Sean Klepper M.D.
Artur Zembowicz M.D....
- Nevi that are present at birth, with histologic features that are distinctive from those of acquired nevi
- Often multiple
- Classified into three categories by size: small (less than 1.5 cm), medium (1.5 to 20 cm) and large (over 20 cm)
- Small and medium-sized congenital nevi have a low risk of malignant transformation, whereas large congenital nevi have a substantial risk.
- They are typically flat and light brown at birth, and over time may increase in pigmentation and thickness, sometimes becoming wart like, and may become increasingly hairy.
- There is often hyperkeratosis, acanthosis and papillomatosis.
- Congenital nevi tend to extend much deeper into the dermis than acquired nevi, and sometimes extend into the subcutaneous fat along the fibrous septa.
- The melanocytic infiltrate tends to be diffuse, with much less tendency to nest than in acquired nevi, except in the junctional and upper papillary dermal components.
- There is often a grenz zone between the epidermis and the dermal nevus cells.
- Nevus cells surround and often involve the skin appendages.
- Perineural involvement is common, and nevus cells often involve the walls of blood vessels. These should not be considered features of malignancy if the typical features of a congenital nevus are present.
- The fibrous stroma seen in acquired nevi is typically not present.
- Pigmentation tends to be most pronounced in the upper aspects of the nevus.
- Occasional mitoses may be seen in the papillary dermis.
- In the deeper aspects of the nevus, single cell and Indian-file patterns are often seen.