To see clearly, the eye must focus the image accurately on the retina. This is not the situation at birth. At birth, the eyeball is short and the refractive power of the anterior segment is not powerful enough for the short eyeball (hyperopia). The process wherein the refractive power of the anterior segment progressively changes to match the natural expected elongation of the eyeball (axial length) is called emmetropization, and it takes years. Thus a newborn does not see 20/20 until emmetropization is complete or nearly there. Only high amounts of hyperopia relative to the age of the child need correction in the form of glasses.
For some individuals, the process of emmetropization does not stop right away. A disruption in the process could occur such that the axial length continues to grow, leaving the refractive power of the anterior segment too strong or too powerful for the size of the eyeball. This is called myopia. Myopic children like to move objects close to their faces to see better. Axial elongation can become progressive (progressive myopia) due to environmental factors such as too much activities at near (reading and use of electronic devices), and the lack of exposure to natural light. Glasses improve patients vision but lifestyle changes and pharmacologic intervention may help decrease the speed of axial elongation.
Astigmatism is another type of refractive error and it is due to optical asymmetries in the anterior segment, either from the cornea or from the lens. Its presence could lead to uncoordinated eye growth and later development of myopia (axial elongation). Significant astigmatism is approximately -1.50 D or higher. It is advisable to treat astigmatism with glasses early in life not only to prevent amblyopia, but also to decrease the risk of developing myopia later.
Full correction of the refractive error is always done when strabismus is co-existent. Refractive surgery is not yet approved as a standard for all cases, but it can be performed on certain cases when amblyopia treatment is going to be a challenge due to high anisometropia.