Definition What exactly is a nevus? According to the authoritative definition of the World Health Organization, a nevus is a localized proliferation of melanocytes and belongs to the category of benign melanocytic tumors. Nevi are divided into congenital nevi and acquired nevi. Nevi that exist at birth are called congenital nevi or fetal nevi, which are the most common type of nevus in clinic, and are called congenital nevocytic nevi (CNN, congenital nevus of nevus) or congenital melanocytic nevi (CMN, congenital melanocytic nevus), which are coded as ICD-O code 87 by the International Classification of Tumors (ICT). The International Classification of Tumors gives it the ICD-O code: 8761/0. Incidence Congenital nevi are equally prevalent in males and females and occur in all races. Statistics from Western countries show that nevi have a prevalence of 1% in newborns and a higher prevalence of 1.8% in blacks. A nevus larger than 20 centimeters in size is called a giant congenital nevus, Giant congenital nevocytic nevus, and has an incidence of 1 in 20,000 Classification If nevi are classified according to their size, they can be divided into small, medium-sized and giant nevi, and their detailed characteristics are shown in the table below. Size: Characteristics Small nevus (area <1.5 cm2), yellowish brown to brown, irregularly shaped macular or papular lesions containing small dots of mottled color. The color deepens after puberty. It may protrude from the skin and develop hair. Medium-sized nevus (between 1.5 cm2 and 20 cm2), similar to a small mole. After birth, congenital nevus cell nevi tend to grow in proportion to the size of the body. Giant nevi (>20 cm2) are dark, hairy, and have wart-like structures. There are often surrounding foci of satellite nevi. Skin lesions have a swimsuit, sleeve or cuff-like appearance or distribution. The lesions may also infiltrate the soft meninges inside the skull and present with neurologic involvement such as seizures. Patients with giant nevi should undergo imaging to rule out central nervous system disease. Nevi that develop on the surface of the spine may be associated with spina bifida or spinal cord bulge. It may also be associated with neurofibromatosis. Common acquired nevus Common acquired nevus is present after early childhood and may be brown, tan, or skin colored and round or ovoid in shape. Pathologic Examination Congenital nevi are usually nevus cells distributed between deep dermal fiber bundles that may invade skin appendages, blood vessels, and nerves, whereas nevus cells in acquired nevi are often confined to capillaries and the upper portion of the dermal reticulum and do not invade skin appendages. The histopathology of a giant nevus is similar to that of a small or medium-sized nevus, but it may infiltrate into muscle, bone, dura mater and skull. Risk of malignancy There are many opinions about the risk of malignancy in nevi, which are inconclusive and in some cases controversial. It is generally accepted that small congenital nevi have a 1-5% lifetime chance of becoming melanomas. The risk of a medium-sized nevus becoming a melanoma is uncertain, and small and medium-sized nevi rarely become malignant before puberty. Small and medium-sized nevi are rarely malignant before puberty. 5-10% of giant nevi will become malignant, and 50% of patients with malignant nevi develop between the ages of 3 and 5 years. The prognosis is very poor if the nevus becomes malignant. Clinical manifestations of malignant changes It is necessary to come to the outpatient clinic if an otherwise calm nevus shows the following manifestations: rapid growth; irregular edges; asymmetrical shape; change in color of the nevus; appearance of satellite foci next to the nevus; change in texture of the nevus. Other skin diseases that can be confused with congenital melanocytic nevi: 1, Epidermal nevus (Epidermal nevus), also known as Verrucous nevus (Verrucous nevus), is present at birth or develops early in early childhood, yellowish-brown or brown patches or pimples, linear in distribution with no plaques or hair; it is most commonly found on the limbs. 2.Nevus sebaceus (sebaceous nevus) Present at birth, manifesting as a single yellow-orange, waxy plaque; often seen on the scalp. It has the possibility of transforming into basal cell carcinoma (10-15%) and excision is recommended. Atypical (dysplastic) nevus (不atypical(发育不良)痣) Appears at puberty or later, usually more than brown in color, irregular in shape, usually larger than 6mm, most common on the trunk. It has the possibility of malignant transformation and requires vigilance. 4.Becker’s nevus (Becker’s nevus) Adolescent onset, lesions are commonly found on one side of the shoulder in males, the color ranges from uniformly yellow-brown to dark brown, the border is usually irregular, there will be hair, the average size of 3125px2. 5.Nevus spilus (Nevus spilus) Usually acquired, but there are also some that are congenital. Appearance of yellowish brown spots, diameter 1-100px, mixed with dark brown papules or spots, diameter in 1-6mm. 6, Halo nevus (Halo nevus) is usually seen in patients younger than 20 years old, the appearance of a white halo around the nevus (which is a lymphocyte reaction), the most common in the upper back of young people in their teens. 7, Spitz nevus (Spitz nevus) usually exists at birth, also called juvenile melanoma, red or black, smooth surface raised papule-like, common capillary dilatation, the average diameter of the head and neck is most common, it is recommended to remove it early for easy diagnosis and treatment. 8.Blue nevus (Blue nevus) often appears in late adolescence, usually less than 25px, tight structure, dark blue to gray-black, clear edge of the round papules, most common on the back of the hand or foot, head and neck. 9.Mongolian spot (Mongolian spot) steel blue spot, born or appear in the lumbosacral region within a few weeks after birth, size from a few centimeters to 20 centimeters or larger, in the dark-skinned newborns are more common, and often disappeared in early childhood. 10.Nevus of Ota (Nevus of Ota) present at birth, or within 1 year after birth, or in adolescence; particularly common in Asians and blacks; blue-brown unilateral periocular spots; size can range from a few centimeters to cover half of the face; distribution is often along the innervation area of the first two branches of the trigeminal nerve. 11, Nevus of Ito (Nevus of Ito shoulder Mongolian spots) (Nevus of Ito) usually appear at birth; common in Asians and blacks; manifested as a large blue-brown spots, located in the back of the shoulder, the posterior branch of the supraclavicular nerve and the area innervated by the lateral epiglottic nerve of the arm. 12.Café au lait spot (Café au lait spot) Can be present at birth, but usually develops during childhood; well defined, uniform color of milk-coffee mixture, ovoid, completely blotchy. It is often associated with neurofibromatosis.