Emmetropization in the rhesus monkey (Macaca mulatta): birth to young adulthood

Abstract

Purpose: To provide baseline measurements on the postnatal changes in refractive error, corneal curvature, and axial elongation of the eyes of normal monkeys. Little is known about the course of normal eye growth from birth to adolescence, particularly how refractive parameters co-vary during development. In animal models of ametropia, usually one eye is manipulated and the fellow eye serves as a control. However, given individual differences, and without baseline data, it is impossible to determine whether either eye develops normally.

Methods: Measurements were obtained on 237 rhesus monkeys, whose ages ranged from birth to 5 years. Examinations included cycloplegic refraction by retinoscopy, keratometry measurements, and A-scan ultrasound measurements of axial length. The time course of development was evaluated using a growth curve analysis appropriate for a mixture of cross-sectional and longitudinal data.

Results: At birth, all three parameters were normally distributed and only weakly correlated. Monkeys had +7 D (SD=2.3 D) of hyperopia, corneal power of 58 D (SD=1 D), and axial length of 13.2 mm (SD=0.4 mm). Refractive error ranged from +0.5 D to +14.5 D, with a mean difference between the two eyes of 0.5 D. Corneal curvature ranged from 61 D to 54 D, with a mean difference between the two eyes of 0.8 D. Axial length ranged from 12.0 mm to 14.2 mm, with a mean difference between the two eyes of 0.1 mm. Although the degree of hyperopia achieved asymptote, of + 2 D, shortly after 1 year of age, corneal curvature and axial length did not achieve asymptote until nearly 5 years of age. By this time, refractive error had declined by 5 D, corneal curvature had declined by 7 D, and axial length had increased by 6 mm.

Conclusions: The magnitude of the individual differences that can occur in a small sample of experimental subjects is large enough to necessitate reference to age norms derived from a large population. Our results provide a baseline for studies of normal and abnormal eye growth and ametropia in primates. Our results also led to the confirmation of a set of "rules" that have been offered as an explanation of how these three parameters interact during emmetropization.