General Medical Information

Physiologically Decreased Vision

Jean Edwards Holt

Refractive Error

A common cause of poor vision is refractive error or change in refractive error. Patients with myopia (nearsightedness) have an eye that is too long for its refractive system. This typically young patient reports not being able to see the blackboard and having to sit in the front of the schoolroom. Simple prescription of concave lenses usually restores visual acuity to normal. Persons with hyperopia, or farsightedness, have an eye that is too short and need simple convex lenses to bring near objects into focus. Aphakia is a special form of hyperopia in which the refractive power of the eye is too weak because of removal of the lens. Astigmatism is nonspheric curvature of the cornea and is extremely common with any refractive error.

Refractive visual surgery, also known as refractive or radial keratotomy, is becoming increasingly common among patients with myopia. Patients seeking elective reduction of physiologic myopia usually are treated with a protocol of therapy that involves performing radial incisions in the cornea to alter its curvature. Newer technology such as manufacture of finer, thinner blades, more accurate blade placement, and computed topography of the corneal surface, have helped to reduce complications and produce more consistent results, including a decreased trend toward a hyperopic drift with time. Some patients need additional incisions, called enhancements.

Presbyopia

Presbyopia is the term used to describe the clinical need for reading glasses or bifocals as the patient enters the fourth and fifth decade of life. The crystalline lens hardens with age and becomes less elastic, decreasing ability to accommodate or focus for near vision. This is a normal physiologic mechanism and should not be considered a sign of disease.