The phase relationships between the diurnal rhythms in axial length and choroidal thickness and the association with ocular growth rate in chicks (original) (raw)
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Visual Influences on Diurnal Rhythms in Ocular Length and Choroidal Thickness in Chick Eyes
Experimental Eye Research, 1998
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) ?
Experimental Eye Research, 2017
Animal models have shown that myopic defocus is a potent inhibitor of ocular growth: brief (1-2 hours) daily periods of defocus are sufficient to counter the effects of much longer periods of hyperopic defocus, or emmetropic vision. While the variables of duration and frequency have been well-documented with regard to effect, we ask whether the efficacy of the exposures might also depend on the time of day that they are given. We also ask whether there are differential effects on the rhythms in axial length or choroidal thickness. 2-week-old chickens were divided into 2 groups: (1) "2-hr lens-wear". Chicks wore monocular +10D lenses for 2 hours per day for 5 days at one of 3 times of day: 5:30 am (n=11), 12 pm (n=8) or 7:30 pm (n=11). (2) "2-hr minus lens-removal". Chicks wore monocular −10D lenses continually for 7 days, except for a 2-hr period when lenses were removed; the removal occurred at one of 2 times: 5:30 am (n=8) or 7:30 pm (n=8). Both paradigms exposed eyes to brief myopic defocus that differed in its magnitude, and in the visual experience for the rest of the day. High frequency A-scan ultrasonography was done at the start of the experiment; on the last day, it was done at 6-hr intervals, starting at noon, over 24-hr, to assess rhythm parameters. Refractive errors were measured using a Hartinger's refractometer at the end.
Experimental Eye Research, 1998
Low coherence laser Doppler interferometry (LDI) allows high precision measurements of the axial length of the eye and of the thickness of the individual layers of the ocular fundus. Here, we used LDI to monitor diurnal changes in these dimensions in eyes of newly hatched chicks and one-year-old chickens with normal or altered visual input. In chicks and chickens with normal visual experience, axial eye length displays diurnal fluctuations increasing during the light phase. Choroidal thickness also exhibits a diurnal pattern, shrinking during the day and expanding during the night. Retinal thickness does not vary. Based on the pressure compliance of the enucleated chick eye, the diurnal intraocular pressure (IOP) fluctuation could contribute both to the increase in axial length and to daytime choroidal shrinkage.
Experimental Eye Research, 2007
In chickens, transient changes in choroidal thickness are found in conditions in which the eye is slowing its growth in response to visual episodes that prevent excessive elongation. To test the hypothesis that the choroidal and ocular growth responses are linked, we used a variety of ''brief daily'' stimuli known to ameliorate the development of myopia and assessed the concurrence of the responses. If the hypothesis is true, they should always be correlated. Form deprivation w/vision or strobe. Diffusers were worn for 5 days and removed for 2 h of ''vision'' each day in: (a) one block of 2-h (n ¼ 16); or (b) two 1-h periods (n ¼ 10). Strobe. Birds were given 0.5 h episodes of 12 Hz strobe at dawn and dusk (12 h apart, n ¼ 11). Negative lenses w/vision or strobe. Lenses (À10D) were worn for 5 days and removed for 2 h of vision each day (n ¼ 14). Strobe. Same as above (n ¼ 11). Darkness/brief vision or myopic defocus. Birds in constant darkness were given 2 daily 0.5 h episodes of light 12 h apart (n ¼ 6) or one daily 0.5 h episode of þ10 D myopic defocus (n ¼ 6) for 4 days. Darkness/''frequent'' or ''infrequent'' myopic defocus. Birds in constant darkness were given frequent (2 min  14) or infrequent (1 min  7) episodes of þ10 D myopic defocus for 4 days. In all experiments a control group had the myopia-inducing treatment but did not receive the visual stimulation. High frequency ultrasonography was done at the start and end of the experiment, and on the last day immediately prior to and 1 h after the period of stimulation. Refractive errors were measured using a Hartinger's refractometer at the end of the experiment. We found that in 7 of the 8 conditions the development of myopia was inhibited. Form deprivation: vision or strobe vs control: À1.2 and À1.8 vs À9.8 D. Negative lenses: vision or strobe vs control: À1.2 and À4.3 vs À8 D. Constant dark: vision or myopic defocus vs control: À0.7 and 1.8 vs À1.8 D. Constant dark: frequent myopic defocus vs control: 4.8 vs À0.4 D (p < 0.05 for all comparisons). In all the effect was axial with growth rate being significantly inhibited. In all cases the choroids showed significant transient increases in thickness as well. Form deprivation: vision or strobe vs control: 58 and 15 vs À3 mm. Negative lenses: vision or strobe vs controls: 74 and 17 vs À17 mm. Dark: vision or myopic defocus vs control: 56 and 46 vs 11 mm. Dark: frequent vs control: 103 vs 5 mm. In the ''infrequent myopic defocus'' condition eyes did not compensate to the defocus, however they did not become myopic. The choroidal response was not significant. These results support the hypothesis that these brief choroidal responses may play a role in ocular growth inhibition.
