Epidemiology and clinical manifestations

Psoriasis occurs in approximately 1–3% of the population¹. Anogenital involvement is common and usually accompanied by disease on other areas of the body. Psoriasis occurs in all races, and at all ages, but most patients exhibit lesions before the age of 30 years. Obesity and alcoholism are more prevalent in patients with psoriasis.

Some patients complain of intense pruritus, others describe pain with fissuring and cracking of the skin, and still others have no symptoms. Most women are troubled by the appearance of the skin and the remarkable proliferation of scale, with embarrassing flakes of skin left on chairs and in the bed.

The clinical appearance of psoriasis varies as this disease occurs in several major patterns (Table 17.1). Most common is psoriasis vulgaris (common psoriasis, or plaque-type psoriasis). Psoriasis vulgaris is manifested by classic well-demarcated, red plaques covered with dense, silvery scale (Figures 17.1 and 17.2). In skin folds, however, the scale is more subtle, often with a shiny or glazed texture (Figure 17.3) Psoriasis vulgaris shows a predilection for elbows, knees, and scalp, although it can occur in all areas. Psoriasis exhibits the Koebner phenomenon, in which the disease preferentially affects areas of trauma or inflammation. This partially explains the distribution of elbows, knees, and genital skin – all areas prone to friction and mild trauma. Many patients also exhibit tiny pits on the fingernails, a relatively specific sign of psoriasis.

Table 17.1 Morphologic Variants of Psoriasis

Figure 17.1 These well-demarcated papules and plaques with heavy white, adherent scale are typical of untreated psoriasis on dry, keratinized skin.

Figure 17.2 Psoriasis vulgaris shows a predilection for the elbows and knees, and the identification of classic disease on extragenital skin such as this helps to guide the diagnosis of nonspecific genital morphology.

(Reproduced from L. Edwards Dermatology in Emergency Care (Fig. 14.1), New York: Churchill Livingstone; 1997.)

Figure 17.3 Psoriasis on the vulva and within other skin folds lacks the dense white scale of psoriasis occurring elsewhere, and is more likely to show shiny or glazed skin.

A second form of psoriasis, guttate psoriasis, appears as 0.5–2-cm round, red, scaling papules and small plaques (Figure 17.4). Scalp and nail disease are less common, and this form of psoriasis does not prominently affect anogenital skin, elbows, knees, or scalp.

Figure 17.4 Small, discrete, scaling papules of guttate psoriasis generally lack the dense white scale of psoriasis vulgaris.

(Reproduced from L. Edwards Dermatology in Emergency Care (Fig. 14.2), Churchill Livingstone.)

Inverse psoriasis consists of disease that preferentially affects skin folds, including the axillae, inframammary skin, crural creases, and anogenital area. Inverse psoriasis consists of moist plaques that are deeply red, with very subtle scale (Figures 17.5–17.7). The thickened skin in skin folds is often macerated and appears white. The gluteal cleft and umbilicus are characteristically also involved. Often, classic elbow, knee, and scalp psoriasis are absent.

Figure 17.5 These well-demarcated, shiny plaques are classic for vulvar psoriasis.

Figure 17.6 The thickened skin of psoriasis appears white in skin folds, where chronic moisture regularly makes thick skin appear white. Skin fold psoriasis both mimics and sometimes coexists with candidiasis. Cultures and response to therapy are often required to differentiate these conditions.

Figure 17.7 Vulvar psoriasis often presents as poorly demarcated red plaques that are indistinguishable from lichen simplex chronicus, contact dermatitis, and seborrheic dermatitis, so that correlation with abnormalities on other skin surfaces, biopsies, and response to therapy may be required for definitive diagnosis.

A fourth form of psoriasis, erythrodermic psoriasis, is any form of psoriasis which generalizes, resulting in widespread, nonspecific erythema and scale. Erythrodermic psoriasis generally lacks the typical heavy, silvery scale. Erythrodermic psoriasis is indistinguishable from any widespread dermatosis, such as generalized eczema or a medication allergy.

The final form of psoriasis is pustular psoriasis, which appears as plaques of pustules that eventuate into crusted plaques. These can be generalized or occur primarily on the hands, feet, and genital skin. The skin findings of pustular psoriasis are indistinguishable from those of Reiter’s syndrome (Figures 17.8 and 17.9). Arthritis is more common with pustular psoriasis than with other forms.

Figure 17.8 Pustular psoriasis consists of plaques or pustules; the fragile pustules rupture almost immediately, leaving small, coalescing erosions (small arrows). Surrounding pustules (bold arrows) are also present in this patient, who was originally misdiagnosed with resistant yeast infection.

Figure 17.9 Acral red plaques of pustules and coalescing crusted papules representing ruptured pustules are characteristic of keratoderma blenorrhagica, the cutaneous eruption of Reiter’s syndrome. This is clinically and histologically identical to skin disease of pustular psoriasis.

