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Diseases of the skin vary from inflammatory disorders to highly malignant neoplasms. This chapter will discuss disorders of pigmentation, melanocytic proliferations (both benign and malignant), other pigmented lesions, non–melanocytic preneoplastic and neoplastic disorders, inherited skin neoplasia syndromes, acute and chronic inflammatory dermatoses, bullous diseases, other inflammatory skin diseases (including erythema nodosum), infectious disorders, and cutaneous manifestations of systemic disorders.

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Overview: The three common disorders of pigmentation discussed below are vitiligo, freckles, and lentigo. These disorders represent predominantly cosmetic problems, but rarely may be mistaken for a premalignant lesion.

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Vitiligo

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Morphology

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  • Gross: Macules or patches of skin with loss of pigment.
  • Areas affected: Hands, wrists, and perioral and anogenital regions.

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Pathogenesis of vitiligo: Partial or complete loss of melanocytes, possibly of autoimmune etiology.

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Important points

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  • Vitiligo versus albinism: Albinism is lack of melanin pigment due to genetic deficiency of tyrosinase.
  • Vitiligo is associated with other autoimmune diseases, and is often seen in patients with Hashimoto thyroiditis, type 1 diabetes mellitus, or Addison disease.

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Freckles

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Basic description: Hyperpigmented lesions occurring as a result of an increased amount of melanin pigment.

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Important point: A freckle darkens with sunlight.

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Lentigo

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Basic description: Hyperpigmented lesions occurring as a result of an increased number of melanocytes along the basement membrane.

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Important point: A lentigo does not darken with sunlight.

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Overview: Melanocytic proliferations vary from the benign melanocytic nevus to the highly aggressive malignant melanoma.

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Melanocytic Nevus

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Basic description: Benign proliferation of nevus cells, which are derived from melanocytes.

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Types of melanocytic nevus

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  • Junctional nevus: Proliferation of nevus cells confined to the basal portion of the epidermis.
  • Compound nevus: Proliferation of nevus cells at the basal portion of the epidermis and upper dermis.
  • Intradermal nevus: Proliferation of nevus cells confined to the dermis (Figure 20-1).

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Figure 20-1.
Graphic Jump Location

Intradermal nevus. Note the proliferation of nevus cells within the dermis only, surrounding the skin appendages. As the nevus cells penetrate deeper into the dermis, they acquire a more “neural” appearance (arrow). Hematoxylin and eosin, 100×.

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Microscopic morphology of melanocytic nevus: Nests of uniform round cells with inconspicuous nucleoli and few if any mitotic figures. As cells get deeper into the dermis, they acquire more of a neural appearance as a result of maturation of the cells.

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Dysplastic Nevus

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Basic description: Proliferation of dysplastic nevus cells; dysplastic nevi are precursors of malignant melanoma.

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Morphology of dysplastic nevus

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  • Gross: Variable pigmentation; > 5 mm in size; irregular borders.
  • Microscopic: Fusion and coalescence of nests of ...

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