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Melanocytic nevus

Section: Skin and soft tissue Sub-section: Benign skin and soft tissue Curriculum: Curriculum, page 52

Definition

  • Groups of melanocytes leading to a pigmented skin lesion
    • Abnormal arrangement of normal cells
  • Natural History
    • Proliferation of melanocytes at dermal-epidermal junction
    • Melanocytes drop into dermis = Naevus
      • Then junctional activity cesses, leaving only dermal elements
      • These then can disappear due to fibrosis in dermis

Incidence

  • Occur from childhood
  • Very common
  • More common in fair-skinned people

Classification

Congenital Melanocytic Naevi

  • Classified according to size
  • Definition
    • Pigmented naevi present at birth
    • Nevus cells often involve lower dermis (amongst collagen) & even fat or muscle
  • Hamartoma
    • Superficial part similar to compound nevus
    • Deep part nests in sheaths of hair follicles, association with sebaceous glands & ducts
  • New York Uni Registry Classification
    • Based on projected size into adulthood
    • Small ≤ 1.5cm in greatest diameter
    • Medium: 1.5-19.9cm
      • Small & medium – small risk of melanoma – mostly in adult life
    • Large / Giant: ≥ 20cm
      • Assoc with increased melanoma risk & also associated with neurocutaneous melanosis (e.g. CNS melanoma)
  • Management

    • 20cm – excise as infant/child

    • < 10cm can observe

Dermoscopic classification

  • Dermatoscopy has given rise to a new classification based on the pigment patterns of melanocytic naevi. Examples include:
    • Reticular naevus
    • Globular naevus
    • Blue Naevi
    • Starburst naevus
    • Facial naevus
    • Acral naevus
    • Naevus with special features
      • Eczematised naevus (illustrated)
      • Irritated naevi
      • Halo Naevi
    • Unclassifiable naevus

Acquired melanocytic naevus

  • Solid brown naevus
  • Solid pink naevus
  • Eclipse naevus
  • Cockade naevus
  • Naevus with perifollicular hypopigmentation
  • Fried-egg naevus
  • Lentiginous naevus
  • Naevus with eccentric pigmentation

Uncommon types of melanocytic naevi

Atypical melanocytic naevus

  • Definition
    • Varies
      • A benign lesion that has some clinical or histopathological characteristics of melanoma
      • A melanocytic naevus with specific characteristics: large (> 5 mm); ill-defined or irregular borders; varying shades of colour; with flat and bumpy components
      • Or, any funny-looking naevus; large, or different from the patient’s other naevi. - Mole with unusual features
  • Cause
  • Dysplastic naevus
    • Best used for a naevus with a specific microscopic appearance
    • Many histologically dysplastic naevi are clinically banale (eg, small in size, and uniform in colour and in structure)
    • Histological dysplasia may be mild, moderate or severe
  • Importance of atypical naevi
    • if >5 then increased risk of melanoma
  • Treatment
    • Do not need removal
    • Remove if unclear if melanoma

Aetiology of Melanocytic Naevus

Pathology of Melanocytic Naevus

  • The pathological classification of melanocytic naevi relates to where naevus cells are found in the skin.
    • Lesions evolve with age
    • Initial lesion being macular with nests of proliferating melanocytes confined to the dermoepidermal junction
    • With time, nests extend into the dermis and lesions become elevated
    • With further maturation junctional activity, is ceased and the naevus becomes intradermal.

Junctional Naevi

  • Well-defined pigmented lesions
  • Appear in infancy / pre-pubertal
  • Flat / slightly elevated, brown, hairless
  • Most typically seen on palms, soles & genitalia
  • Proliferation of melanocytes at epidermal-dermal junction
  • Low malignant potential

Compound Naevi

  • Elevated, dome-shaped & brown ± hair
  • Adolescents
  • Melanocytes lie both in the epidermal-dermal junction & within the dermis
  • Low malignant potential

Intradermal Naevi

  • Dome-shaped (sometimes pedunculated)
  • Fleshy to brownish pigmented moles
  • Characteristically seen in adults
  • Frequently contain hairs
  • Micro:
    • Melanocytes are entirely within the dermis
    • Nests superficial, diffuse deeper
  • Very low malignant potential

Management

  • Should be excised if changes occur:
    • Itching
    • Inflammation
    • Change in colour
    • Halo formation
    • ↑ Size
    • Bleeding / ulceration
    • If congenital naevi require excision, it has to be down to fascia
      • Even then there is no guarantee that all the melanocytes have been removed

Prognosis

  • Small risk of progression to melanoma
    • True risk unknown ? 5%
  • Congenital nevus > 20cm (10% risk of melanoma)
  • If a melanoma is going to occur – 60% within first decade

Follow-up

  • Lifetime surveillance for congenital naevi > 20cm
  • Or if involves 2-5% of body area

Graph View

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