Corneal and crystalline lens dimensions before and after myopia onset

Abstract

Purpose: To describe corneal and crystalline lens dimensions before, during, and after myopia onset compared with age-matched emmetropic values.

Methods: Subjects were 732 children aged 6 to 14 years who became myopic and 596 emmetropic children participating between 1989 and 2007 in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study. Refractive error was measured using cycloplegic autorefraction, corneal power using a hand-held autokeratometer, crystalline lens parameters using video-based phakometry, and vitreous chamber depth (VCD) using A-scan ultrasonography. Corneal and crystalline lens parameters in children who became myopic were compared with age-, gender-, and ethnicity-matched model estimates of emmetrope values annually from 5 years before through 5 years after the onset of myopia. The comparison was made without and then with statistical adjustment of emmetrope component values to compensate for the effects of longer VCDs in children who became myopic.

Results: Before myopia onset, the crystalline lens thinned, flattened, and lost power at similar rates for emmetropes and children who became myopic. The crystalline lens stopped thinning, flattening, and losing power within ±1 year of onset in children who became myopic compared with emmetropes statistically adjusted to match the longer VCDs of children who became myopic. In contrast, the cornea was only slightly steeper in children who became myopic compared with emmetropes (<0.25 D) and underwent little change across visits.

Conclusions: Myopia onset is characterized by an abrupt loss of compensatory changes in the crystalline lens that continue in emmetropes throughout childhood axial elongation. The mechanism responsible for this decoupling remains speculative but might include restricted equatorial growth from internal mechanical factors.

Figure 1

Corneal power as a function of annual visit relative to the onset of myopia (−5 years before to +5 years after, with onset designated as year 0). (A) Data are shown for became-myopic children (■) and emmetropes (○). Error bars are 95% confidence intervals. (B) The difference between became-myopic and emmetrope data unadjusted for VCD. *Significant differences between became-myopic and emmetropic children (i.e., the difference between groups is significant relative to a value of 0).

Figure 2

Lens thickness in the became-myopic and emmetrope groups (A). The dashed lines represent emmetrope values unadjusted for vitreous chamber depth and the solid lines are following adjustment for vitreous chamber depth. Error bars are 95% confidence intervals. The difference in lens thickness between became-myopic children and emmetropes adjusted for VCD are shown in (B). Symbols are as in Figure 1.

Figure 3

Calculated lens equivalent power in the became-myopic and emmetrope groups (A); difference in calculated lens equivalent power between became-myopic children and emmetropes adjusted for VCD (B). Symbols are as in Figure 1.

Figure 4

Gullstrand lens power in the became-myopic and emmetrope groups (A); difference in Gullstrand lens power between became-myopic children and emmetropes adjusted for VCD (B). Symbols are as in Figure 1.

Figure 5

Crystalline lens equivalent refractive index in the became-myopic and emmetrope groups (A); difference in calculated lens equivalent refractive index between became-myopic children and emmetropes adjusted for VCD (B). Symbols are as in Figure 1.

Figure 6

Cubic multilevel growth curves for lens thickness as a function of age and three age of myopia onset groups (≤10 years, 10 years to ≤12 years, and >12 years). The growth curves for myopes indicate thinner lenses at all ages with later ages of minimum lens thickness the older the age of onset.

Figure 7

Ocular component differences between became-myopic and emmetrope groups as a function of visit relative to the onset of myopia and ethnicity. Error was similar between ethnic groups. Representative error bars (±95% confidence intervals) are included for Asians only for clarity.

Figure 8

Ocular component differences between became-myopic and emmetrope groups as a function of visit relative to the onset of myopia and gender. Error bars are ±95% confidence intervals.