vitiligo A condition in which the skin turns white due to the loss of pigment from the melanocytes, cells that produce the pigment melanin that gives the skin color. In vitiligo, the melanocytes are destroyed, leaving depigmented patches of skin. The hair that grows in areas affected by vitiligo may also turn white. The skin…
vitelline duct See yolk stalk.
vitamin therapy The use of vitamins to prevent or cure disease, often as a complementary therapy to accompany medication or other treatments. One variant on this theme, megavitamin therapy, is still rather controversial. It is important to consult a physician before adding vitamin supplements to a health regimen. See also Appendix C, “Vitamins.”
vitamin An organic substance that naturally occurs in plants or animal tissue that is essential for normal metabolism of the body and to life. Vitamins play a part in dozens of crucial activities in the body: Some are antioxidants, preventing oxidation of cells and potentially preventing cancer; others permit or deny chemical reactions involved in…
vital Necessary to maintain life. For example, breathing is a vital function.
visual nerve pathways See optic nerve pathways.
visual nerve See optic nerve.
visual field test A test that measures the extent and distribution of the field of vision. A visual field test may be done via a number of methods, including termed confrontation, tangent screen exam, and automated perimetry. These tests are not painful or uncomfortable. Many diseases can adversely affect the visual field, including glaucoma, strokes,…
visual acuity test See eye chart test.
visual acuity The clarity or clearness of vision, a measure of how well a person sees.
vision therapy The use of special eye exercises to address eye defects, such as strabismus. Some vision therapists claim that eye exercises can help people with neurological or learning disabilities. Vision therapy is not proven for the latter use, although some patients do report improvement.
vision, macular See central vision.