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Thomas T Norton, Alexander H. Ward, Regina Rab; Myopia in juvenile tree shrews born and raised in total darkness. Invest. Ophthalmol. Vis. Sci. 2020;61(7):3408.
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© ARVO (1962-2015); The Authors (2016-present)
To learn if juvenile shrews that never experienced light develop myopia as do tree shrews raised in light and then placed in darkness (Amedo et al., IOVS 2006).
Two pregnant northern tree shrews (Tupaia belangeri) were placed in total darkness (a darkroom with a rotating light-tight door) shortly before each delivered three pups. The animals were observed at least twice per day with an infrared viewer and infrared LED flashlight. The mothers ate and drank water normally. The day of eye opening was determined by daily examination of the pups using the infrared viewer so that “days after eye opening (DAEO)”, comparable to “days of visual experience (DVE)” in colony-raised animals could be determined. Refractive state (Nidek autorefractor, mean of right and left eyes) was measured in a darkened room (litter #1 at 21 DAEO, litter #2 at 24). Ocular component dimensions (Lenstar optical biometer) were then measured in dim lighting (litter #1 at 23 DAEO, litter #2 at 25). These values were compared with a group of 7 animals raised in colony light (Gawne et al., Optom Vis Sci 2018).
All six dark-reared animals were more myopic than the colony-raised animals (Fig. 1A, mean ± SEM, litter #1, –2.4 ± 0.7 D; litter #2, −5.7 ± 0.2 D). At 24 DVE, colony-raised tree shrews at 24 DVE were slightly hyperopic (1.3 ± 0.4 D). Vitreous chamber depth in the dark-reared animals (Fig. 1B, litter #1, 2.98 ± 0.02 mm; litter #2, 2.99 ± 0.01 mm) was larger than any of the colony-raised animals (2.80 ± 0.03 mm). Subsequent measures with cycloplegia (tropicamide or atropine) confirmed the results. Other ocular component dimensions were similar to those of the colony-raised group. Four animals exhibited slowed recovery toward emmetropia.
In a lighted environment, the emmetropization mechanism uses the initial hyperopic defocus of young tree shrews to guide axial elongation, establishing and maintaining emmetropia. Darkness removes the defocus information needed to slow axial elongation at emmetropia, resulting in elongated, myopic eyes. Prior exposure to a visual environment is not needed for dark treatment to have a myopiagenic effect.
This is a 2020 ARVO Annual Meeting abstract.
Fig. 1. A: Daily refractions of individual colony-reared tree shrew and the refractions measured in the six animals born and raised in darkness. B: Vitreous chamber depth of the colony- and five dark-reared animals.
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