What is normal vision?

In order to better understand how certain problems can affect your child's vision, it is important to understand how normal vision occurs. For children with normal vision, the following sequence takes place:

1. Light enters the eye through the cornea, the clear, dome-shaped surface that covers the front of the eye.
2. From the cornea, the light passes through the pupil. The amount of light passing through is regulated by the iris, or the colored part of your eye.
3. From there, the light then hits the lens, the transparent structure inside the eye that focuses light rays onto the retina.
4. Next, it passes through the vitreous humor, the clear, jelly-like substance that fills the center of the eye and helps to keep the eye round in shape.
5. Finally, it reaches the retina, the light-sensitive nerve layer that lines the back of the eye, where the image appears inverted.
6. The optic nerve carries signals of light, dark, and colors to the area of the brain (the visual cortex), which assembles the signals into images (our vision).

What are refractive errors?

The following are the most common refractive errors, all of which affect vision and may require corrective lenses for correction or improvement:

• Astigmatism. Astigmatism is a condition in which an abnormal curvature of the cornea can cause two focal points to fall in two different locations--making objects up close and at a distance appear blurry. Astigmatisms may cause eye strain and may be combined with nearsightedness or farsightedness. Astigmatism can start in childhood or in adulthood. Some symptoms include headache, eye strain, and/or fatigue. Eye rubbing, lack of interest in school, and difficulty in reading are often seen in children with astigmatism. Depending on the severity, eyeglasses or contact lenses may be required.

• Hyperopia. Commonly known as farsightedness, hyperopia is the refractive error in which an image of a distant object becomes focused behind the retina, either because the eyeball axis is too short, or because the refractive power of the eye is too weak. This condition makes close objects appear out of focus and may cause headaches, eye strain, and/or fatigue. Squinting, eye rubbing, lack of interest in school, and difficulty in reading are often seen in children with hyperopia.

• Myopia. Commonly known as nearsightedness, myopia (the opposite of hyperopia) is a condition in which an image of a distant object becomes focused in front the retina, either because the eyeball axis is too long, or because the refractive power of the eye is too strong. Myopia is the most common refractive error requiring correction seen in children. This condition makes distant objects appear out of focus and may cause headaches and/or eye strain.

  Eyeglasses or contact lenses may help to correct or improve myopia by adjusting the focusing power to the retina.

What causes refractive errors?

Refractive errors (myopia and hyperopia) have been found to cluster in families. A variety of inheritance patterns have been observed including dominant (one gene passed from a parent with a refractive error to a child), recessive (caused by two genes, one inherited from each parent who may or may not have a refractive error), and multifactorial (combination of genes and environment). Refractive errors are present in a number of genetic disorders, such as Marfan syndrome and Down syndrome.

Eyeglasses or contact lenses may help to correct or improve hyperopia by adjusting the focusing power to the retina.