近視的最新研究進展-眼正視化和正常生長(Emmetropization and Normal Ocular Growth)

When incident parallel rays of light from distant objects are brought to a focus upon the retina without accommodation, it is known as emmetropia. During postnatal eye growth, the precise matching of the axial length (the distance from the anterior corneal surface to the retina along the visual axis) and the optical power of the eye brings the eye to emmetropia. This active regulatory process that harmonizes the expansion of the eye with the optical power of the cornea and the crystalline lens is known as emmetropization. Any disruption to these highly coordinated ocular changes results in the development of refractive errors, wherein distant images are focused either behind (hyperopia) or in front (myopia) of the retina.

當來自遠處物體的平行光線在不調節的情況下聚焦在視網膜上時，稱為正視。在出生後的眼球生長過程中，眼軸長度(從角膜前表面沿視軸到視網膜的距離)與眼屈光力的精確匹配，使眼睛趨向正視。這種眼的生長與角膜和晶狀體屈光力相協調的主動調節過程被稱為正視化。對這些高度協調的眼部變化的任何破壞都會導致屈光不正，其中遠處的圖像聚焦在視網膜的後方（近視）或前方（近視）。 

Human eyes exhibit a distinctive pattern of eye growth during the early period of visual development. The distribution of refractive errors at birth appears to be normally distributed. Apart from some exceptions, the majority of new- born infants are moderately hyperopic (~+2.00 to +4.00 D) and this refractive error reduces significantly during the first 18 months of life (Fig. 4.1). By about 2–5 years of age, the distribution becomes leptokurtic with a peak around emmetro- pia to low hyperopia of about +0.50 to +1.00 D. Although studies have reported small reductions in hyperopic refraction until the middle to late teen years, emmetropization is believed to be largely completed by 5–6 years of age.

人眼在視覺發育的早期，表現出一種獨特的生長模式。出生時屈光不正的分布類似正態分布的。除了一些例外，大多數新生兒都有中度遠視(~+2.00~+4.00D)，在出生後18個月，屈光不正明顯減少(圖1)。到大約2-5歲時，這種分布變得尖峰化，在正視眼到低度遠視的位置有一個峰值，大約在+0.50~+1.00D。儘管有研究報告稱，在青少年中後期的遠視屈光會有輕微的降低，但正視化被認為在5-6歲時就基本完成了。

Fig.1 Comparison of refractive error distribution among newborns and 6–8-year-old chil- dren. The distribution of refractive errors narrows between infancy to early childhood during the process of emmetropization. Adapted from FitzGerald and Duckman 圖1 新生兒和6-8歲兒童屈光不正分布的比較。在正視化過程中，屈光不正的分布在嬰兒至兒童早期之間變窄。摘自FitzGerald and Duckman
Based on the visually guided ocular growth observed in a variety of animal models, the growth of the human eye is also believed to be modu- lated by an active visual feedback from the hyperopic refractive error in neonatal eyes. Studies have found a strong correlation between the rapid reduction in hyperopia and the changes in axial length during early ocular development. Human eyes are ~17 mm long after birth and grow to about 20 mm after the first year. This rapid expansion of the eye is largely attributed to the expansion of the vitreous chamber. From 2–3 years of age, axial elonga- tion slows to approximately 0.4 mm/year until preschool age. Consistent with changes in ocular refraction, the growth of the eye stabilizes further at 5–6 years, and only increases by 1–1.5 mm through the teenage years. Together, these studies suggest that axial length is the most influential factor for emmetropiza- tion in human eyes.

根據在各種動物模型中觀察到的視覺引導的眼球生長，人眼的生長也被認為是由新生兒眼的遠視屈光不正的主動視覺反饋調節的。研究發現，遠視的快速減少與早期眼發育中眼軸長度的變化有很強的相關性。人眼出生後長約17毫米，一年後長到20毫米左右。眼的迅速生長主要是由於玻璃體腔的擴張。從2-3歲開始，直到學齡前，軸向伸長減緩到大約0.4毫米/年。與眼屈光度的變化一致，眼睛的生長在5-6歲時會進一步穩定，在整個青少年時期僅增加1-1.5 毫米。總之，這些研究表明眼軸的長度是影響人眼正視的最重要因素。

In addition to changes in axial length, there is also a significant reduction in the refractive power of the cornea and the crystalline lens that contributes to the overall reduction in hyperopia in the first year of life. Mutti et al. reported a reduction of 1.07 and 3.62 D in corneal and crystalline lens powers, respectively, associated with flattening of the corneal and lens radii in newborn infants, between the ages of 3 and 9 months. Studies have also found higher degrees of corneal astig- matism in newborn infants, which reduces during the first 4 years of life and is associated with corneal flattening. Overall, these studies sug- gest that emmetropization in human eyes is largely attributed to the changes in axial length with minor contributions from corneal and crystalline lens powers.

除了眼軸長度的變化，角膜和晶狀體的屈光力也有顯著降低，這有助於在出生後第一年全面減少遠視。Mutti等人報告，3-9個月大的新生兒角膜和晶狀體曲率半徑變大，角膜和晶狀體屈光度分別降低1.07和3.62D。研究還發現，新生兒角膜散光程度較高，在出生後的頭4年減少，並與角膜扁平化有關。總體而言，這些研究認為人眼正視化主要歸因於眼軸長度的變化，而角膜和晶狀體屈光力的變化影響較小。

Refractive errors occur as a result of either variations in (a) axial length with respect to the total refractive power of the eye (termed axial myopia or hyperopia) or (b) refractive power of the cornea and the crystalline lens with respect to the axial length of the eye (termed refractive myopia or hyperopia). 

任何一種變化都可導致屈光不正：(一)眼軸長度的變化相對於眼的總屈光力佔優(稱為軸性近視或遠視)；(二)角膜和晶狀體的屈光力變化相對於眼軸長度佔優(稱為屈光性近視或遠視)。