Vulval psoriasis is common and is often under‐reported even in patients with psoriasis elsewhere because of many factors, including embarrassment [1]. The pattern of psoriasis most commonly affecting the vulva is flexural psoriasis, and other flexural sites and the scalp may also be affected. Although psoriasis is genetically determined, various trigger factors will provoke it in susceptible individuals. It is thought that friction, occlusion, and the isomorphic response (Koebner phenomenon) may be involved in flexural psoriasis.

It is well reported that patients with genital psoriasis experience significantly worse quality of life than patients with psoriasis on any other areas. Therefore, the ability to manage expectations and address the sexual distress caused by vulval psoriasis is an important part of optimal management [2–6].

Epidemiology

Psoriasis is a common skin disease which affects 2–3% of the world population but up to 8–11% of the populations of some northern European countries [7]. Genital psoriasis may occur in up to 63% of patients with psoriasis elsewhere [8] and up to 79% in patients with inverse psoriasis [9], but may also be the cause of vulval symptoms in those without psoriasis in up to 5% of women [10].

Genetics

It is understood that psoriasis has a genetic susceptibility, particularly in the presence of the HLA‐C*0602 risk allele, but in addition environmental factors such as stress, streptococcal infection, and injury to the skin are thought to be relevant. More recently, it is understood that IL‐17 and IL‐23 are key drivers in the pathogenesis of psoriasis [11]. Several genetic loci have been linked with the development of psoriasis, but to date there have been no reported studies of specific genetic loci leading to a predisposition to vulval or flexural psoriasis.

Pathophysiology

Psoriasis occurs because of immunological and environmental factors in a genetically susceptible individual. The isomorphic response or Koebner phenomenon, first reported by Heinrich Koebner in 1876, is the development of a skin disorder in areas of skin trauma and is often seen in patients with psoriasis, such as in surgical scars or tattoos [12]. This is likely to be of importance in the development of vulval psoriasis in susceptible individuals, particularly in obese patients where there is more likely to be skin‐to‐skin contact, friction, and occlusion. In addition, the possible role of bacterial or fungal colonisation or superinfection of the flexures as a possible trigger is debated [13].

Histological features

Vulval psoriasis is a clinical diagnosis, and a biopsy is not usually required. The typical histology in psoriasis shows parakeratosis, acanthosis with elongation of the rete ridges, and a reduced or absent granular layer, often with collections of neutrophils in the epidermis. However, flexural psoriasis does not always have the typical histological features of psoriasis seen elsewhere, and there may be marked spongiosis and papillary oedema with less epidermal hyperplasia (Figure 21.1) [13]. Clinicopathological correlation is therefore often required.

Clinical features

In contrast to patients with chronic plaque psoriasis, patients with vulval psoriasis often complain of itch. They may get burning, discomfort plus superficial dyspareunia, particularly if there is associated fissuring of the affected skin. This can, in turn, have an impact on sexual function [14] and quality of life [15]. Patients may also complain of malodour without vaginal discharge and of soreness in the perianal area due to extension of psoriasis into the natal cleft with fissuring.

The characteristic silvery scale often seen in psoriasis elsewhere is lost in flexural areas, but can sometimes still be seen on the mons pubis, labia majora, and buttocks (Figure 21.2). There are beefy‐red well‐demarcated symmetrical plaques with peripheral scale on the labia majora (Figure 21.3) and in the genitocrural folds with extension to the perineum and natal cleft (Figure 21.4). The plaques are less indurated than those seen elsewhere, and excoriations are often visible. Superficial fissuring at the fourchette or over the perineum is common.

Rarely, there may be some scarring associated with vulval psoriasis, with loss of the labia minora [16]. Patients with lichen sclerosus have a higher incidence of coexisting psoriasis [17,18] or a family history of psoriasis so it is possible that this observation was due to previous lichen sclerosus or lichen planus.

Psoriasis does not generally affect mucosal surfaces, except in the rare pustular form and in the lesions of Reiter’s disease. Geographic tongue is thought to occur more frequently in patients with psoriasis, and has some clinical and histopathological similarities [19]. In additional to the oral mucosa, it is important to examine the rest of the skin, paying particular attention to the hairline and scalp; behind the ears, elbows, and knees; other flexural sites; and the nails, for signs of psoriasis elsewhere. These signs may be subtle and may not have been noticed by the patient.

Some patients who have definite psoriasis elsewhere, and perhaps a perianal rash, complain of itching of the inner aspects of the vulva. In a series of 93 women with psoriasis, 44% complained of vulval symptoms but lesions were only seen in 23.7% [2]. In clinical practice, sometimes all there is to see is mild erythema of the interlabial sulci and a build‐up of keratinous debris. This highlights the importance of a thorough examination of the rest of the skin, including the scalp and nails, for clues to the correct diagnosis.

Differential diagnosis

Seborrhoeic eczema can have very similar clinical features. The distribution of the rash in the vulva is similar, including the erythema and debris in the interlabial sulcus, but the patches are less well defined and any scaling tends to be confluent rather than just at the peripheral edge. The presence of fine scaling throughout the scalp rather than erythematous scaly plaques, erythema, and scaling of the nasolabial folds and lack of nail changes or evidence of psoriatic plaques elsewhere is helpful.

Intertrigo is a descriptive term for a rash in the flexures, and may be inflammatory, due to an underlying dermatosis such as psoriasis or seborrhoeic dermatitis; or infective, due to bacterial or fungal superinfection.

Extramammary Paget’s disease and high‐grade squamous intraepithelial lesions (HSIL) may have psoriasiform features when seen in the vulva, so these should be kept in the differential diagnosis, particularly if the patient has no evidence of psoriasis elsewhere or does not appear to respond to treatment.

The skin signs seen in Reiter’s disease can be similar to that seen in psoriasis; however, the associated features of arthritis and urethritis make differentiation easier (see Chapter 27).