Both nearsightedness (myopia) and farsightedness (hyperopia) are caused by minute structural differences in the size and shape of the eyeball.
HYPEROPIA ( FARSIGHTEDNESS)
With farsightedness, the eyeball is no longer perfectly spherical. It is somewhat vertically oval and the lens is closer to the retina than in an eye with normal vision. This makes it difficult for the lens to clearly focus close-up objects on the retina, but does not hinder the eye's ability to focus on faraway objects -- hence the term farsightedness. Farsightedness is also sometimes, although rarely, caused by a thinning of the lens or flattening of the cornea.
Farsightedness is often an inherited trait. Normally, all young children are farsighted to some degree, but the condition usually decreases during the teens, since a child's eyes lengthen as they grow. Children who are farsighted generally see well near and far because the lens is strong enough, at this age, to compensate for the shape of the eye, and clearly focus images on the retina. Since the muscles of the eye have to contract tightly to see up close, farsightedness in children is sometimes associated with crossed eyes.
Treatment of Hyperopia
Farsightedness is usually corrected with glasses or contact lenses. It is not necessary to correct small degrees of farsightedness in children if other visual or non-visual symptoms are not present. Only children with severe hyperopia or crossed eyes need glasses for this condition. LASIK -- Laser in-Situ Keratomileusis -- is a surgical procedure where a laser is used to remove microscopic layers of corneal tissue to change its shape and allow light rays to focus more directly on the retina. To treat farsightedness, the center of the cornea is made steeper by removing tissue around the center.
With nearsightedness, the eyeball is no longer perfectly spherical. It is somewhat horizontally oval and the lens is farther from the retina than in an eye with normal vision. This makes it difficult for the lens to clearly focus distant objects on the retina, but does not hinder the eye's ability to focus on objects that are near -- hence the term nearsightedness. Nearsightedness is also sometimes, although rarely, caused by a thickening of the lens or the cornea becoming more spherical.
Nearsightedness is most often an inherited trait that becomes evident in children at around eight to twelve years of age (although an adult form of myopia may occur in the early twenties). It typically increases during the teens and levels off in adulthood, with very little change from age 20 to 40. Few factors other than heredity influence nearsightedness. Things like ready in dim light or poor nutrition do not cause or affect nearsightedness.
Treatment of Myopia
Nearsightedness is usually corrected with glasses or contact lenses. The lenses compensate for the longer shape of the eye by bending light rays to more clearly focus them on the retina. Although some people have been fitted with a series of hard contact lenses, in a process called orthokeratology, sight improvement is only temporary. When the lenses are removed, the cornea returns to its original shape and the nearsightedness returns. There is no scientific evidence that contact lenses or eye exercises can stop the progression of myopia. The use of special eye drops (atropine), and bifocal glasses can also be an effective treatment for nearsightedness in some cases.
LASIK -- Laser in-Situ Keratomileusis -- is a surgical procedure where a laser is used to remove microscopic layers of corneal tissue to change its shape and allow light rays to focus more directly on the retina. To treat nearsightedness, the cornea is made flatter by removing tissue from its center.
With normal vision, the cornea is smooth and equally curved in all directions. But with astigmatism, the cornea is warped and curves more in one direction than another. This distorts your vision similar to what you might see in the funhouse mirrors that make you look too tall, too short, or wavy.
Like myopia and hyperopia, astigmatism is usually inherited. It may even be present at birth, and frequently remains unchanged throughout your life. Small amounts of astigmatism are very common and rarely require vision correction.
Treatment of Astigmatism
The distortion caused by astigmatism is usually corrected with glasses or contact lenses. Hard or gas-permeable contact lenses usually correct astigmatism better than soft lenses, but there are special soft lenses for astigmatism that are sometimes helpful. Very large amounts of astigmatism are not easily corrected. Glasses that correct it can sometimes cause distortion of side vision, and contacts may wobble on the wavy surface of the irregular cornea. In these cases, a special contact lens, called a "toric" contact lens, can be ground with a curve in the back of it that follows the surface of the irregular cornea and stabilizes the contact on the cornea so it does not wobble.
If the distortion follows a straight line across the cornea, glasses or contacts with a similar wave will compensate for the distortion of the cornea. But in some cases, like with a scar due to an eye injury, the distortion may be irregular and not follow a straight line. This "irregular" astigmatism can be difficult to correct and often requires making a totally new surface on the cornea. This is usually accomplished either by applying hard contact lenses, or with a corneal transplant to replace the scarred corneal tissue with a donated cornea.
LASIK -- Laser in-Situ Keratomileusis -- is a surgical procedure where a laser is used to remove microscopic layers of corneal tissue to change its shape and allow light rays to focus more directly on the retina. To treat astigmatism, the cornea is made more spherical by removing tissue more in one direction than another.
PRESBIOPIA (AGING EYES)
As your eye ages, the lenses gradually harden and lose some of their flexibility, thereby limiting their ability to focus clearly on close-up objects. This condition is called presbyopia -- or aging eyes. Presbyopia is an inevitable companion of middle age, usually occurring after the age of 40.
Treatment of Presbyopia
Presbyopia is usually corrected with reading glasses, and no other treatments -- including diet or exercise -- have been shown to increase or decrease its progress. Bifocal or trifocal lenses may be necessary for patients with presbyopia who also have other refractive errors like myopia, hyperopia, or astigmatism.