Psoriasis is a common chronic inflammatory dermatosis, affecting 1% to 2% of individuals residing in the United States. Recent epidemiologic studies have shown that psoriasis is associated with an increased risk for heart attack and stroke, a relationship that may be related to a chronic inflammatory state. Psoriasis also is associated in up to 10% of patients with arthritis, which in some cases may be severe.
Psoriasis is a T cell-mediated inflammatory disease, presumed to be autoimmune in origin, although the antigens are not well described. Both genetic (HLA types and other susceptibility loci) and environmental factors contribute to the risk. It is unclear whether the inciting antigens are self-antigens, environmental antigens, or some combination of the two. Sensitized populations of T cells home to the dermis, including CD4+ TH17 and TH1 cells and CD8+ T cells, and accumulate in the epidermis. These cells secrete cytokines and growth factors that induce keratinocyte hyperproliferation, resulting in the characteristic lesions. Psoriatic lesions can be induced in susceptible individuals by local trauma (Koebner phenomenon), which may induce a local inflammatory response that promotes lesion development. Genome-wide association studies (GWAS) have linked an increased risk for psoriasis to polymorphisms in HLA loci and genes affecting antigen presentation, TNF signaling, and skin barrier function. Several loci also are associated with the development of psoriatic arthritis, a more severe complication of this disease.
MORPHOLOGYThetypicallesionisawell-demarcated, pink to salmon– colored plaque covered by loosely adherent silver-white scale (
Fig.24.3A ).Thereismarkedepidermalthickening(acanthosis), withregulardownwardelongationofthereteridges(see
Fig.24.3B ).Thepatternofthisdownwardgrowthhasbeenlikenedto“testtubesinarack.”Increasedepidermalcellturnoverandlackofmaturationresultsinloss of the stratum granulosum and extensive parakeratotic scale. Alsoseenisthinningoftheepidermalcelllayeroverlyingthetipsofdermalpapillae(suprapapillaryplates),anddilatedandtortuousbloodvesselswithinthepapillae.Thesevesselsbleedreadilywhenthescaleisremoved,givingrisetomultiplepunctatebleedingpoints(Auspitz sign). Neutrophilsformsmallaggregateswithinboththespongioticsuperficialepidermisandtheparakeratoticstratumcorneum.Similarchangescanbeseeninsuperficialfungalinfections,whichneedtobeexcludedwithappropriatespecialstains.
Psoriasis most frequently affects the skin of the elbows, knees, scalp, lumbosacral areas, intergluteal cleft, glans penis, and vulva. Nail changes on the fingers and toes occur in 30% of cases. In most cases psoriasis is limited in distribution, but it can be widespread and severe. The clinical subtypes are defined by pattern of involvement and severity. Treatment is aimed at preventing the release or actions of inflammatory mediators. Mild disease is treated topically with ointments containing corticosteroids or other immunomodulatory agents, whereas more severe disease is treated with phototherapy (which has immunosuppressive effects) or systemic therapy with immunosuppressive agents such as methotrexate or TNF antagonists.
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Fig. 24.3 Chronicpsoriasis.(A)Erythematouspsoriaticplaquescoveredbysilvery-whitescale.(B)Microscopicexaminationshowsmarkedepidermalhyperplasia,downwardextensionofreteridges(psoriasiformhyperplasia),andprominentparakeratoticscalewithinfiltratingneutrophils.