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D.L. Nickla, C. Johnson, G. Lytle; Phase Relationships Between The Diurnal Rhythms In Axial Length And Choroidal Thickness And Their Association With Ocular Growth Rates In Chicks . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1158.
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Purpose: In chickens, myopic defocus induced by either prior form deprivation or by wearing positive spectacle lenses results in large phase shifts in the diurnal rhythms in axial length and choroidal thickness, bringing the two into phase (Nickla et al., 1996; 1998). It has been hypothesized that these phase shifts play a role in ocular growth regulation. To test this we did a longitudinal study in which we examined these rhythms in eyes undergoing successive changes in growth rate induced by various visual manipulations. Methods: Eyes were measured using high frequency A–scan ultrasonography at 6 hr intervals over 24 hrs on various days. Form deprivation: Birds were monocularly deprived for 1 week; eyes were measured on days (d) 1, 2 and 5. The diffusers were removed and eyes measured on d2, d3, d6 and d8 (myopic defocus). Negative lenses: –10D lenses were worn on one eye. Eyes were measured on d1, d2 and d5 of lens wear (hyperopic defocus), and d2, d3 and d6 after lens removal (myopic defocus). Positive lenses: +10D lenses were worn. Eyes were measured on d1, d2, d5 and d6 of lens wear (myopic defocus) and on d2 and d3 after lens removal (hyperopic defocus). Results: In all 3 cases, myopic defocus resulted in shifts into phase of the two rhythms; this shift was not always temporally coincident with the slowest growth rates. In plus lens–wearing eyes and eyes "recovering" from negative lens–induced myopia the shift was transient (24 hrs) and preceded the slowest growth (plus lens: d1; mean phase difference=2 hrs, rate=91µm/d; negative lens recovery: d2; mean phase diff=3 hrs, rate=100µm/d; rates not different from normal). In previously form deprived eyes however, the shift was more sustained and concurrent with a reduction in growth (d3, d6, d8; mean phase diff= 4hrs, 0 hrs, 0 hrs respectively; rate=43 µm/d, 54 µm/d, 49 µm/d, respectively). The shift was largely the result of a phase advance in the choroidal rhythm. In eyes responding to hyperopic defocus and form deprivation the rhythms were in approximate antiphase. Of note, both parameters sometimes showed higher frequency oscillations (omitted from analyses). Conclusions: The myopic defocus–associated shift into phase of the two rhythms precedes the reduction in growth rate both in eyes wearing positive lenses and in eyes "recovering" from negative lens wear, but is concomitant with the slowest growth in eyes recovering from form deprivation. The lack of consistent concurrence between phase and rate suggests that the phase relationships per se do not have a direct causal effect on ocular growth rate.
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