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. Specificall y, the chick eye elongates with a daily rhythm, said to be absent in f orm-deprived eyes. The present study asks: (1) Which components of the eye have daily rhythms-only the overall eye size, or also choroidal t hickness 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-depriv ation myopia)? Using high-frequency A-scan ultrasonography that allowe d fine (8-20 mu m) resolution of anterior chamber depth, vitreous cham ber depth, choroidal thickness and axial length, we measured normal ey es, form-deprived eyes and eyes recovering from form-deprivation myopi a at 6 hour intervals for 5 days and 4 nights. All eyes showed daily r hythms in axial elongation and choroidal thickness. In both normal and form-deprived eyes, the axial length was greatest in the afternoon wh en the choroid was thinnest, and hence, these rhythms were approximate ly in anti-phase to one another; in addition, there is some evidence t hat the axial length rhythm in form-deprived eyes is phase-advanced re lative to that of their fellow control eyes. The amplitude of the rhyt hm in choroidal thickness in form-deprived eyes was significantly larg er than in normal eyes. In recovering eyes in which elongation is slow ed, the rhythm in axial length was significantly phase-delayed relativ e 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 approxim ately 100 mu m/day. In all three groups, the rhythm in anterior chambe r depth appears to differ in phase from the rhythm in axial length (an d hence from the rhythm at the posterior wall of the eye). We propose that the phase relationship between these choroidal and eye length rhy thms influence the rate of growth of the eye, and conclude that diurna l ocular rhythms may be important in eye growth regulation. (C) 1998 A cademic Press Limited.