Overview of Eye Function and Focusing
The eye is like a camera. The front of the eye is called the cornea. It is a clear structure that allows light into the eye, much like a watch crystal, and also begins the process of focusing the light. Inside the eye, behind the cornea, is the iris. This is the part of the eye which gives it color, such as blue or brown. The iris controls how much light enters the eye depending on the brightness of the environment. It does this by opening and closing the small hole in its center, the pupil. Behind the iris and pupil is the focusing lens of the eye. This lens completes the focusing of light. It is flexible and allows the eye to focus at different distances. After passing through these structures at the front of the eye, the focused light forms an image on the retina, a layer of tissue on the back of the eye, analogous to the film of a camera. If the image is not focused on the retina correctly, then the image is blurred and a refractive error is present.
Types of Refractive Error
Myopia: Also known as nearsightedness, myopia is the inability to see clearly at distance while objects at near are clear. The focused image in the eye is in front of the retina because the eye is too long or because the lens is too strong. Individuals will “squint” or move closer to the object they are viewing to see clearly. Myopia generally progresses into the late teens. Just as a child becomes taller or needs progressively larger shoes with increased body size through puberty, the eyes grow and lengthen as well. Therefore, an eye that is longer in infancy will proportionally lengthen or “worsen”. Therefore, myopic individuals are less likely to “grow out of” glasses.
Hyperopia: Also known as farsightedness, hyperopia is the inability to see clearly at near, but there is also difficulty at distance. The image is never focused in hyperopia since the eye is too short or the lens has too little focusing strength. In this case, the image is actually trying to focus behind the retina. It is actually normal for infants and young children to have mild degrees of hyperopia. As they grow and the eye becomes longer, this disappears. These children do not require correction with glasses. However, high degrees of hyperopia may create too much blur and lead to amblyopia, or a form of strabismus called accommodative esotropia, both of which are discussed elsewhere. In such patients, glasses are necessary to properly focus the images, and sometimes to also straighten the eyes. Some hyperopic children may actually “grow out” of their glasses when they are older because of the eye growth mentioned above.
Astigmatism: Astigmatism is an irregular shape of the refractive surfaces of the eye, most commonly the cornea but sometimes also the lens. This is often described as the eye being shaped like an oval, such as an egg or an American football rather than a sphere, like a basketball. Because of the irregular shape, light rays are distorted because light entering at the front of the eye is bent at a different angle or direction than light entering at another spot. Astigmatism can affect both distance and near vision and may be present alone, or with either hyperopia or myopia.
Anisometropia: Most children have similar readings in each eye. However, some may have very unequal eyes, with refractive error in one eye only, refractive error in both eyes but much stronger in one, or even different types of refractive error in each eye. This is known as anisometropia. These children are at risk of developing “lazy eye” or amblyopia in the eye with a higher refractive error. Therefore, glasses are extremely important not only to focus the image, but to prevent vision loss. These children are often not discovered until a later age, and often complain that they do not need glasses because they see well through one eye and seem to function normally even though they have poor vision in the other eye.
Presbyopia: Presbyopia is a condition affecting older patients, usually past the age of about 40 years. The lens of the eye becomes stiffer over time and gradually reduces the ability to focus on near objects. Patients find that they must hold things farther away in order to see them. Because of this, it is often confused with hyperopia since the symptoms are similar. This is not an issue faced by the pediatric population.
Glasses and Other Refractive Topics
Although most people have a consistent type of refractive error, such as being “near sighted”, refractive errors may also be a combination of problems such as myopic astigmatism or hyperopic astigmatism. An individual does not have to have solely myopia or hyperopia or astigmatism.
Corrective lenses, whether as glasses or contact lenses, are necessary for children with refractive errors to correct vision and ensure that a child will achieve the best vision as possible as an adult. As with many other medical issues, there is a genetic predisposition to having refractive error and needing glasses, but it is not a hard and fast rule.
An important thing to understand about glasses is that they do not change the eyes. In other words, they do not “get rid” of the problem or make it worse. (However, amblyopia or “lazy eye” can get worse if a child does not use prescribed glasses. This is discussed elsewhere.) The need for updated glasses over time in children is caused by eye growth.
Common, incorrect myths concerning glasses include:
“Don’t read in the dark, because it will make your eyes worse.” There is no evidence to support this
“Eat lots of carrots so you don’t need glasses.” Although carrots are very healthy and supply nutritional substances for retinal function, eating them will have no influence on the size and shape of the eye, and therefore has no effect on the need for glasses.
“I don’t wear glasses, so my children will not wear glasses.” Although the tendency for glasses is hereditary, it is influenced by many genes and there is no hard and fast hereditary pattern. There are many instances of children needing glasses when the parents do not and vice versa. It depends on which genes a child gets from each parent.
“Wearing glasses will only make your eyes worse.” As mentioned above, glasses do not change the size or shape of the eye. Changes in the refractive error in children are primarily caused by eye growth.
“Wearing glasses will only make you more dependent on them.” Many people report that they become dependent on their glasses and cannot go back to functioning without them. Actually, after realizing how good vision can be, it is difficult to go back to being blurry all day.