White patch of hair birthmark, also known as poliosis, is a condition that causes a patch of hair to grow white or gray. It is caused by a lack of melanin, the pigment that gives hair its color. Poliosis can be congenital, meaning it is present at birth, or it can develop later in life. It can affect any part of the body, but it is most commonly seen on the scalp, eyebrows, or eyelashes.
Poliosis is usually a harmless condition, but it can sometimes be a sign of an underlying medical condition, such as vitiligo or alopecia areata. Treatment for poliosis is not typically necessary, but there are some cosmetic treatments available to help disguise the white patch of hair.