Emmetropization and eye growth in young aphakic chickens

Abstract

Purpose: To establish a chick model to investigate the trends of eye growth and emmetropization after early lensectomy for congenital cataract.

Methods: Four monocular treatments were applied: lens extraction (LX); sham surgery/-30 D lens; LX/+20 D lens; and LX/+30-D lens (nine per group). Lens powers were selected to slightly undercorrect or overcorrect the induced hyperopia in LX eyes and to induce comparable hyperopia in sham-surgery eyes. Refractive errors and axial ocular dimensions were measured over a 28-day period. External ocular dimensions were obtained when the eyes were enucleated on the last day.

Results: The growth patterns of experimental (Exp) eyes varied with the type of manipulation. All eyes experiencing hyperopia initially grew more than their fellow eyes and exhibited myopic shifts in refraction. The sham/-30 D lens group showed the greatest increase in optical axial length, followed by the LX group, and then the LX/+20 D lens group. The Exp eyes of the LX/+30 D lens group, which were initially slightly myopic, grew least, and showed a small hyperopic shift. Lensectomized eyes enlarged more equatorially than axially (i.e., oblate), irrespective of the optical treatment applied.

Conclusions: The refractive changes observed in young, aphakic eyes are consistent with compensation for the defocus experienced, and thus emmetropization. However, differences in the effects of lensectomy compared to those of sham surgery raise the possibility that the lens is a source of essential growth factors. Alterative optical and mechanical explanations are offered for the oblate shapes of aphakic eyes.

Figure 1

Photograph of a representative aphakic eye, after enucleation on postoperative day 28. The posterior capsule is intact and the pupillary zone is optically transparent.

Figure 2

Mean IOP (±SEM) for lens extraction (LX), sham surgery, and fellow eyes. Symbols indicate statistically significant differences between experimental and fellow eyes (*P < 0.05; †P < 0.01; LSD multiple comparisons).

Figure 3

Refractive error data (mean ± SEM) for the four treatment groups. Immediately after the lensectomy surgery, experimental eyes showed large hyperopic refractive errors (A), and over the experimental period they underwent further refractive error changes that varied according the optical manipulation used. Pre- and postoperative values for fellow eyes are similar, as these eyes changed minimally. (B) Eyes subjected to sham surgery and fitted with -30 D lenses showed much larger myopic shifts in their refractions than eyes undergoing lensectomy and left uncorrected, although they experienced similar amounts of defocus at the start of the monitoring period. Eyes undergoing lensectomy and fitted with corrective lenses showed either minimal change (+20 D lens group) or became more hyperopic (+30 D lens group).

Figure 4

(A) Optical axial lengths and (B) outer axial lengths (mean ± SEM), obtained by A-scan ultrasonography for experimental and fellow eyes of the four treatment groups, plotted as a function of time over the study period. The experimental eyes of both lens extraction (LX) and sham/-30 D lens groups elongated faster than their fellows, while the experimental eyes of the LX/+30 D group elongated more slowly than their fellows.

Figure 5

(A) Retinal thickness and (B) choroidal thickness (mean ± SEM), obtained by A-scan ultrasonography for experimental and fellow eyes of the four treatment groups, plotted as a function of time over the study period. Both the retinas and choroids of experimental eyes tended to be thinner than their fellows, for all groups, with one exception involving the LX/+30 D group for which the choroids of experimental eyes were thicker than those of their fellows.

Figure 6

External axial length (anterior to posterior, A-P), and horizontal (H) and vertical (V) equatorial diameters (mean ± SEM), obtained from enucleated eyes with digital calipers at the end of the study (A). Equatorial dimensions were larger than axial dimensions for all groups and both experimental eyes and their fellows. However, experimental eyes were more oblate than their fellows: The mean ratios of A-P to H-V (average of horizontal and vertical diameters) were smaller than those of their fellows (B). Eyes undergoing lensectomy surgery (LX) were also more oblate than those undergoing sham surgery. Symbols indicate statistically significant differences between experimental and fellow eyes (*P < 0.05; †P < 0.01; paired t-test).