Photorefraction

Introduction:
Photorefraction is the newest retinoscopy technique
Its main application is the screening of infants and young children

1. Photorefraction Mohammad Almasi - Mojtaba Rajabpour Shamim Yaghoubian Optometry S. in SBMU 2016 Photorefraction of Eye Mohammad Almasi Mojtaba Rajabpour Shamim Yaghoubian Optometry S. in S.B.M.U 2016

2. Introduction Photorefraction is the newest retinoscopy technique Its main application is the screening of infants and young children It is useful in the detection of anisometropia and strabismus In essence, a flash photograph is taken of both eyes together, with the flash source near the camera Size and location of fundus reflection as seen in the pupil in the camera determines refraction

3. Forms of Photorefraction Photorefraction based on point-spread (on-axis) : Orthogonal Isotropic Photorefraction based on retinoscopic -like (off-axis) : eccentric

4. How to do photorefraction ? In practice, the subject is seated in a darken room (ambient light between 2 and 20 lx) at a distance of 0.75 to 1.5 m from the camera The photographer ensures that the subject is fixating a point near the camera lens before the picture With infants this is usually done by the phothographer placing his or her head near the camera lens and establishing eye contact with the subject

5. Point-spread method

6. Orthogonal photorefraction Orthogonal photorefraction was the first pointspread photorefraction method to be developed In this technique, the camera lens is focused on the patient’s eyes A small flash source of light is mounted centrally in front of the camera lens The light returning from the fundus to the camera lens falls on four cylindrical lenses arranged radially around the source at 90 degrees intervals

7. Orthogonal photorefraction The image on the camera plane is a cross The length of the cross arms is proportional to the size of the point spread image, which in turn is proportional to the refractive error relative to the camera Two meridians are measured simultaneously Photograph are taken with the lens elements at 90/180, at 45/135 These two images are taken to give reasonable information about astigmatism

8. Orthogonal photorefraction

9. Isotropic photorefraction A second pointspread method eliminates the need for the cylander lenses of orthogonal photorefraction This method provides a means of assessing the sign of defocus 3 separate pictures are taken of the subject with a flash source centred in the lens of the camera One picture is taken with the camera focused on the eyes of the subject. This picture may be used to measure pupil diameters Then two other pictures are taken with the camera focused 0.5 D in front of and behind the subject.

10. Isotropic photorefraction Myopic eye : In myopic eye, the real image of the fundus reflex is between the camera lens and the subject’s eye If the camera is focused in front of the eye, this real image is in better focus at the camera and appear smaller than it would if the camera were to be focused beyond the eye By comparison of the size and brightness of the image, the sign of refractive error can be ascertained Focused In front of the eyes Focused behind the eyes

11. Isotropic photorefraction Hyperopic eye : In hyperopic eye, the virtual image of the fundus reflex is located behind the subject’s eye When the camera is focused behind the subject, this virtual image will be in focus and small bright spot will be recorded at the camera’s image plane When the camera is focused in front of the subject,this virtual image of the retina is now defocused,and a difuse,large spot will be recorded By comparison of the size and brightness of the image, the sign of refractive error can be ascertained Focused In front of the eyes Focused behind the eyes

12. Rtinoscopic -like method

13. Eccentric photorefraction ( photoretionscopy ) 2 versions of photoretinoscope are exist : A- One version of photoretinoscope consists of a centered fiber light guide and a shield that surrounds the light source.the shield provides an eccentric knife edge to the source and a finite ‘eccentricity’ or distance between the knife edge and the center of the photorefractor’s camera B- Infrared light-emitting diodes arranged in rows that can be separately illuminated in sequence

14. Eccentric photorefraction ( photoretionscopy ) If the pupil of the subject is photographed : In emmetropic eyes : The retinal reflex is uniform red reflex in the pupil (as seen as in this figure) → I n ametropic eyes : A whitish crescent will be captured within the red reflex. This is a picture of the reflected blur circle at the plane of the pupil

15. Crescent size Crescent size indicates the magnitude of the refractive error, whereas crescent location indicates the type of the ametropia The size of crescent is dependent on: pupil size eccentricity of light source camera to subject distance distance at which the eye is focused

16. Eccentric photorefraction ( photoretionscopy ) If the light source is placed below the camera aperture and the pupil of the subject is photographed : In myopic eyes : The crescent will be appear in the bottom of the pupil In hyperopic eyes : The crescent will be appear in the top of the pupil Myopic eye Hyperopic eye

17. Video-retinoscopy The phtoretinoscopic technique has developed into video-retinoscopy using rows of infra-red LEDs within the shield Rows can be illuminated sequentially, and the rate of movement of the detected crescent measured t determine refraction A more recent development is to have all the LEDs on simultaneously, and to measure the slope of image intensity at right angle to the edge of the shield The slope is linearly related to the refractive error for a considerable range

18. Advantages the photorefractive techniques use a very large camera to subject distance (typically 1 m) They refract both eyes simultaneously They don’t require fixation of the subject They are optimally suited for noncooperative subjects ( for example Infants )

19. disadvantages The results of these methods are not very accurate The range of refractive error that can be measured by orthogonal and isotropic photorefraction is limited

20. Currently Available Photoscreeners MTI PlusoptiX iScreen3000 Spot

21. Refrences William J.Benjamin , Clinical Refraction, 618-626 Mark Rosenfield , Nicola Logan,Optometry : Science, Technique AND Clinical Management, Second Edition, 205-207 Bruce Moore, Eye care for infants and young children,1 st Edition, 52-53, 116-117 W. R. BOBIER & O. J. BRADDICK, American Journal, Eccentric Photorefraction : Optical Analysis and Empirical Measures Howland HC., Braddick O. Atkinson J. Howland B.; Optics of Photorefraction : Orthogonal and Isotropic methods; J opt Soc Am. 1983 Dec;73(12):1701-8. W. Neil Charman , Visual Optics and Instrumentation,1991, 407-410 http:// eyewiki.aao.org/Photoscreening Howard C. Howland, Optometry and Vision Science, 2009 , 603-606