Blue nevus as described by Tieche, to which little can be added, is a dermal-based, benign melanocytic lesion histopathologically made up by variable proportions of oval/spindle and bipolar, usually heavily pigmented dendritic cells. The aggregation of oval/spindle melanocytes with pale cytoplasm into discrete expansile nodules features a “cellular” blue nevus.
The cell components of blue nevus consist of arrested embryonal melanocytes migrating from the neural crest into the epidermis during embryonic development.
Immunohistochemically, they usually express melanoma-associated antigen HMB45, together with S100 protein and Melan A/Mart-1. These cells are the sole or the main components of several other melanocytic lesions named “dermal dendritic melanocytic proliferations”.
Three categories of dermal dendritic melanocytic proliferations have been identified in the classical dermatopathology literature:
- hamartomatous dermal melanocytoses (which include Mongolian spot, nevus of Ota, and nevus of Ito);
- classic and cellular blue nevus; and
- malignant blue nevus.
The common blue nevus constitutes a clinical and morphological continuum with the so called “cellular” blue nevus. The dichotomy, common versus cellular is entrenched in literature, but ignores the great variety of appearances that blue nevi have.
Forms of Blue Nevus and Associated Clinical Features
Many clinical forms of blue nevus reported in the literature include:
- Jadassohn-Tièche blue nevus
Jadassohn-Tièche blue nevus, the most frequently encountered clinical form, consists of a small, glabrous, painless, blue-black papule, which occurs on the dorsal surfaces of the extremities and on the head and neck of young, female patients with dark complexions.
Blue nevus usually develops in adolescents or in young adults, but may also appear in middle-aged individuals. Half of the lesion occurs on, or near, the dorsa of the hands and feet.
The clinical diagnosis is, as a rule, immediately obvious and correctly made. Basically, blue nevus remains static throughout life but may undergo fibrosis with subsequent flattening, lightening of color and gradual involution.
- Congenital form
Blue nevus can be congenital and in this case is usually found on the scalp; though other sites are also possible (thorax, calf). It is a large lesion and may massively occupy the deep dermis and subcutaneous fatty tissue.
The lesion remains stable until adolescence, then gradually enlarges and keeps growing. Evolution into melanoma has been described in congenital scalp lesions.
- Eruptive (or agminated or multinodular plaque-type) blue nevus
These forms of blue nevus consist of multiple nevi grouped together. These nevi appear as bluish macules and nodules in a circumscribed area, which is either normal skin or a pigmented macule (blue nevus on nevus spilus); this peculiar form of blue nevus is usually present at birth but can also appear in young adults.
Agminated blue nevus has been described in association with cutaneous or visceral alterations. A combination of agminated-type blue nevus of the face with melanoma of the chorid has been reported.
- Patch-type and plaque-type blue nevus
The patch type blue nevus consists of a close aggregation of non-palpable circumscribed macules that historically correspond to dermal melanocytosis.
The plaque type consists of an aggregation of macules and palpable papules grouped together in a single lesion, varying in diameter from millimeters to several centimeters.
Sometimes the lesion may be very large. The plaque type is often congenital (but can also appear in adult life) and may involve deep dermis, subcutis, fascia and striated muscle.
Lesions vary histologically from dermal melanocytosis to common blue nevus. These variants are mainly localized on the trunk (but can be found elsewhere). The distinction between this variant and the above mentioned eruptive blue nevus is sometimes arbitrary.
- Halo blue nevus
This lesion is clinically a blue nevus surrounded by a whitish halo. The halo histologically corresponds to a dense dermal lymphocytic infiltrate that also involves the melanocytic lesion.
- Linear blue nevus
This lesion consitst of a series of small blue nevi linearly arranged and located on one side of the body. Sometime these lesions show a zoosteriform distribution or may be located along the length of a nerve.
Occasionally, linear blue nevi appear after a trauma or sunburn. Linear blue nevus commonly occurs in young adults (congenital forms are also possible). Lesions vary histologically from sparse dermal melanocytosis to blue nevus.