Experimental Eye Research, 1998
Recent investigations have shown that growing chicken eyes elongate during the day and shorten during the night. We asked whether the chick, like a number of other animals, exhibits a rhythm in intraocular pressure (IOP) and whether this rhythm might be associated with this rhythm in elongation. We find that the intraocular pressure in normal eyes is high during the day and low in the middle of the night, similar to the rhythm in ocular elongation. The amplitude of this rhythm in IOP is approximately 8 mm Hg ; it persists in constant darkness, albeit with a reduced amplitude, implying that the rhythm has a circadian component. Form deprivation by translucent diffusers does not affect the amplitude of the rhythm in IOP, but makes the phase of the rhythm more variable, such that the trough no longer consistently occurs at night.
Ocular diurnal rhythms and eye growth regulation: Where we are 50 years after Lauber
Experimental Eye Research, 2013
Many ocular processes show diurnal oscillations that optimize retinal function under the different conditions of ambient illumination encountered over the course of the 24 h light/dark cycle. Abolishing the diurnal cues by the use of constant darkness or constant light results in excessive ocular elongation, corneal flattening, and attendant refractive errors. A prevailing hypothesis is that the absence of the Zeitgeber of light and dark alters ocular circadian rhythms in some manner, and results in an inability of the eye to regulate its growth in order to achieve emmetropia, the matching of the front optics to eye length. Another visual manipulation that results in the eye growth system going into a "default" mode of excessive growth is form deprivation, in which a translucent diffuser deprives the eye of visual transients (spatial or temporal) while not significantly reducing light levels; these eyes rapidly elongate and become myopic. It has been hypothesized that form deprivation might constitute a type of "constant condition" whereby the absence of visual transients drives the eye into a similar default mode as that in response to constant light or dark. Interest in the potential influence of light cycles and ambient lighting in human myopia development has been spurred by a recent study showing a positive association between the amount of time that children spent outdoors and a reduced prevalence of myopia. The growing eyes of chickens and monkeys show a diurnal rhythm in axial length: Eyes elongate more during the day than during the night. There is also a rhythm in choroidal thickness that is in approximate anti-phase to the rhythm in eye length. The phases are altered in eyes growing too fast, in response to form deprivation or negative lenses, or too slowly, in response to myopic defocus, suggesting an influence of phase on the emmetropization system. Other potential rhythmic influences include dopamine and melatonin, which form a reciprocal feedback loop, and signal "day" and "night" respectively. Retinal dopamine is reduced during the day in form deprived myopic eyes, and dopamine D2 agonists inhibit ocular growth in animal models. Rhythms in intraocular pressure as well, may influence eye growth, perhaps as a mechanical stimulus triggering changes in scleral extracellular matrix synthesis. Finally, evidence shows varying influences of environmental lighting parameters on the emmetropization system, such as high intensity light being protective against myopia in chickens. This review will cover the evidence for the possible influence of these various factors on ocular growth. The recognition that ocular rhythms may play a role in emmetropization is a first step toward understanding how they may be manipulated in treatment therapies to prevent myopia in humans.