(Reproduced from L. Edwards Dermatology in Emergency Care (Fig. 7.8), Churchill Livingstone.)

Except for guttate psoriasis, all variants of psoriasis frequently exhibit anogenital involvement, and inverse psoriasis nearly always affects the vulva and crural creases. Psoriasis usually affects hair-bearing, nonmucous membrane anogenital skin. However, pustular psoriasis rarely occurs on the modified mucous membranes of the vulva, the vaginal epithelium, and ectocervix.

Differential diagnosis

Diseases that mimic genital psoriasis include candidiasis, which is also characterized by red, intertriginous plaques, but candidiasis is usually accompanied by small satellite papules and collarettes, and a positive fungal preparation or culture. Clearing with anticandidal therapy confirms that diagnosis. Tinea cruris, although uncommon in women, sometimes appears indistinguishable from anogenital psoriasis. Lichen simplex chronicus of the labia majora and mons is morphologically similar to psoriasis but generally is less well demarcated. Seborrheic dermatitis is very uncommon on the vulva, but appears similar clinically and histologically, with red, moist, finely scaling red plaques on the vulva and crural creases. Seborrheic dermatitis is regularly accompanied by typical seborrhea of the scalp, central face, and other skin folds. The red, rough plaques of irritant and allergic contact dermatitis sometimes resemble psoriasis, but are classically less well demarcated and unaccompanied by signs of extragenital psoriasis. The above diseases are differentiated by culture, the identification of characteristic lesions elsewhere, and response to therapy. Often, psoriasis coexists with one of these diseases, especially candidiasis, lichen simplex chronicus, and contact dermatitis (Table 17.2).

Table 17.2 Psoriasis

Laboratory findings and histology

Mild psoriasis is usually unaccompanied by laboratory abnormalities. Patients with pustular psoriasis are likely to exhibit a leukocytosis, and those with psoriatic arthritis may exhibit characteristic radiographs.

Early lesions and the edges of progressing lesions of psoriasis are the most likely to show diagnostic changes of psoriasis, whereas late lesions are often nonspecific. Classic histology shows regular elongation of the rete pegs with thickening of the lower portion of the pegs, as well as thinning of the suprapapillary epidermis, hyperkeratosis, and parakeratosis. A chronic inflammatory infiltrate is present in the dermis, but the diagnostic finding is that of small collections of neutrophils (microabscesses) in the epidermis and within the parakeratotic stratum corneum. Pustular psoriasis exhibits much larger pustules that are clinically visible. Often, however, psoriasis is chronic and the histology is nonspecific, yielding a histologic “diagnosis” of psoriasiform dermatitis. This is a description rather than a diagnosis, indicating features of both psoriasis and eczema, but diagnositic of neither. This biopsy report neither proves nor disproves the diagnosis of psoriasis.

Pathogenesis

Psoriasis is an immunologically mediated disease characterized by a T-cell inflammatory infiltrate². This T-cell-mediated type 1 autoimmune process and the resulting cytokines ultimately produce an increased turnover of keratinocytes, thickening of the epidermis, and scale. The predilection for the development of psoriasis is partly inherited.

Therapy and prognosis

Vulvar psoriasis is usually controllable with topical therapy. Corticosteroids, the elimination of irritants, and careful attention to secondary infection, particularly Candida, produce sufficient benefit for most women with anogenital psoriasis. An ultrapotent topical corticosteroid such as clobetasol ointment is applied very sparingly once or twice daily, tapering the application to the least frequent dosing that controls the disease, usually about three times a week. The areas most often affected with psoriasis, the hair-bearing labia majora, crural crease, perianal skin, and medial thighs, are the areas that atrophy very easily. Patients who require ongoing daily therapy should be managed with a lower-potency topical corticosteroid, and, if needed, calcipotriene (Dovonex) cream. Other topical agents, such as tar, salicylic acid, and anthralin, are too irritating for use on anogenital skin. Tacrolimus and pimecrolimus are useful in some patients, and although somewhat irritating, some women tolerate them well, particularly after the skin is partially controlled with a topical corticosteroid.

Patients who are not adequately controlled with topical therapy require treatment with systemic agents such as methotrexate, acitretin, or one of the new biological agents, including etanercept, infliximab, efalizumab, adalimumab, and alefacept.

Psoriasis cannot be cured, but it can be controlled. Psoriasis of the anogenital skin is usually more easily controlled than psoriasis of other areas, but corticosteroid side-effects are also more prominent here so that ongoing surveillance is indicated. Secondary anogenital malignancy has not been reported, but deforming arthritis is a known sequela and there is recent evidence that the inflammation of psoriasis may predispose to coronary artery disease³.