NIGHT BLINDNESS

also called Nyctalopia, failure of the eye to adapt promptly from light to darkness and reduced ability to see in dim light or at night. It occurs as a symptom of numerous diseases that cause degeneration of the rods of the retina (the sensory cells responsible for vision in dim light); as an inherited deficiency in visual purple, or rhodopsin, which is the pigment of the rods; or as a result of vitamin A deficiency. Congenital night blindness with or without myopia (nearsightedness) occurs either as a dominant, recessive, or sex-linked hereditary trait and usually remains stable throughout life. Night blindness developing during childhood or adolescence may be an early sign of retinitis pigmentosa, in which continued deterioration of vision often leads to blindness. Vitamin A deficiency causes night blindness that is usually not severe, and vision recovers when adequate levels of the vitamin are administered. Night blindness also occurs in the light-sensitive condition known as xerophthalmia but is treatable with vitamin A.