Skin deep

Psoriasis is a non-contagious chronic skin disease affecting the keratinocytes, the cells that predominantly form the epidermis. During outbreaks of the disease, the keratinocytes proliferate at ten times the rate of non-diseased skin cells and fail to mature properly, resulting in raised, inflamed, scaly red skin lesions known as plaques, which can be itchy and painful (see Figure 1).

Figure 1
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Psoriasis is common

Global epidemiological data are sparse. There is a lack of uniformity of data collection, so worldwide incidence and prevalence are poorly documented. Some trends do emerge — for example, prevalence is higher closer to the poles (see Figure 2). A combination of genetic and environmental factors are thought to be behind this pattern.

Figure 2
figure 2

SOURCE: PARISI, R. ET AL. J. INVEST. DERMATOL. DOI:10.1038/JID.2012.339 (2012)

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Age of onset

Although plaque psoriasis is considered a single disease, its severity, progression and response to treatment can vary markedly between patients — suggesting the existence of multiple underlying subtypes. For example, incidence data by age show two spikes, suggesting early- and late-onset forms of the disease (see Figure 3).

Figure 3
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SOURCE: PARISI, R. ET AL. J. INVEST. DERMATOL. DOI:10.1038/JID.2012.339 (2012)

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Psoriasis is highly heterogeneous

Psoriasis comes in several distinct forms (see Figure 4). Patients usually display a single form at any one time, although forms can coexist, and one form can be followed by another. Around 80% of psoriasis cases are classified as mild.

Figure 4
figure 4

SOURCE: THE PSORIASIS AND PSORIATIC ARTHRITIS POCKET GUIDE, NATIONAL PSORIASIS FOUNDATION CREDIT: DR P. MARAZZI/SCIENCE PHOTO LIBRARY / CNRI/SCIENCE PHOTO LIBRARY / NATIONAL PSORIASIS FOUNDATION / DR P. MARAZZI/SCIENCE PHOTO LIBRARY / SCIENCE PHOTO LIBRARY

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Treatment

A broad spectrum of therapies are available to treat psoriasis, depending on the severity of the disease. Newer biologic drugs are assessed by a 'PASI 75' score, representing the percentage of patients achieving at least a 75% reduction in their Psoriasis Area and Severity Index (PASI; see Figure 5). Despite considerable progress (see 'Silencing psoriasis', page S58), a cure remains elusive.

Figure 5
figure 5

SOURCE: FARBER, E. M. J. AM. ACAD. DERMATOL. 27, 640–645 (1992); LEONARDI, C. L. ET AL. N ENGL J MED 349, 2014–2022 (2003); GOTTLIEB, A. B. ET AL. J. AM. ACAD. DERMATOL. 48, 829–835 (2003); PAPP, K. A. ARCH DERMATOL RES. 298, 7–15 (2006); MENTER, A. ET AL. J. AM. ACAD. DERMATOL. 58, 106–115 (2008); LEONARDI, C.L. ET AL. LANCET 371, 1665–1674 (2008); FDA.

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