Slit-lamp studies of the rhesus monkey eye: II. Changes in crystalline lens shape, thickness and position during accommodation and aging.

Changes in crystalline lens shape and axial thickness, anterior chamber depth and anterior cornea-posterior lens distance during accommodation induced by corneal iontophoresis of carbachol or electrical stimulation of the Edinger-Westphal nucleus were studied in 25 living, surgically aniridic rhesus monkey eyes, aged 1-25 years. Intraocular distances and anterior and posterior lens surface curvatures were evaluated from slit-lamp Scheimpflug photographs; distances were also determined by A-scan ultrasonography. With increasing accommodation, both lens surfaces become more sharply curved, the lens thickens and the anterior chamber shallows, while the posterior lens surface remains fixed relative to the cornea. Within statistical limits, the respective curvature and distance changes are the same for a given dioptric accommodation induced by either stimulation technique. The respective intraocular distance-accommodation relationships are identical whether derived from photographic or ultrasonographic measurements. Temporal and contralateral reproducibility of all measurements is excellent. Each parameter-accommodation relationship is strikingly linear in all eyes, although above 20 D the slopes of the lens surface curvature-accommodation relationships may have decreased. The curvature change per D of accommodation averages approximately 20% more for the posterior than for the anterior lens surface. There is relatively little interindividual variation in the slope of each relationship despite the significant interindividual differences in age and accommodative amplitude, indicating that the relationships are independent of age. However, when extrapolated back to the non-accommodated resting state, the data indicate that the lens thickens, both its surfaces become more sharply curved, and the anterior chamber shallows with age in adult greater than 5 years, while opposite trends are seen in younger animals.