The phase relationships between the diurnal rhythms in axial length and choroidal thickness and the association with ocular growth rate in chicks.

Eyes of young chickens show diurnal oscillations in axial length and choroidal thickness that are out of phase. In eyes responding to myopic defocus induced by prior form deprivation, the two rhythms shift into phase. In order to elucidate the possible role for these rhythms in ocular growth regulation, they were measured under visual conditions that altered ocular growth rate. (1) Form deprivation to myopic defocus. Eyes of chicks were monocularly deprived for 5 days. Diffusers were removed. (2) Myopic defocus to hyperopic defocus. Eyes wore positive lenses for 6 days; lenses were removed. (3) Hyperopic to myopic defocus. Eyes wore negative lenses for 5 days; lenses were removed. Eyes were measured using A-scan ultrasonography at 6-h intervals for 24 h over various cycles. The rhythms shift into phase in eyes slowing their growth in response to myopic defocus in all three conditions. This shift precedes by 1 day the decrease in growth in both lens conditions, and is concomitant with it in recovering eyes. There is a positive correlation between the phase difference and growth rate. In conclusion, there is a consistent association between growth rate and phase relationships of the rhythms in axial elongation and choroidal thickness.