Clinical features and sub-classifications

Psoriasis is characterised by and diagnosed according to its clinical appearance. The classic presentation of psoriasis is distinctive: patches or plaques appear on the skin as well-

demarcated, raised, inflamed lesions covered in silvery-white flaky scales (Figure 1A).

Psoriatic lesions can appear anywhere on the body, with the elbows, knees, lumbar and umbilicus area and/or scalp the most commonly affected sites (Griffiths & Barker, 2007) (Figure 1B, 1C). They often appear in symmetrical and bilateral fashion, that is, opposing sides of the body may have plaques that are near mirror images of each other (Figure 1C) (Luba & Stulberg, 2006). Plaques exhibit pin-prick bleeding upon removal of scales, which is known as the Auspitz sign, and occurs due to the proximity of dilated capillaries to the surface of the skin (Raychaudhuri et al., 2014). Around 20% of psoriasis patients experience the Koebner phenomenon, which is the development of new psoriatic plaques on skin following irritation or trauma (Myers et al., 2006). Other common symptoms of psoriasis are itch, and skin that is painful, burning, stinging, sensitive, cracked and bleeding (Globe, Bayliss & Harrison, 2009). The visible manifestations of psoriasis can also extend to the nails, with an estimated 80% – 90% of people with psoriasis developing nail disease (onchodystrophy) during their lifetime (Figure 1D) (Reich, 2009). This tends to affect fingernails more often than toenails, and takes

15 the form of pitting, separation of the nail plate from the nail bed (onycholysis), yellow or brown marks caused by an accumulation of cellular debris under the nail (known as oil spots) and/or nail dystrophy (Griffiths & Barker, 2007).

While the aforementioned features are considered ‘classic’ features of psoriasis, in practice the term encompasses a variety of presentations. For instance, the size, thickness and type of lesion (macule, papule or plaque) can vary greatly between, or even within, individuals (Raychaudhuri et al., 2014). For some, psoriasis is widespread across the body, whereas for others it is localised to one particular region (Raychaudhuri et al., 2014). Similarly, the site of plaque development can vary, as can the pattern of distribution; for example, a sub-type called flexural

A B

Figure 1. The clinical appearance of psoriasis1

(A) Characteristic silvery-white flaky scale on a psoriasis plaque; (B) and (C) the elbows and knees are typical sites of psoriasis development. Figure (C) shows symmetrical and bilateral plaque development. (D) Onchodystrophy (nail disease) in psoriasis: nail pitting and onycholysis (separation of the nail from the nail bed); (E) flexural psoriasis occurs in skin folds and has a shiny appearance without scales.

1

Images A, B and C are reproduced with permission from Chronic Plaque Psoriasis, February 15, 2006, Vol. 73, No. 4, issue of American Family Physician, Copyright © 2006 American Academy of Family Physicians. All Rights Reserved. Images D and E are used with permission from S. N. Cohen, S. E. Baron, C. B. Archer, Clinical and Experimental Dermatology, John Wiley and Sons. © 2012 The Authors.

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psoriasis occurs only within skin folds and is without scales (Figure 1E), whereas another sub-

type, sebopsoriasis, tends to develop on the face, behind the ears and on the sternum, and scales are greasy and yellow (Griffiths & Barker, 2007). Furthermore, the severity of psoriasis ranges from mild to severe based on the degree of visible inflammation, thickness and scaling (Schmitt & Wozel, 2005).

A further point of variation amongst those with psoriasis is the age at which psoriasis first develops. Age of onset, like much of the condition, is unpredictable, and can occur anytime between infancy and old age (Naldi & Gambini, 2007). However, Henseler and Christophers noted a bimodal pattern of age of onset, with the first peak occurring between 16 and 22 years and the second between 57 and 60 years (as cited in Griffiths & Barker, 2007). Early onset of psoriasis (< 40 years) is much more common than late onset (Naldi & Gambini, 2007) and women are thought to develop it slightly earlier than men (Griffiths & Barker, 2007). Further investigation in this area has identified clinical and genetic differences between those who develop psoriasis early in life (defined as onset < 40 years) compared to those who develop it later (> 40 years), leading to the concepts of Type I and Type II psoriasis, which describe early and late onset psoriasis, respectively (Naldi & Gambini, 2007). Those with Type I psoriasis, which is more common, are thought to be more likely to have severe, recurrent disease, more frequent nail disease, and to have at least one first-degree relative with psoriasis, compared to those with Type II psoriasis, which tends to be a milder, more stable condition and in which the suggestion of heredity is not as clear (Naldi & Gambini, 2007). However, age of onset does not appear to be a reliable indicator of psoriasis characteristics in its own right, as these associations are not consistently found (Guinot, Latreille, Perrussel, Doss, & Dubertret, 2009).

In one of the more comprehensive explorations into the distinctions between phenotypes of psoriasis, Guinot et al. (2009) also proposed six different phenotypes based on observed clusters of self-reported clinical characteristics, including age of onset and severity, in 1,484 participants. The most common of these were Type 1, which involved late onset development, few lesions involving mainly the scalp and elbows, no associated arthritis, continuous

17 development and no apparent family history or sensitivity to environmental factors; and Type 4, which involved very severe psoriasis affecting all areas of the body except soles of feet, palms of hands and nails, no association with age of onset, lower sensitivity to the Koebner phenomenon, and less association with itch as a symptom, and with personal and family history of arthritis, atopic dermatitis, asthma or allergic rhinitis (Guinot et al., 2009). While still unlikely to be definitive, these findings illustrate the large variability that is found between forms of the disease that are currently classified as psoriasis. Due to this variety in the presentation and occurrence of psoriasis, it has been proposed that what is currently defined as chronic plaque psoriasis is actually a number of closely related, but geno- and phenotypically distinct conditions (Griffiths & Barker, 2007). At the very least, psoriasis may be best thought of as a spectrum of disease rather than a clearly delineated disease entity (Raychaudhuri et al.,

2014). A summary of ways in which psoriasis has been sub-classified is presented in Table 1.

Some of these classifications have been developed in an attempt to explain the significant variation in course and response to treatment that exists amongst people with psoriasis, and as these attempts have usually focused on one binary pair (e.g., thick vs. thin plaque psoriasis, Type I vs. Type II psoriasis), significant overlap exists between classifications.