Intraocular pressure of rhesus monkey (Macaca mulatta). I. An initial survey of two free-breeding colonies.

As part of an ophthalmic and anthropometric survey of two free-breeding rhesus monkey (Macaca mulatta) populations, intraocular pressure (IOP) of 114 animals was measured under ketamine catalepsia with the use of a floating-tip pneumatonometer. The mean IOP of the 102 animals included in the main analyses was 14.9 +/- 2.1 (S.D.) mm Hg. The age distribution of animals in this sample ranged from 7 months to 21 years and reflected reasonably well the estimated age distribution in the total population (approximately 1600 macaques). Since ketamine does not have a barbiturate-like effect on IOP, this value can be regarded as a good estimate of the normal IOP of rhesus monkeys. No significant differences were found between left and right eyes nor between males and females. However, the mean IOP of infants and juveniles (7 months to 3 years) was significantly higher (15.7 +/- 2.0 mm Hg; n = 33) than that of young adult and adult rhesus monkeys (14.5 +/- 2.0; n = 69). The IOP of young animals (less than or equal to 6 years) showed a decline between 9 A.M. and 2 P.M., whereas the IOPs of older animals showed only small fluctuations between 8 A.M. and 5 P.M. This study shows that the normal IOP of macaques is remarkably similar to that of humans and demonstrates the fesibility of surveying IOP in free-ranging primates.