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Points de Vue, International Review of Ophthalmic Optics, N66, Spring 2012

Myopia in young adulthood

Online publication :
07/2012
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5 min

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The reasons why myopia develops in early childhood are not well understood but one might consider that strong familiar and environmental influences and a highly active eye growth system would be critically involved. Why myopia first develops and continues to progress years later, in the teenage years and young adulthood is even more obscure, as normal eye growth has slowed many years previously. It is generally considered that the eye reaches 90% of adult size by 3 years of age, 95% by 6 yr and 98-99% by 12 yr [12], with the fastest growth between birth and 3 yr (axial length increases from approximately 17 mm to 22 mm over this time). Whether these two forms of myopia are the same, but just with variation in timing, or are completely different eye conditions is debated.

Based on the experience of ophthalmic practitioners and vision researchers Brisbane, Australia is a city with relatively low levels of myopia in children. A possible reason for this is the sunny, warm weather and outdoor lifestyle adopted by the people who live there. By necessity our research has focussed on the young adult University student population who are more prone to myopia development than the general community [13].

Major questions we have studied involve (i) determination of the impact of reading and role of the accommodation system on myopia progression, (ii) determination of the variation in eye shape and peripheral refraction with refractive error, and (iii) measurement of retinal processing in myopic and emmetropic individuals.

Accommodation system

Accommodation errors that occur during reading have received much attention amongst myopia researchers [15, 6]. The general hypothesis is that lags of accommodation (too little accommodation) during reading create hyperopic retinal defocus which stimulates myopic eye growth, as in animal models [16]. As was reported for children [11], we found that young adults with progressing myopia (>0.50 D over the past 2 yr) had larger accommodation lags than either emmetropes or stable myopes [1]. In particular, progressing myopes showed reduced accommodation response to negative lens-induced accommodative demand. This work revealed a link between inaccurate accommodation and myopia in this age group.

In the course of their studies University students will have to read many articles and often they will choose to photocopy reduce the material to fit two pages onto one. A possibility is that reading small lower contrast text might produced greater accommodative effects and be involved in myopia - however we found no evidence for this [17].

Like others [10, 18] we measured larger near work induced transient myopia (NITM) values and longer decay times to baseline in myopes than emmetropes: these effects were similar across the combinations of letter size and contrast that we tested (Fig. 1 for data of 60% contrast text).


Fig. 1: The effect of letter size of 60% contrast letters on the magnitude of near work-induced transient myopia (NITM) in emmetropes (EMM, n=19), stable myopes (SM, n=17), and progressing myopes (PM, n=17). The two myopic groups (SMs and PMs) showed greater near work-induced adaptation effects than emmetropes for all targets. Participants were aged 18 to 25 yrs. Text was read for 3 min at a 4 D demand for each font size. Adapted from Schmid et al., 2005 [17].

Eye shape and peripheral refraction

There is great interest in peripheral refraction, because of the idea that defocus in the retinal periphery might influence the development of myopia and the advent of new myopia treatments based on this premise [5]. Atchison et al. (2004) [2] found that length, height and width of young adult eyes increase as myopia increases in the approximate ratio 3:2:1 - the increases in length corresponded to that required for the development of myopia. Consistent with this the shape of the posterior retinal surface resembled asymmetrical ellipsoids whereas myopia increased the width semidiameter changed only slightly in comparison to increases in vertical semi-diameter [3]. We also showed in young adults that myopia had some effect on peripheral refraction along the horizontal field (Fig. 2) but very little affect along the vertical field [4] – the peripheral variations in refraction matched the variations in shapes of emmetropic and myopic eyes. In children myopia appears to alter eye shape and peripheral refraction similarly [5]. The fact that changes in peripheral refraction are asymmetric across the field needs to be taken into account in lens designs aimed at correcting these errors.


Fig. 2: Spherical equivalent as a function of visual angle across the horizontal field. Emmetropes show relative peripheral myopia (~0.25- 0.75 D). Myopes of about 2D have no relative peripheral refraction error. Myopes of 3.5 to 6.5 D have up to 1.5 D of relative peripheral hyperopia. Adapted from Atchison et al., 2006 [4].

