Poliosis is a condition in which there is a patch of white or gray hair in a person who has otherwise maintained their normal hair color. It is caused by a decrease or absence of the pigment melanin in hair follicles, leading to patches of white in the hair, eyebrows, eyelashes, or any other hairy area. Less commonly, the condition may cause multiple white patches in the hair. Some mistake these white patches for simple birthmarks. It is also known as poliosis circumscripta.
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.