Visual Influences on Diurnal Rhythms in Ocular Length and Choroidal Thickness in Chick Eyes

Recent investigations have raised the possibility that ocular diurnal rhythms might be involved in the regulation of eye growth. Specifically, the chick eye elongates with a daily rhythm, said to be absent in form-deprived eyes. The present study asks: (1) Which components of the eye have daily rhythms-only the overall eye size, or also choroidal thickness or anterior chamber depth? (2) Does the phase or amplitude of these rhythms differ in eyes growing either faster than normal (form-deprived eyes) or slower than normal (eyes recovering from form-deprivation myopia)? Using high-frequency A-scan ultrasonography that allowed fine (8-20 micron) resolution of anterior chamber depth, vitreous chamber depth, choroidal thickness and axial length, we measured normal eyes, form-deprived eyes and eyes recovering from form-deprivation myopia at 6 hour intervals for 5 days and 4 nights. All eyes showed daily rhythms in axial elongation and choroidal thickness. In both normal and form-deprived eyes, the axial length was greatest in the afternoon when the choroid was thinnest, and hence, these rhythms were approximately in anti-phase to one another; in addition, there is some evidence that the axial length rhythm in form-deprived eyes is phase-advanced relative to that of their fellow control eyes. The amplitude of the rhythm in choroidal thickness in form-deprived eyes was significantly larger than in normal eyes. In recovering eyes in which elongation is slowed, the rhythm in axial length was significantly phase-delayed relative to normal eyes (peak at 8 pm) and the rhythm in choroidal thickness was phase-advanced (peak at 8 pm); thus in these eyes, the two rhythms are in phase. In these eyes, the choroids were thickening by approximately 100 micron/day. In all three groups, the rhythm in anterior chamber depth appears to differ in phase from the rhythm in axial length (and hence from the rhythm at the posterior wall of the eye). We propose that the phase relationship between these choroidal and eye length rhythms influence the rate of growth of the eye, and conclude that diurnal ocular rhythms may be important in eye growth regulation.