Retinal processing

Differences in retinal processing in eyes of emmetropic and myopic individuals have been measured; differences appear greater in young adults than children [14]. Of interest is whether these differences are involved in myopia development or are caused by the anatomical and physiological changes that occur in an elongated eye. The increased axial length that accompanies myopia development must cause structural changes to the eye and retina that will affect retinal electrophysiology. We measured the multifocal electroretinogram (mfERG) responses in young adult emmetropes and myopes [7]. P1 implicit time (stimulus onset to the first prominent response peak) was longer in myopes than in emmetropes, indicating delayed retinal responses in myopia (Fig. 3). Axial length contributed 15% of total variance to the implicit time while refractive error accounted for 27%. This meant that the delayed mfERG responses observed in myopes were not attributable to the anatomical change that accompanies myopia suggesting there are underlying differences in retinal function that result from being myopic. Subsequent testing [8] using more complicated multifocal test stimuli with the slow flash paradigm, revealed that the late components (P1 and N2) were affected in myopia, suggesting possible reduced ON- and OFF-bipolar cell activity. Using this paradigm we have shown that the oscillatory potentials are affected in myopia [9]. Oscillatory potentials are predominantly generated by the amacrine cells and interplexiform cells, with neuronal events that cause their generation involving the initiation of the inhibitory feedback pathways utlilising dopaminergic, GABAergic, and glycine-mediated systems. It is thus concluded that minor retinal processing system dysfunction may result in the development of refractive error.


Fig. 3: P1 implicit time data for emmetropic and myopic young adults across the 5 concentric test rings. Implicit time was significantly longer in myopes than in emmetropes indicating delayed retinal responses in myopia. Ring 1
represents the foveal response, rings 2–5 correspond to the successive retinal annuli (ring 2: 2º–7.6º, ring 3: 7.6º -14.8º, ring 4: 14.8º -23º, ring 5: 23º -30º). Adapted from Chen et al., 2006 [7, 8, 9].

Concluding remarks

Myopia development and progression that occurs in young adults is a fascinating issue for investigation. To return to the posed question of whether childhood and young adult myopia are fundamentally the same eye condition. Many of the characteristics described here that vary with myopia are affected by both the myopia of children and young adults.

References

01. Abbott ML, Schmid KL, Strang NC. Differences in the accommodation stimulus response curves of adult myopes and emmetropes. Ophthal Physiol Opt. 1998; 18, 13-20.
02. Atchison DA, Jones CE, Schmid KL, Pritchard N, Pope JM, Strugnell WE, Riley RA. Eye shape in emmetropia and myopia. Invest Ophthalmol Vis Sc. 2004, 45, 3380-6.
03. Atchison DA, Pritchard N, Schmid KL, Scott DH, Jones CE, Pope JM. Shape of the retinal surface in emmetropia and myopia. Invest Ophthalmol Vis Sc. 2005, 46, 2698-707.
04. Atchison DA, Pritchard N, Schmid KL. Peripheral refraction along the horizontal and vertical visual fields in myopia. Vision Res. 2006, 46, 1450-58.
05. Charman WN, Radhakrishnan H, Peripheral refraction and the development of refractive error: a review. Ophthalmic Physiol Opt. 2010, 30, 321-38.
06. Chen JC, Schmid KL, Brown B. The autonomic control of accommodation and implications for myopia development: a review. Ophthal Physiol Opt. 2003, 23, 401-22.
07. Chen JC, Brown B, Schmid, KL. Delayed multifocal electroretinogram responses in myopia. Vision Res. 2006a, 46,1221-9.
08. Chen JC, Brown B, Schmid KL.Slow flash multifocal electroretinogram in myopia. Vision Res. 2006b, 46, 2869-76.
09. Chen JC, Brown B, Schmid KL. Evaluation of inner retinal function in myopia using oscillatory potentials of the multifocal electroretinogram Vision Res. 2006c, 46, 4096-103.
10. Ciuffreda KJ, Wallis DM. Myopes show increased susceptibility to nearwork aftereffects. Invest Ophthalmol Vis Sci. 1998,39,1797–803.

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Points de Vue, International Review of Ophthalmic Optics, N66, Spring 2012

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