Purpose: In chickens, there are circadian rhythms in axial length, choroidal thickness, and intraocular pressure (IOP), the phases of which differ, depending on visual manipulations that alter ocular growth rate. In this study, these same rhythms were sought in the common marmoset, a primate model of eye growth, to establish whether these may play a role in ocular growth regulation across species.
Methods: IOP was measured by applanation tonometry in 14 untreated marmosets ranging in age from 24 to 259 days. High-frequency A-scan ultrasonography was used to measure ocular dimensions (axial length and choroidal thickness) in 12 marmosets ranging in age from 24 to 572 days. Four monkeys were measured when they were juveniles (<110 days of age) and again later, when they were adolescents. Measurements were typically made at 12-hour intervals, although three animals were measured at 6-hour intervals. Nine monkeys had both IOP and axial dimensions measured in the same experiment.
Results: There was a diurnal rhythm in IOP in the marmosets: IOP was higher during the dark period and lower during the light period (mean change, 3.6 mm Hg; P < 0.005). There were also rhythms in axial length and choroidal thickness. The rhythm in axial length was dependent on age, with faster-growing eyes of juveniles increasing in length during the day and decreasing at night (+25 microm vs. -22 microm; P < 0.001) and slower-growing eyes of adolescents showing the opposite pattern (-27 microm vs. +46 microm; P < 0.0001). The choroid thickened during the night and thinned during the day, at all ages measured (+19 microm vs. -16 microm; P < 0.0001).
Conclusions: Diurnal rhythms in IOP, axial length, and choroidal thickness exist in primates. Age-related differences in the phase relationships of these rhythms may be associated with differences in the rates of ocular growth.