Inverse Psoriasis

Inverse psoriasis (flexural psoriasis) is a variant of psoriasis that affects skin folds, with smooth, well demarcated, glistening erythematous plaques that lack the usual scale. It involves the axillae, groin, genitals, inframammary and natal cleft regions.

It occurs in about 3 to 36% of people with psoriasis, with wide variation between series, and it is more common in older people and in people who are overweight. It can occur alone or with plaque disease at other sites.

Obesity, sweating, friction and secondary infection with Candida or bacteria are aggravating factors. Genital involvement carries a high impact on quality of life and sexual function.

Pathophysiology.

Psoriasis arises from interleukin-23 and interleukin-17 driven keratinocyte proliferation. In folds, moisture and friction macerate the scale, which gives a glazed, red surface. Mechanical stress and skin colonisation add to inflammation.

• Bright red, smooth, sharply defined plaques with little or no scale in the axillae, groin, genitals, inframammary folds, umbilicus, natal cleft and behind the ears.
• Fissures, soreness and itch, with maceration and malodour.
• Frequently accompanied by scalp, nail or plaque disease at other sites.
• Secondary candidal or bacterial infection causes satellite pustules and sudden worsening.

Red flags.

• Unilateral, asymmetrical or non-responding plaque in the groin, raising concern for Bowen disease or extramammary Paget disease.
• Rapid spread, pustules or fever.
• Erosions with pain, suggesting herpes simplex or an erosive dermatosis.

• Clinical diagnosis from the distribution and morphology, with examination of the scalp, nails, umbilicus and natal cleft for corroborating psoriasis.
• Swabs and skin scrapings for bacteria and yeasts, and potassium hydroxide microscopy and culture for dermatophytes.
• Wood lamp examination for coral red fluorescence (erythrasma).
• Skin biopsy for atypical or treatment resistant disease.

Differential diagnosis. Candidal intertrigo, tinea cruris, erythrasma, seborrhoeic dermatitis, allergic or irritant contact dermatitis, familial benign chronic pemphigus, Langerhans cell histiocytosis, extramammary Paget disease, Bowen disease and lichen planus.

• Mild to moderate potency topical corticosteroid (hydrocortisone 1% or clobetasone butyrate 0.05%) once daily for up to 2 weeks, then intermittent use. Avoid potent preparations in folds because of atrophy and striae.
• Tacrolimus 0.1% ointment or pimecrolimus 1% cream twice daily as steroid sparing agents (off label).
• Calcipotriol 50 micrograms/g ointment once or twice daily, used cautiously because it irritates folds, or in the combination of calcipotriol and betamethasone once daily for up to 4 weeks.
• Treat infection: topical clotrimazole 1% or miconazole 2% cream twice daily, or the combination with hydrocortisone 1%. Oral fluconazole 150 mg once weekly for 2 to 4 weeks for refractory candidiasis. Topical or oral erythromycin for erythrasma.
• Weight loss, loose cotton clothing, drying agents and barrier creams.
• Extensive or resistant disease: narrowband ultraviolet B, methotrexate 7.5 to 15 mg once weekly (maximum 25 mg) with folic acid 5 mg weekly, or biologic therapy (adalimumab 80 mg then 40 mg every 2 weeks from 1 week later, secukinumab 300 mg or ixekizumab). Screen for tuberculosis and hepatitis B, hepatitis C and HIV before starting biologics.

Monitoring. Review at 2 to 4 weeks for response and signs of atrophy.

Referral and escalation. Refer to dermatology for diagnostic doubt, resistant disease or need for systemic or biologic therapy.

Special groups. In pregnancy use topical therapy only. In children use mild steroids and tacrolimus 0.03% for short periods.