An atypical nevus is a benign melanocytic lesion that appears clinically atypical with asymmetry, large size, irregular or ill-defined borders, and/or varied coloration. A dysplastic nevus (also referred to as a Clark nevus) is a histopathologic term that describes a melanocytic proliferation showing architectural disorder and cytologic atypia and sharing some features of melanoma. Dysplastic nevi usually appear clinically atypical and atypical nevi may be dysplastic; however, the clinical and histopathologic terms do not always completely correlate.
There is controversy around the term dysplastic nevus, including a lack of consensus on how it is defined and what it represents biologically. Originally described in melanoma-prone families, dysplastic nevi were part of a clinical phenotype. Patients characteristically had numerous nevi, which often appeared clinically atypical; on biopsy (as initially described by Dr. Wallace Clark), 4 main histologic features were observed: 1) atypical melanocytic hyperplasia, 2) melanocytes with cytologic features characteristic of malignant melanocytes, 3) mesenchymal changes in the papillary dermis, and 4) lymphocytic infiltrate. Since then, dysplastic nevi have been recognized to occur spontaneously, and different variations of the above diagnostic criteria (including grading of cytologic atypia as mild, moderate, or severe) have been proposed without a clear diagnostic consensus.
Dysplastic nevi usually begin to appear in childhood through early adult years and have an estimated prevalence ranging from 2%-10%. They are thought to have a genetic component and are more common in phototype I-III individuals. While less prevalent than common nevi, dysplastic nevi are believed to correlate with the overall number of melanocytic lesions in an individual and are thought to confer a 4- to 15-fold increased risk of melanoma.
While there is an increased relative risk of melanoma in individuals with multiple dysplastic nevi and dysplastic nevi can share clinical and histologic features with melanoma, current evidence does not support a dysplastic nevus as a definite premalignant lesion. Studies have shown that most melanomas appear to arise de novo without an associated nevus, and even when an associated nevus is present in histologic contiguity with melanoma, it is more frequently a common nevus than a dysplastic nevus.
ICD10CM: D23.9 – Other benign neoplasm of skin, unspecified
SNOMEDCT: 254818000 – Atypical nevus of skin
Differential Diagnosis & Pitfalls
Seborrheic keratosis – Many may fulfill one or more of the ABCDE criteria; thus, awareness of typical features of these exceedingly common skin lesions will prevent unnecessary biopsies. Notably, they are a "stuck on-" or waxy-appearing tan-to-dark-brown papule.
Recurrent melanocytic nevus – History of the initial biopsy is often critical for the dermatologist and/or pathologist. Recurrent nevi characteristically occur in women aged 20-30 years on the trunk within 6 months of the original biopsy. While many different clinical morphologies may be seen, it often manifests as a scar with variegated hyper-or hypopigmentation, linear streaking, and halo, stippled, and/or diffuse pigmentation patterns. While most cases do not pose a diagnostic challenge on histopathology, some specimens, especially partial biopsies, may look indistinguishable from melanoma on histopathologic grounds alone.
Desmoplastic nevus – Typically small (< 1 cm) skin-colored to erythematous to mildly pigmented papule or nodule on the extremity of a young adult (average age 30 years). Histopathology reveals spindled to epithelioid melanocytes within a fibrotic stroma.
Spitz nevus – Solitary; pinkish, tan, or red-brown; smooth-surfaced; dome-shaped papule, often with surface telangiectasia.