Understanding Vision Problems -- the Basics
Light rays entering the eye first cross the clear cornea. Surprisingly, nearly two-thirds of the eye's focusing power occurs along its front surface (tear film or cornea). The normal cornea should have a semi-spherical contour similar to a soup spoon. This permits the eye to create a single focused image. If the central cornea is not symmetrical or uniform we say it is "astigmatic."
Astigmatism, often combined with nearsightedness or farsightedness, occurs when the clear cornea has a non-round curvature -- more like a teaspoon or football. Because of that, the eye lacks a single point of focus. People with astigmatism may have a random, inconsistent vision pattern, wherein some objects appear clear and others blurry. The next time you hold some shiny silverware, compare your reflection in a soup spoon to that produced by the teaspoon -- that's astigmatism! Astigmatism is usually present from birth but is sometimes not recognized until later in life. Most astigmatism is fully correctable. Also, it changes very little over time.
Near vision requires focusing or accommodation. The amount of near focusing power decreases throughout life. Presbyopia is blurred vision at a normal reading distance in someone with otherwise normal distance vision (with or without gasses). It occurs when the eye develops insufficient focusing power for reading and other near tasks. Presbyopia typically starts at about age 40 and is the reason most older adults rely on reading glasses. Bifocal spectacles permit the wearer to see objects clearly both near and distant.
Visible light rays form images that reach the brain. In order to do that, the retina converts the light signal into a nerve impulse. Think of the retina as silky wallpaper that lines the inside of the eyeball. Unlike wallpaper, however, there is no glue. Small holes can develop in areas where the retina is exceptionally thin or damaged. If that happens, the clear liquid vitreous that fills the eye can seep behind the retina and cause the wallpaper to come off. This is retinal detachment.
Although a detached retina is not painful, it is an urgent medical situation. If the retina is not reattached to the eye wall promptly, retinal cells starve and permanent blindness can result. Risk factors for retinal detachment include the following:
- Moderate or extreme nearsightedness
- Previous eye surgery or injury
- Previous retinal detachment
- Inherited thinness of retinal tissue