The following article is presented to you by Petros Papadogiannis
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
What is peripheral vision
Peripheral vision is the part of our vision that is outside the center of our gaze, and it is the largest portion of our visual field. For both eyes the combined visual field is 130°–135° vertical and 200°–220° horizontal with 180-200 degrees comprising the peripheral vision. It is weaker in humans than in many other species, and this disparity is even greater where it concerns our ability to distinguish color and shape. This is due to the density of the receptor cells on the retina and the enlargement of optical errors in the periphery. As a result, reduced visual acuity and contrast sensitivity occurs.
Retinal shape and myopia
Myopic eyes have multiple variations on their retinal shape. This phenomenon is related to the potential models of retinal stretching that occurs during axial elongation. The picture below represents the 4 models of retinal stretching that can occur in myopia. The solid circles represent the shape of the retina of an emmetropic eye, the dashed shapes represent the myopic retinas, and the arrows indicate the regions of stretching. (1,2)
It was found that despite the existence of myopia in both the central and peripheral retina, myopic error in the periphery is smaller. (1)
Also, in 2009 Tabernero and Schaeffel found that myopes (even those with medium refractive error) appear to have more irregular shape than emmetropes, on the peripheral retina. (8)
What do animal studies show?
Animal studies have shown that the peripheral retina can trigger or stop the growth of the eye depending on the location of the peripheral image relative to the retina. When an image is focused on the central retina and for the peripheral retina, the image is focused behind, this results in a relatively hypermetropic periphery and a defocused image. This defocused image sends a growing signal to the eye and makes the eye myopic.
By their experiments in laboratory animals, Smith et all found that visual signals from the peripheral retina can dominate against the visual signals from the central retina in terms of regulation of eye’s refractive status. (3)
The concept that dominates is that cones are more involved than rods(they are located in the peripheral retina) in the detection of visual signals that contribute to eye growth. But a study of 2010 in mice, shows that rods are important for the detection of the signals that are involved in the procedure of emmetropization and the development of myopia.(4)
Does peripheral refractive status affect the onset and progression of myopia?
A number of studies in humans, have shown that peripheral refractive errors are ante-dated to the onset of central myopia and can, therefore, be a risk factor for the onset and progression of myopia.
In a 1971 study in young trainee pilots, Hoogerheide found that emmetropes with peripheral hypermetropic refraction had greater possibilities to develop myopia, compared to emmetropes that appeared to have myopic astigmatism in the periphery. (5)
More recently, Schmid (2011) verified an important association between the greater steepness of the retina (more prolate eye shape) and the central myopic shift in children.(6)
On the other hand, Mutti in 2011 didn’t manage to verify the influence of peripheral hypermetropia in the onset of myopia. Particularly, despite the fact that he found a correlation between the magnitude of the peripheral hypermetropia and myopia progression, the total influence of peripheral hypermetropic state in central refraction was limited. (7)
To conclude with, although the hypothesis that a relatively hypermetropic periphery can drive the development of human myopia remains unproven, the existing research support the possibility of an interaction between the states of focus on axis and in the periphery.
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1) Pavan K Verkicharla,Ankit Mathur,Edward AH Mallen,James M Pope,David A Atchison. Eye shape and retinal shape, and their relation to peripheral refraction. OPO 2012; 32: 184–199 2) Strang NC, Winn B & Bradley A. The role of neural and optical factors in limiting visual resolution in myopia. Vision Res 1998; 38: 1713–1721. 3) Earl L. Smith. The Charles F. Prentice Award Lecture 2010: A Case for Peripheral Optical Treatment Strategies for Myopia Optom Vis Sci. 2011 September ; 88(9): 1029–1044 4) S. B. Jabbar; A. E. Faulkner; G. F. Schmid; F. Schaeffel; J. Abey; P. M. Iuvone; M. T. Pardue. Rod Photoreceptor Contributions to Refractive Development and Form Deprivation Myopia in Mice. Investigative Ophthalmology & Visual Science 2010; 51: 1726 5) Hoogerheide J. · Rempt F. · Hoogenboom W.P.H. Acquired Myopia in Young Pilots. Ophthalmologica 1971;163:209–215 6) Schmid GF. Association between retinal steepness and central myopic shift in children. Optom Vis Sci. 2011 Jun;88(6):684-90. 7) Donald O. Mutti; Loraine T. Sinnott; G. Lynn Mitchell; Lisa A. Jones-Jordan; Melvin L. Moeschberger; Susan A. Cotter; Robert N. Kleinstein; Ruth E. Manny; J. Daniel Twelker; Karla Zadnik Relative Peripheral Refractive Error and the Risk of Onset and Progression of Myopia in Children Investigative Ophthalmology & Visual Science.2011;52:199-205 8) Juan Tabernero; Frank Schaeffel. More Irregular Eye Shape in Low Myopia Than in Emmetropia. Investigative Ophthalmology & Visual Science.2009;50:4516-4522.