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Banal common nevi evolve through a process from a junctional nevus to a compound nevus to a dermal nevus. Junctional nevi are the result of a single abnormal melanocyte that begins to proliferate at the dermal epidermal junction, and are often seen clinically as a dark flat nevus.
A junctional nevus will typically evolve over time into a compound nevus, which is a nevus with both epidermal and dermal melanocytes. Compound nevi become elevated and usually are more pale. Junctional nevi will typically grow to be 3 or 4 mm across, stop growing sideways, and begin to evolve into a compound nevus. This evolution continues through the compound nevus stage to the dermal nevus stage. In the dermal nevus all of the melanocytes are in the dermis. Dermal nevi are clinically manifested as pale soft wrinkled polyps on the skin. This sequence is the normal life pattern of a nevus. Dermal nevi can eventually drop off the skin.
Dysplastic nevi do not follow this pattern of maturation and self-destruction. Dysplastic nevi do not age properly. The junctional phase can be prolonged, so that the lesion grows beyond 6 mm. Very often, with increasing size, the lesion shows increasing variation in colour and edge. Variable colour, irregularly irregular within a dysplastic nevus, is indicative of genetic instability for the production of melanin. An irregular edge suggests a genetic instability in terms of lateral growth rate. Either are indicative of a tendency to progress to invasive melanoma.
Dysplastic nevi can be identified clinically as mild, moderate, or severe.
Mildly dysplastic nevi are nevi that show this tendency to become very much larger than a normal nevus yet there is some central maturation. This can produce a "fried egg" appearance to the nevus. Mildly dysplastic nevi do not show asymmetrical colour variation or irregularity or edge.
Moderately atypical nevi do not show the central maturation of the fried egg nevus. They tend to be larger than normal junctional nevi with some variation of colour and edge. Colour is often regularly irregular.
A severely dysplastic nevus is indistinguishable from early melanoma. Very often these lesions show considerable irregularity of edge and irregularly irregular colour. The differentiation between a moderate and severely dysplastic nevus is one of degree.
Mildly dysplastic nevi can be observed. Severely dysplastic nevi should certainly be surgically removed. Depending upon the degree of clinical abnormality, moderately atypical nevi can be followed closely if there are many, or surgically removed if there are only a few.