**3. When psoriasis coexists with other skin diseases**

Often chronic skin inflammatory diseases coexist with psoriasis suggesting common pathogenic pathways.

Among them, hidradenitis suppurativa (HS) also exhibits a systemic inflammatory nature with systemic comorbidities similar to psoriasis. The systemic inflammation might explain the observation of a recent study, where it was found that in patients with both HS and psoriasis, the disease diagnosed first tended to take a more severe course than the later diagnosed (**Figure 2**) [33].

Pityriasis rubra pilaris (PRP) is an inflammatory dermatologic disorder of unknown cause, which often is misdiagnosed as psoriasis. However, differentiating between erythrodermic PRP and pustular psoriasis is challenging even histologically. The same treatment is indicated in both diseases, despite the absence of standard

#### **Figure 2.**

*A 65-year-old male patient. Hidradenitis suppurativa Hurley III coexists with psoriatic plaques. Photo from personal collection.*

recommended treatment algorithms for PRP. According to recently published data, we must consider the coexistence of psoriasis and autoimmune diseases in patients with PRP [34]. From our personal experience, a patient can present with erythrodermic PRP and the improvement of the rash might be followed by the appearance of classical plaque psoriasis after treatment with an anti-TNF-α biological agent.

One of the most paradoxical relationships is between psoriasis and atopic dermatitis (AD). The Th17 immune response is dominant in psoriasis and causes neutrophil migration, induction of innate immunity, and increased epithelial metabolism, while Th2 immunity that characterizes AD is dominated by IL-4 and IL-13 cytokines leading to an impaired epidermal barrier, dampened innate immunity, and eosinophil migration. However, the association of AD with psoriasis is not so rare. Both diseases share many characteristics: high prevalence, chronicity, primary skin inflammation, associated comorbidities, important impact on the quality of patient's life due to itch and stigmatization. Some authors consider that the co-occurrence is an overlapping syndrome [35] and others found bidirectional association [36]. In Bozek's study [35] the patients with concomitant AD and psoriasis were frequently boys and overweight and had skin lesions equally distributed throughout the body. Despite the fact that the pathogenesis of psoriasis and AD is different, a family history of atopic disease is a more frequent finding in children with concomitant AD and psoriasis and in children with AD than in children with only AD or psoriasis [35]. Genetics and epigenetics studies with a focus on this topic might provide useful data regarding the particular management of patients with AD and psoriasis.
