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Psoriasis is a hereditary disorder of skin characterized by chronic
scaling papules and plaques in a characteristic distribution, largely
at sites of repeated minor trauma. The HLA types most frequently
associated with psoriasis are HLA-B13, -B17, -Bw16, -B37, -Cw6,
and DR7.
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It can be difficult to distinguish between atopic dermatitis
and psoriasis in infancy. If family history and cutaneous findings
are not helpful, one hint is atopic dermatitis usually spares the
diaper area and psoriasis favors that location.
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Age 10% have onset of
lesions before age 10, and 37% have onset before age 20.
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Prevalence 2% of the world’s
population. United States and Canada: 4% to 5% of
the population.
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Race Low incidence in Asians, Africans,
African Americans, American Indians, and Japanese.
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Other Features Multifactorial inheritance.
Minor trauma is a major factor (45% of patients) in eliciting
lesions (Koebner’s phenomenon). Infection (particularly
streptococcal) also plays a role. Many episodes of psoriasis follow
sore throats or upper respiratory infections. Stress, cold weather,
hypocalcemia, and lack of sunlight exposure aggravate the condition.
Certain drugs (lithium, interferon, β-blockers, alcohol, antimalarials,
corticosteroid withdrawal) can also precipitate psoriasis.
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Psoriasis is likely a polygenic disease caused by the inappropriate
activation of T cells (the adaptive immune system) as well as abnormal
keratinocyte proliferation (the innate immune system).
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Onset of Lesions Usually, months
but may be sudden as in acute guttate psoriasis and generalized
pustular psoriasis (von Zumbusch).
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Skin Symptoms Pruritus is reasonably
common, especially in scalp and anogenital psoriasis.
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Constitutional Symptoms In 5% of
cases, psoriasis can be associated with arthritis, fever, and/or
an “acute illness” syndrome (weakness, chills,
fever) with generalized erythroderma.
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Type Well-delineated, erythematous,
thickened plaques with a characteristic silvery-white scale (Fig. 4-1A).
Removal of scale results in the appearance of miniscule blood droplets
(Auspitz sign).
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Color Salmon pink to red.
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