What is (are) epidermal nevus ?
An epidermal nevus (plural: nevi) is an abnormal, noncancerous (benign) patch of skin caused by an overgrowth of skin cells. Epidermal nevi are typically seen at birth or develop in early childhood. They can be flat, tan patches of skin or raised, velvety patches. As the affected individual ages, the nevus can become thicker and darker and develop a wart-like (verrucous) appearance. Often, epidermal nevi follow a pattern on the skin known as the lines of Blaschko. The lines of Blaschko, which are invisible on skin, are thought to follow the paths along which cells migrate as the skin develops before birth.  There are several types of epidermal nevi that are defined in part by the type of skin cell involved. The epidermis is the outermost layer of skin and is composed primarily of a specific cell type called a keratinocyte. One group of epidermal nevi, called keratinocytic or nonorganoid epidermal nevi, includes nevi that involve only keratinocytes. Other types of epidermal nevi involve additional types of epidermal cells, such as the cells that make up the hair follicles or the sebaceous glands (glands in the skin that produce a substance that protects the skin and hair). These nevi comprise a group called organoid epidermal nevi.  Some affected individuals have only an epidermal nevus and no other abnormalities. However, sometimes people with an epidermal nevus also have problems in other body systems, such as the brain, eyes, or bones. In these cases, the affected individual has a condition called an epidermal nevus syndrome. There are several different epidermal nevus syndromes characterized by the type of epidermal nevus involved.