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

PURPOSE. TO provide baseline measurements on the postnatal changes in refra ctive error, corneal curvature, and axial elongation of the eyes of normal monkeys. Little is known about the course of normal eye growth from birth t o adolescence, particularly how refractive parameters co-vary during develo pment. In animal models of ametropia, usually one eye is manipulated and th e fellow eye serves as a control. However, given individual differences, an d without baseline data, it is impossible to determine whether either eye d evelops normally. METHODS. Measurements were obtained on 237 rhesus monkeys, whose ages range d from birth to 5 pears. Examinations included cycloplegic refraction by re tinoscopy, keratometry measurements, and A-scan ultrasound measurements of axial length. The time course of development was evaluated using a growth c urve 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 powe r 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 tw o eyes of 0.5 D. Corneal curvature ranged from 61 D to 54 D, with a mean di fference 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 t he degree of hyperopia achieved asymptote, of +2 D, shortly after 1 year of age, corneal curvature and axial length did not achieve asymptote until ne arly 5 years of age. By this time, refractive error had declined by 5 D, co rneal 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 refe rence to age norms derived from a large population. Our results provide a b aseline for studies of normal and abnormal eye growth and ametropia in prim ates. Our results also led to the confirmation of a set of "rules" that hav e been offered as an explanation of how these three parameters interact dur ing emmetropization.