Nonhuman Primate Ocular Biometry.

Nonhuman primates (NHPs), including baboons, cynomolgus and rhesus monkeys, have been used in numerous studies as models for human visual functions such as accommodation and its loss with increasing age.1–13 However, relatively little is known about the growth of the nonhuman primate eye and in particular the lens, which is a key component of the accommodative system. Limited data are available for rhesus monkeys and virtually none for baboons and cynomolgus monkeys. Several studies on accommodation in the rhesus monkey have yielded data for in vivo lens dimensions and surface curvatures.3–7,10–13 Unfortunately, there is little agreement between them. Estimates of disaccommodated lens thickness vary from 3 to 4.5 mm at any age between 1 and 30 years, with no clearly discernible age-related trend in many of the reported data sets. Some data were interpreted as showing an increase with age: others as a decrease. Similar high variability was observed in a study on lens diameters.13 It is not obvious why there are such large variations, not only between investigators, but also within the same study. Some may reflect individual differences while others may be due to differences in measuring techniques. Consequently, it is not possible to state with certainty what the dimensions of different NHP lenses may be. In addition, it has not been established how the dimensions change with age, information which is critical to understanding presbyopia development. The variability and lack of certainty makes it very difficult to compare humans and other primates. It has been pointed out previously that while in vivo observations can provide important information on the functioning of the lens in the ocular environment, they are less useful for determining biometric properties.14 Some of these would be influenced by the surrounding tissues, the accommodative state of the eye and any corrections applied for optical or sonic distortion during measurements. Age-related changes in lens properties can only be studied effectively with ex vivo tissues. There are potential sources of error with in vitro measurements, as well. These relate predominantly to postmortem changes in the lens resulting from inappropriate handling or prolonged storage of the tissue as observed with human15 and sheep16 lenses. Furthermore, postmortem changes due to inappropriate handling may also alter the dimensions of the globe and cornea. As part of a study on isolated lens power, it was observed that lens diameter increases with increasing age while thickness decreases in baboons, cynomolgus and rhesus monkeys.17 This differs from humans where both diameter and thickness increase.14,18 However, most of the data were obtained from animals aged under 10 years, equivalent to humans younger than 30 to 40 years and, therefore, not particularly relevant to presbyopia development. Over a large number of years, it has been possible to accumulate ex vivo data in our laboratory, albeit limited in some cases, on changes in NHP eye dimensions as a function of age. There appears little chance that significantly more can be collected. Ethical and financial considerations make it extremely difficult to obtain such information these days. Therefore, the currently available data are presented in the hope that they may be of value to others and/or stimulate further research in this area.