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Zheng Shao, Marsha Kisilak, Elizabeth L Irving, Melanie C W Campbell; Growth and completion of emmetropization in the normally developing chick eye. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2173.
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© ARVO (1962-2015); The Authors (2016-present)
Normal emmetropization has long been assumed to result in zero refractive error, but recently this has been questioned. In normally growing chick eyes, we are interested in objectively determining when emmetropization is complete. We are also interested in whether growth during and following normal emmetropization differs.
From literature values of chick eye parameters, functions were fit to MOR (mean ocular refraction or spherical equivalent) and optical axial length (OAL; anterior cornea to anterior retina) vs. age. Dioptric length (K’) and eye power (F) were derived up to day 75 using our previously reported method to calculate eye power. Pupil size data were also used to calculate the angular and linear retinal blurs (EB and LRB) due to defocus.
Eye power and K’ decrease exponentially with age at slightly different rates until power and K’ reach almost equal values about day 35. Subsequently, power and K’ decrease almost identically with age. This gives an initial rapid exponential decrease in MOR, which reaches a relatively stable value of 1.0 D of hyperopia beyond day 35. The completion of emmetropization is defined as the first time point beyond which MOR remains relatively stable, estimated as between 30 and 35 days. EB and LRB decrease almost exponentially until day 35. After emmetropization is complete, MOR changes little, EB remains almost constant while LRB increases slowly from about day 45 to the end of available measurements on day 75, in agreement with predictions of an almost uniformly expanding eye model. The radius of the blur on the retina is larger than cone spacing prior to completion of emmetropization, and approaches cone spacing as emmetropization is completed.
Concurrent variations in eye power and length combine to produce the smaller, more rapid changes in MOR during emmetropization. The time point at which emmetropization is complete can be defined as the first time point after which MOR and angular retinal blur are stable. Emmetropization appears to be driven by an active reduction of EB to a value close to cone resolution. After emmetropization is complete, the subsequent change in retinal blur is consistent with a slow, almost uniform ocular expansion. However, after the age when normal emmetropization is complete, an emmetropization response to additional imposed defocus blur has been observed.
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