Optometry - Journal of the American Optometric Association
Volume 81, Issue 3 , Pages 153-156, March 2010

Effect of nearwork-induced transient myopia on distance retinal defocus patterns

  • Kenneth J. Ciuffreda, O.D., Ph.D.

      Affiliations

    • Corresponding Author InformationCorresponding author: Kenneth J Ciuffreda, O.D., State University of New York State College of Optometry, Department of Vision Sciences, 33 West 42nd Street, New York, New York 10036.
    • ,
  • Balamurali Vasudevan, B.S. Optom., Ph.D.

State University of New York State College of Optometry, New York, New York

Article Outline

Abstract 

The purpose of the current study was to assess the effect of nearwork-induced transient myopia (NITM) on retinal defocus patterns during distance viewing. An empirically derived, conceptual model of human blur perception and related retinal defocus patterns has been extended to determine the effect of NITM on the relative contributions of myopic and hyperopic retinal defocus during distance viewing. Under the normal hyperfocal refractive condition during distance viewing with isolated stimulus conditions, there is very small myopic defocus (∼0.25 diopters), and no hyperopic defocus, present. After sustained nearwork generating NITM, a moderate increase in this myopic defocus contribution takes place. In the normal isolated distance viewing situation with only very small myopic defocus present, as would occur with many far outdoor activities, the paucity of overall retinal defocus may provide a “protected” condition against myopia development. In contrast, with the addition of NITM producing increased myopic retinal defocus only, there is an imbalance of retinal defocus that may be myopigenic, especially in the context of foveal and peripheral retinal interactions.

Keywords: Retinal defocus, Hyperfocal refraction, Refraction, Blur, Myopia, Nearwork-induced transient myopia (NITM)

 

It is well-established that nearwork is a primary, environmentally based factor in the development and progression of myopia. There is sufficient supportive evidence from clinical, laboratory, modeling, and epidemiologic studies in human beings using a meta-analysis approach.1, 2 Furthermore, it has been proposed that chronically increased hyperopic retinal defocus is a likely related myopigenic mechanism.1, 2, 3, 4, 5 Moreover, the link between chronically increased retinal defocus at near and permanent myopia has been supported by studies in several nonhuman species over the last several decades (see Ong and Ciuffreda1 for a review).

Nearwork-induced transient myopia (NITM) is one of many possible environmentally based, myopigenic nearwork contributory factors. It refers to the small and transient, pseudomyopic shift in the far point of the eye after a period of sustained nearwork. It reflects an inability of the crystalline lens to reduce its power appropriately and rapidly, thus demonstrating an accommodative after-effect/hysteresis phenomenon of pharmacologic origin involving interaction of the parasympathetic and sympathetic systems.4, 5 With respect to NITM magnitude, several studies (see Chen et al.6 for a review) have reported it to be greater in late-onset myopes (LOMs) and early-onset myopes (EOMs) than in emmetropes (EMMs).6 It has also been reported that progressive myopes (PMs) were more susceptible to NITM than were stable myopes (SMs).2, 7 With respect to NITM decay characteristics, some studies have reported complete decay, whereas others have found only partial decay (e.g., in young children) to baseline after a sustained period of nearwork.4, 5

Blur perception and related retinal defocus in the near and far retinal periphery recently have been considered to be important factors in myopia development with respect to interaction with the fovea.3, 8 Thus, to understand human blur perception, and its possible relation to myopic development, the retinal defocus patterns must be considered as a function of both viewing distance and stimulus array with respect to the distal and proximal depth intervals within the context of the depth-of-focus (DOF) and equiblur zones.8

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Blur perception model 

An empirically derived, conceptual model of human blur perception and related retinal defocus patterns includes both blur detection and blur processing at the fovea and near retinal periphery (up to 8°).8 This model was extended to determine the effect of NITM on the relative summed contributions of myopic and hyperopic retinal defocus across the visual field during distance viewing.