- Multiple blue nevus
A form of multiple blue nevi disseminated throughout the body has been described. This multiple blue nevus can be isolated or associated with Carney complex Opens in new window. Familiar cases are also on record.
- Target blue nevus
This lesion consists of a blue nevus with a central macule surrounded by a whitish halo, further surrounded by an outer blue ring. Histologically, the halo is composed of fibrous tissue with only a sparse cellular infiltrate.
- Combined blue nevus
Blue nevus can be present occasionally inside or in combination with other forms of nevus. Although many scenarios are possible, the most frequent situation is that of a common nevus where a black or bluish macule or nodule appears, even after decades of stability.
The sudden appearance of this blue spot inside a previously stable lesion is always of great concern for the patient and the dermatologist as well, and the lesion is frequently excised.
- Hypopigmented blue nevus
This entity is clinically grey-white, skin-colored or pink and is usually confused with a neurofibroma, a scar or other skin neoplasms.
The patient’s age and site of the lesion are similar to the conventional pigmented form of blue nevus. it occurs predominantly in females.
- Blue nevus in association with other anomalies
The association of blue nevus of the face or the scalp and leptomeningeal melanosis has been described with or without intracranial pigmented tumors.
- Mucosal blue nevus
Blue nevus can be found in oral and genital mucosae, on the sclera, the conjucntiva, the uterine cervix, the prostate, the nail bed or nail apparatus and in lymph nodes.
- Epithelioid blue nevus
In the clinical complex that bears his name, Carney recently described a peculiar form of blue nevus. lesions are multiple in almost half of the cases and have a familiar occurrence in one third of the patients.
In the Carney complex Opens in new window, blue nevi are associated with mucosal lentigines and cutaneous and atrial myxomas, among other signs.
The blue nevi were said to be clinically typical but histopathologically composed of epithelioid cells. The epithelioid blue nevus has been reported also in patients without Carney syndrome.
Histologically, the common blue nevus contains elongated, deeply pigmented, bipolar melanocytes whose size is larger than epidermal junctional melanocytes.
These cells resemble the melanocytes of hair follicle bulbs in dark-haired subjects: in fact, they have an oval cell body and long thread-like cytoplasmatic dendrites originating from the two poles.
The cells are engorged with melanin pigment, which is often so abundant that the fine cellular details are blurred.
In the early evolutionary phase, the lesion is composed only of the above mentioned cells and appears to be situated entirely within the dermis: in blue nevus, a significant junctional component is lacking.
A maturation gradient is absent in blue nevus and therefore all cells within the lesion are similar, and no “zoning” can be appreciated: this is probably because the dendritic cells of blue nevus represent, per se, a mature cellular phenotype.
The dendritic cells are often arranged almost in parallel to the epidermis except around the cutaneous appendages where they may follow the contour of the lining follicular or glandular epithelium.
Mitotic figures are exceedingly rare. There is no significant connective tissue reaction around or in the lesion and the neoplastic growth does not seem to alter the normal dermal architecture.
The histological pattern of cellular blue nevus is defined as spindle/fascicular. It is composed by dendritic melanocytes together with islands of epithelioid and plump spindle cells with abundant pale cytoplasm and usually little pigment.
Melanophages are found between the cellular islands. The tumor often bulges into the subcutaneous fat as a nodular downgrowth with a typical clapper-like silhouette. Stromal desmoplasia and balloon-cell changes are rare occurrences.
A peculiar lesion which places somewhat in between combined cellular blue and Spitz (Blitz) nevus and epithelioid blue nevus is deep penetrating nevus.
It is a dermal V-shaped lesion that bulges into the subcutis; typically dendritic and spindle melanocytes are its main components, with some interspersed epithelioid (spitzoid) cells whose morphological hallmark is a finely vacuolated (sebocyte-like) cytoplasm. The dermoscopic appearance of this lesion is often “polychromous”.
Neither surgical procedures nor further clinical controls are needed for most cases of blue nevi, because their clinical recognition is obvious. In the presence of atypical features (e.g., recent onset and/or recent changes, peripheral halo of inflammation, unusual dermoscopic features) surgical excision is mandatory.
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