Blue nevus (BN) is a benign dermal melanocytic proliferation arising from neural-crest melanocytes that likely became arrested in migration during embryogenesis. Also known as "blue neuronevus", "dermal melanocytoma", and "nevus bleu", it usually first appears in adolescence and young adulthood. The blue colour is caused by the melanin pigment being deeper in the skin than in ordinary nevi.
Diffuse melanocytosis are clinical variants of blue nevi characterized by the development of blue diffuse pigmentation, many years after birth, on the face (Ota nevus), on the trunk or arms (Ito nevus), on the sacrum area (Mongolian spot).
Common BN present as a solitary small, well-defined, round to oval shaped, flat to slightly-elevated papule with a predominant steel blue color (spectrum gray-blue-black). They are most often located on the distal extensor aspects of limbs, followed by the face and scalp, and the sacral region. Other less common locations include genitalia, oral cavity, and lymph nodes (Zembowicz 2017). The histopathologic features seen in common BN are highly pigmented bipolar spindle cells parallel to the epidermis intermingled with bundles of collagen in the mid to deep dermis, sometimes even extending to the hypodermis. Typically, there is a grenz zone at the dermoepidermal junction, which remains unaffected. When faced with blue lesions, clinical history remains of great importance. Blue nevi lesions arising in childhood and with no history of change are highly suggestive of common BN. In contrast, a new onset and fast-growing lesion in a middle-aged adult, or in the context of a previous melanoma, should raise concern for the possibility of melanoma.
The most characteristic pattern of BN is the absence of a pigment network, with homogeneous and structureless blue pigmentation or a combination of two colors: white-blue, steel blue-gray, blue-brown or blue-black. The pigment is confluent throughout the lesion and fades toward the periphery (figures 12). Multichromatic pigmentation has also been described, always including blue in the spectrum (figure 4) (Di Cesare et al. 2012). With the use of polarized light dermoscopes additional structures are seen: bluish globules/dots, network-like structures or peripheral out-of-focus streaks corresponding to pigmented projections of the nevus cells or melanophages, both deep within the dermis.
A combined nevus with a blue nevus component and a dermal nevus component shows a characteristic image with brown homogeneous pigmentation associated with blue homogeneous pigmentation in the same lesion. In blue and combined nevus streaks at the periphery can be seen by the presence of radially oriented bundles of melanocytic cells or melanin-laden macrophages. In contrast with the streaks at the periphery of melanoma or Spitzoid/Reed type melanocytic lesions in blue nevus these structures show a slate-blue pigmentation and are out of focus due to their depth.
In the nevus of Ota and nevus of Ito a diffuse irregular blue-brownish pigmentation is the main dermoscopic feature.
Dermoscopic Differential diagnosis of blue nevus
- Some nodular melanomas and malignant blue nevus can clinically and dermoscopically resemble a blue nevus. In the case of the malignant tumours the diameter is usually larger due to their fast growth.
- A subtype of melanoma epidermotropic metastasis are undistinguishable of blue nevus due to the pigmentation at the dermal level. In this malignant condition the clinical history of previous melanoma and the presentation of multiple new blue lesions are relevant for the differential diagnosis.
- Tattoos used in radiotherapy and by cosmetics reasons exhibit a blue coloration that in dermoscopy is composed by blue dots that can be distinguished of the blue homogeneous pigmentation of blue nevi.
- Vascular lesions exhibit a typical dermoscopy pattern with red, blue or purple black clods without vascular features. However, some vascular lesions can exhibit a blue homogeneous pattern.
- Finally, some rare pigmented basal cell carcinomas with large blue ovoid nests may mimic a bleu nevus. Usually in these lesions multiple clods and other dermoscopic features characteristic of basal cell carcinoma such as branched vessels are present
Blue nevi are benign lesions and no treatment or follow-up is needed. The history of recent change or the evidence of increase in size or change in structure demonstrated by digital follow up is the clue for not missing melanoma blue nevus like.
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