Figure 1A represents an isolated far viewing condition in which there are few, if any, intervening intermediate and near targets present. This might occur when observing either a plane or bird against a cloudless sky, when aiming at the hoop during a “free throw” in basketball, or during rifle target practice. The isolated object of interest would be seen clearly, as it would lie within the distal edge of the DOF. There would be no blurred stimuli present and only a maximum potential of 1 diopter (D) of combined distal (i.e., myopic) and proximal (i.e., hyperopic) foveal retinal defocus present.9

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  • Figure 1 

    Two-dimensional, schematic representation of 2 far-viewing conditions with related DOF (heavy solid lines) and proximal equiblur planes (heavy dashed lines). Grey region represents clarity zone beyond infinity. A, Isolated stimulus far-viewing condition; B, Complex stimulus far-viewing condition. T = target. Figure not to scale.

Figure 1B presents a far viewing condition in which numerous intermediate and near targets are present. For example, this might occur when searching for a person in a long and crowded supermarket aisle or looking at a bird in a large leafy tree in a dense forest. In such cases, there is the clearly perceived object of interest, but now it is embedded within an array of blur stimuli at a range of distances within each successive blur zone. In this case, however, most of the blurred stimuli would be of the hyperopic retinal defocus variety, i.e., objects located closer than the eye's focal plane compared with the more isolated far viewing condition (Figure 1A). Thus, in this latter case, there would be an “imbalance” between the 2 directions of retinal defocus with considerably more hyperopic than myopic defocus present and related amount of perceived blur. In the former case, however, the amount of retinal defocus present would be relatively small and with only myopic defocus present. For example, in Figure 1A, there would be no hyperopic defocus and only approximately 0.25 D of myopic defocus present.10 In contrast, in Figure 1B, there may be 3 D or more of hyperopic defocus, but still only 0.25 D of myopic defocus present. That is, objects positioned within the multiple blur planes at the various dioptric levels located closer than infinity would create considerable hyperopic defocus, whereas the myopic retinal defocus (shaded area in Figures 1A and 1B) located beyond infinity remains the same. The only difference is the addition of objects at various positions in the visual field in Figure 1B. This large amount of hyperopic defocus is presumably potentially myopigenic.

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Model of NITM and permanent myopia based on hyperfocal refraction 

Under normal hyperfocal refractive conditions during distance viewing, there is very small myopic defocus (<0.25 D10) and no hyperopic defocus present (see Figure 2A). This would occur with many far outdoor activities, such as sports. The paucity of overall retinal defocus may provide a “protected” condition against myopia development, which is consistent with recent findings in human beings.11, 12, 13

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  • Figure 2 

    Plot of hyperfocal clarity zones/planes without (A) and with (B) NITM at far/infinity (∞). C in free space represents the point conjugate with the retina (C1). Small dotted lines represent the distal edge of the depth-of-focus, and the heavy dashed lines represent the proximal edge of the depth-of-focus. The thin solid line represents infinity (∞).

After sustained nearwork generating NITM of up to 0.6 D, as found in some visually normal young adults (especially myopes) and children,5 there would now be a moderate increase in the myopic defocus contribution only (compare length of arrow from C to ∞ in Figure 2B versus Figure 2A). With this addition of NITM to the baseline hyperfocal refraction producing increased myopic retinal defocus only, the directional imbalance of retinal defocus may be myopigenic. This notion is supported by recent findings in 2 clinical trials14, 15 in pediatric human myopes involving purposeful undercorrection of 0.5 to 0.75 D, which created increased myopic defocus at far. The undercorrection situation (see Figure 2B) resulted in increased myopic progression relative to that found for the full-distance hyperfocal refractive correction with its reduced level of myopic retinal defocus (see Figure 2A). Related to this suggestion that small amounts of myopic defocus are myopigenic, Chung et al.15 stated that “…presence of blurred vision at any distance may stimulate the progression of myopia regardless of the sign of defocus, in eyes which are susceptible,” and Adler and Millodot14 stated that “the fact that undercorrection produces an increase in the rate of myopic progression, or no statistically significant increase as found here, casts doubt on the validity of applying the results of animal studies to humans.” That is, although the animal studies suggest that only hyperopic defocus is myopigenic,1 this appears not to be the case in human beings.