Induction of axial eye elongation and myopic refractive shift in one-year-old chickens
Vision Research, 1998
Depriving the eyes of neonatal animals of form vision induces axial eye elongation and ipsilateral myopia. We studied one-year-old chickens, an age at which full body growth has been attained, to learn if form deprivation myopia can develop at a later stage. We found that ocular reactivity to visual form deprivation continues in one-year-old chickens; but both the growth stimulation and the myopic shift in refraction are attenuated compared with newly hatched birds. Neurochemical changes in visually deprived eyes of one-year-old chickens parallel those in newly hatched chicks: ipsilateral decreases in retinal dopamine and in the activity of ciliary ganglion and uveal choline acetyltransferase. These findings strengthen the relevance of the form deprivation model to more common human myopia and suggest a common eye growth control mechanism at both ages.
Experimental eye research, 2016
Changes in ocular growth that lead to myopia or hyperopia are associated with alterations in the circadian rhythms in eye growth, choroidal thickness and intraocular pressure in animal models of emmetropization. Recent studies have shown that light at night has deleterious effects on human health, acting via "circadian disruptions" of various diurnal rhythms, including changes in phase or amplitude. The purpose of this study was to determine the effects of brief, 2-hour episodes of light in the middle of the night on the rhythms in axial length and choroidal thickness, and whether these alter eye growth and refractive error in the chick model of myopia. Starting at 2 weeks of age, birds received 2 hours of light between 12:00 am and 2:00 am for 7 days (n=12; total hours of light: 14 hrs). Age-matched controls had a continuous dark night (n=14; 14L/10D). Ocular dimensions were measured using high-frequency A-scan ultrasonography on the first day of the experiment, and again...
Experimental Eye Research, 2002
When form deprived, young chicks rapidly develop axial myopia, from which they recover if the treatment is ceased at a suf®ciently early age. The increased axial growth of the eye is accompanied by choroidal thinning and decreased choroidal blood¯ow (ChBF). In contrast, during the early part of the recovery process, the choroid thickens, shifting the retina towards the new plane of focus. Little information is available about ChBF during recovery from myopia. Because of the possibility that choroidal thickening during recovery from myopia might be driven by an increase in ChBF, the temporal relationship of ChBF and choroidal thickness changes was examined during such recovery. White Leghorn chicks were form deprived from 3 days of age for 2±3 weeks using detachable plastic diffusers. Axial ocular dimensions, including choroidal thickness, were then measured by high frequency A-scan ultrasonography at various times after the diffusers were removed up to 240 hr. ChBF was measured transclerally immediately following the A-scan ultrasonography, using laser Doppler owmetry. In the chicks measured immediately after diffuser removal, the vitreous chamber was 29. 9 % longer, the choroid was 6. 4 % thinner and ChBF was 13. 7 % less in the treated than in the non-treated control eyes. These changes are characteristic of myopic chick eyes and are reversible in young eyes. Thus, in chicks examined 7 hr after diffuser removal, the ChBF in recovering eyes was now greater than that in control eyes. This ChBF increase peaked about 19 hr after the diffusers were removed. The mean increase in ChBF in treated eyes for the 7±30 hr monitoring period was 187 %, relative to control eyes. ChBF in the treated eyes gradually returned to the control level after this time. By contrast to the early, transient increase in ChBF, signi®cant choroidal thickening was not observed in treated eyes until 30 hr after diffuser removal, and continued to increase relative to control eyes over the remainder of the monitoring period, reaching a ®nal mean value of 182 %. This study demonstrates, in chick eyes recovering from form deprivation myopia, large increases in ChBF that preceded increases in choroidal thickness and were also more transient than the latter. These results raise the possibility that the increase in ChBF may trigger or even drive the subsequent onset of choroidal expansion, perhaps by facilitating the ®lling of the choroidal lymphatic lacunae that are well developed in the avian eye.
Experimental Eye Research, 2017
It is generally accepted that myopic defocus is a more potent signal to the emmetropization system than hyperopic defocus: one hour per day of myopic defocus cancels out 11 hours of hyperopic defocus. However, we have recently shown that the potency of brief episodes of myopic defocus at inhibiting eye growth depends on the time of day of exposure. We here ask if this will also be true of the responses to brief periods of hyperopic defocus: may integration of the signal depend on time of day? If so, are the rhythms in axial length and choroidal thickness altered? Hyperopic defocus: Birds had one eye exposed to hyperopic defocus by the wearing of −10D lenses for 2 or 6