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Skin color is genetically determined and is caused by the total
amount of melanin pigment in the skin. Normal constitutive melanin
pigmentation determines skin type, which is currently classified
by the Fitzpatrick phototypes as follows:
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Disorders of hypopigmentation are caused by decreased melanin
content in the skin owing to decreased or absent melanin production
or melanocytes. Disorders of hyperpigmentation are caused by increased
melanin content in the skin owing to an increase in melanin production
or melanocytes.
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Decreased or absent melanin in the skin can lead to hypomelanosis
and can occur by two main mechanisms.
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1. Melanocytopenic hypomelanosis: absent or decreased number
of melanocytes (e.g., vitiligo).
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2. Melanopenic hypomelanosis: absent or decreased melanin production,
but normal number of melanocytes (e.g., oculocutaneous albinism).
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Pityriasis alba is a common asymptomatic, sometimes scaly, hypopigmentation
of the face, neck, and body.
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Pityriasis alba entails both mild dermatitis and pigment alteration;
generally, the former responds quickly while the latter continues
to be an issue for many months or years.
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Age Young children, often between
the ages of 3 and 16 years.
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Race All races, more noticeable
in darker skin types.
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Etiology Likely a form of atopic
dermatitis.
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Thought to be an eczematous dermatosis, with hypomelanosis resulting
from postinflammatory changes and ultraviolet screening effects
of the hyperkeratotic and parakeratotic epidermis.
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Hypopigmented areas are usually stable then gradually disappear
with age. Some lesions may persist into adulthood. The areas are
typically asymptomatic, but can sometimes burn or itch.
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Type Macules, may have slight scale.
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Number One to twenty lesions may
be present.
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Color Pink, then off-white to tan-white.
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Size and Shape 5 to 30 mm or larger.
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Distribution Face (malar region),
neck, trunk, extremities.
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Sites of Predilection Face, especially
the cheeks (Fig. 12-1), midforehead, and around the eyes and mouth.
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