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Blur perception, nearwork, the retinal periphery, and myopia development 

Myopes have relative hyperopia in the retinal periphery (i.e., larger axial length compared with equatorial diameter) when compared with their central foveal refractive state.16 It has been proposed3, 8 that the axial elongation typically observed in myopic eyes may result from the interactive effects of retinal defocus between the central foveal and near/far retinal periphery, with both myopic and relative/actual hyperopic retinal defocus being present simultaneously throughout different regions of the posterior pole of the eye. A weighting of those retinal defocus-based components may take place, which, in turn, contributes to the overall growth of the eye. When myopia is fully corrected at the fovea, as done clinically per their hyperfocal refraction (i.e., maximum plus for maximum visual acuity17), the central residual refractive error is minimal (i.e., only small myopic defocus is present, ∼0.25 D), while there remains relative/actual hyperopic defocus in the periphery (up to 0.75 D or so) because of the difference in curvature between the posterior globe and peripheral retinal refractive plane18 (see Figure 3). In agreement with the Smith et al.3 animal work, it is suggested that the myopigenic effect is related to the interaction between central versus near retinal periphery. Hence, 0.25 D of myopic defocus centrally and 0.75 D of hyperopic defocus peripherally would presumably create a sufficient retinal defocus directional imbalance.

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  • Figure 3 

    Schematic model represents the relative contribution of the center and near/far peripheral retinal defocus and its relation to NITM and the hyperfocal refraction. With either NITM added or with myopic undercorrection, the image plane is transiently displaced anteriorly, and the relative myopic defocus increases across the retinal extent. (These planes are depicted to be flatter than assumed for purposes of graphical clarity.)

However, the above relationship is altered with the addition of either NITM or purposeful myopic undercorrection. In both cases, increased myopic defocus would be produced at the fovea and the immediately contiguous near peripheral areas, but now with relatively less hyperopic defocus in the far retinal periphery, thereby giving rise to overall reduced hyperopic defocus compared with that found with the conventional full hyperfocal correction. This effect is produced by a myopic shift in the entire retinal refractive plane, thereby creating a new weighting of hyperopic versus myopic defocus, with more retinal area now having myopic defocus in the foveal center and near retinal periphery than in the far retinal periphery. The increased amount of myopic defocus across the retina with NITM added would likely be an important factor in myopigenesis.

Interestingly, recent studies14, 15, 16 have suggested that an extended period of far viewing after nearwork may play a critical role in myopia development; it appears to significantly inhibit myopigenesis. This situation is graphically depicted in Figure 1A for the isolated far-viewing condition. It has minimal retinal defocus and thus appears to produce a “protected” condition from myopic development.

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Summary 

This study presents a conceptual, laboratory-based model of human blur perception, with clinical ramifications involving retinal defocus patterns. A key feature is the relative imbalance in hyperopic versus myopic defocus and its potential for myopia development, especially in susceptible children and adolescents. Recent animal models of central versus peripheral retinal defocus and myopia development are consistent with our models and speculations in the human realm.

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References 

  1. Ong E, Ciuffreda KJ. Accommodation, nearwork, and myopia. Santa Ana Optometric Extension Program Foundation Press; 1997;
  2. Vasudevan B, Ciuffreda KJ. Additivity of near work-induced transient myopia and its decay characteristics in different refractive groups. Invest Ophthalmol Vis Sci. 2008;49:836–841
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PII: S1529-1839(09)00661-7

doi:10.1016/j.optm.2009.03.022

Optometry - Journal of the American Optometric Association
Volume 81, Issue 3 , Pages 153-156, March 2010

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