dulled and irregular. This young girl had cataract surgery to that eye 1 year previously with a lens implant. In Figures 6a-c our patient reveals a

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Two photographs were taken per patient. These show reflexes in two meridians (vertical and horizontal). The camera flash was placed above the lens for one and to the left of the lens (from the photographer's standpoint) for the other photograph. A reflex at 6 o'clock indicates a + refraction result (hypermetropia) in the vertical meridian. If the horizontal reflex is at 3 o'clock, a+result in the horizontal meridian is read. Once again the photographs are assessed for sym-

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Eccentric photorefraction (Figures 7a and b)

Figure 5. Isotropic photograph focused on the plane of the eye. Note the diminished left reflex.

Figure 3. Emmetropic dilated control subject holding a -4.0 Dioptre corrective lens in front of the right eye. a

non-alignment of her visual axes, astigmatism and a low hypermetropia. Measurements of the reflexes suggest a refraction at + 3 D at 120", + 1 D at 30" for the right eye and + 1 D sphere for the left. This equates to a refraction of + 1 / + 2 x 30" right, + 1 D sphere left (actual refraction was + 1/ + 2 x 30" right, +0.5 D sphere left). This patient had a right amblyopic eye and right convergent squint.

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Figures 4a-c. Calibration plots for a eccentric, b isotropic focused at 0 . 5 m and c isotropic focused at 1.5m photorefractions.

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Deursch et al.

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metrical reflexes, lid position, corneal reflexes and media clarity. The patient in Figures 7a and b has a left ptosis (drooping eye lid) - a photographic record of ptosis is useful as worsening of the ptosis can be monitored. In addition, mild ptosis may be more obvious on a photograph. She also has a right convergent squint. The refractive error measures at + 4 D horizontal and + 3 D vertical, for the right eye. This is equivalent to a refraction of + 31 + 1 x 180" keeping in mind that this system will only assess the axis photographed. The patient's actual refraction was + 2.751+ 1.5 x 50". The left reflex is more difficult to interpret.

Discussion The technique of photorefraction is an effective means of screening for refractive error and squint. It will in addition give information on the clarity of the ocular media and eye lid position. It provides a permanent record that can be kept by the screener or the patient. It is easy to use particularly in children, and can be used without dilation of the pupil although it is likely that more information is obtained with dilation. Photographs can be read effectively and rapidly by any trained individual. The cost of equipment is that of a single reflex camera and 50mm lens

with a fibreoptic cone adaptation. The cost of the fibreoptic adaptation should be no more than 650. The eccentric system is slightly more expensive as the catadioptric lens costs in the region of E530. We hired a lens at a cost of El0 per day, 20 patients per session. The instillation of drops, photography, and reading of results in total took under 5 min per patient. The drops require a working time of about 10-15 min. At the above rates the cost of screening would be in the region of E l per patient. Screening and reading of results should take in the region of 5 min per patient. We have discussed the use of photo-

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Figures 6a-c (composite photos and line drawings). An example of an isotropic photorefraction result. This patient has a right convergent squint, with some astigmatism and hypermetropia of the right eye. Screening result: refer. Line drawing a: +, position of visual axis; 0, actual position of corneal reflex. Line drawing b: large blurred reflexes overlapping image of pupil. Line drawing c: shows the axis of reflex.

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Figures 7a and b (composite photos and line drawings). An example of an eccentric photorefraction result. This patient has a left ptosis, convergent right squint and a reasonably spherical hypermetropic refractive error of the right eye. Screening result: refer.

refraction in screening for amblyogenic factors in childhood. There is no doubt that this is an invaluable research tool and has helped clarify many aspects of infant visual development. Screening for amblyogenic factors in infancy is undoubtedly a worthwhile procedure and has yet to be fully exploited.

References Atkinson J., Braddick O., Durden K., Watson P., Atkinson S. (1984) Screening for refractive errors in 6-9 month

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old infants by photorefraction Br. J. Ophthalmol. 68,105-12. Atkinson J., Braddick O., Ayling L., PirnrnSrnith E., Howland H., lngrarn R. (1981) Isotropic photorefraction. Doc. Ophthalmol. Proc. Ser. 30, 217-23. Day S., Norcia A. (1986) Photographic detection of arnblyogenic factors. Ophthalmology 93,25-8. Hof-van Duin J. (1989) The development and study of visual acuity. Developmental Medicine and Child

Neurology31,543-52. Howland C., Sayles N. (1987) A photorefractive characterization of focusing ability of infants and young children. Invest. Ophthalmol. Vis. Sci. 28, 1005-15. Hoyt C. S. (1987) Photorefraction. Arch. Ophthalmol. 105,1497-8. Kennedy R., Sheps S. (1989) A cornparison of photoscreening techniques for arnblyogenic factors in children. Can. J. Ophthalmol. 24,259-64.

Deutsch et al.

Photorefraction: two methods and their clinical applications.

Photorefraction is an increasingly used research screening tool and numerous studies have demonstrated the simple and effective techniques available. ...
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