Classification

• Melanocytic nevus (nevomelanocytic nevus, nevocellular nevus): benign proliferation of melanocytes, the skin cells that make the brown pigment melanin. Hence, most nevi are brown to black. They are very common; almost all adults have at least one, usually more. They may be congenital or acquired, usually at puberty. The melanocytic nevi are classified as such:
  
  • Junctional nevus: the nevus cells are located along the junction of the epithelium and the underlying dermis. A junctional nevus is flat and brown to black.
  • Compound nevus: a mixture of junctional and intradermal proliferation. Compound nevi are slightly raised and brown to black.
  • Intradermal nevus: the nevus cells are located in the dermis only. Intradermal nevi are raised; most are flesh-colored (not pigmented).
  • Dysplastic nevus (nevus of Clark): usually a compound nevus with cellular and architectural dysplasia. They tend to be larger (more than 6 mm), with irregular borders and irregular coloration. Hence, they ressemble melanoma, appear worrisome and are often removed to clarify the diagnosis. Dysplastic nevi don't transform into melanoma but are a marker of risk when they're numerous (atypical mole syndrome).
  • Blue nevus: the nevus cells are very deep in the dermis.
  • Spitz nevus: a distinct variant of intradermal nevus, usually in a child. They are raised and reddish (non-pigmented). A pigmented variant, called the nevus of Reed, typically appears on the leg of young women.
  • Giant Hairy Nevus: these large, pigmented, often hairy congenital nevi can be a source of much psychological and social suffering. They are also important because melanoma may occasionally (10 to 15%) appear in them.
  • Intramucosal nevus: junctional nevus of the mucosa of the mouth or genital areas. In the mouth, they're found most frequently on the hard palate.
  • Nevus of Ito and Nevus of Ota: congenital, flat brownish lesions on the face or shoulder.
  • Mongolian spot: congenital large, deep, bluish discoloration on the back of Asian babies.
• Epidermal lesions:
  
  • Epidermal nevus: congenital, flesh-colored, raised or warty, often linear lesion, usually on the upper half of the body.
  • Nevus sebaceus: variant of epidermal nevus on the scalp presenting as a hairless, fleshy or yellowish area.
• Connective tissue lesions:
  
  • Connective tissue nevus: fleshy, deep nodules. Rare.
• Vascular lesions. See birthmark for a more complete discussion:
  • Hemangioma (strawberry mark or nevus).
  • Nevus flammeus (port-wine stain).
  • Spider angioma (nevus araneus).
  • Blue Rubber Bleb Nevus Syndrome: dilatation of veins anywhere in the body (both skin and internal organs), usually lethal from internal hemorrhage. It is a very rare genetic disorder.

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