Difficult-to-Treat Sites: Nails, Palmoplantar, Scalp, and Intertriginous Psoriasis

Section 10 covers the clinical presentation of psoriasis subtypes, but certain anatomical sites present unique diagnostic and therapeutic challenges that warrant their own discussion. These “difficult-to-treat” sites (nails, palms and soles, scalp, and skin folds including genital skin) disproportionately affect quality of life relative to their surface area and often require treatment strategies distinct from generalised plaque psoriasis.

11.1 Nail Psoriasis

Nail psoriasis affects approximately 50% of psoriasis patients at any given time and up to 80-90% over a lifetime (Jiaravuthisan et al., 2007). In patients with psoriatic arthritis, nail involvement is even more common (66-80%) and serves as an independent predictor of PsA development, reflecting the anatomical connection between the nail matrix and the enthesis of the distal interphalangeal (DIP) joint extensor tendon (Edwards & de Berker, 2009). Despite its high prevalence, nail psoriasis is frequently undertreated and can cause significant functional impairment and psychological distress.

Clinical features. Nail psoriasis produces distinct signs depending on whether the nail matrix or nail bed is affected. Matrix disease causes pitting (the hallmark sign, small depressions in the nail plate), leukonychia (white spots), crumbling, and red spots in the lunula. Nail bed disease causes onycholysis (separation of the nail plate from the bed), oil drop discolouration (translucent yellow-red patches), splinter haemorrhages, and subungual hyperkeratosis (thickening under the nail). The mNAPSI scoring system (see Section 10) assesses all 8 features across 10 fingernails on a 0-130 scale; mNAPSI = 0 represents complete nail clearance, the nail equivalent of PASI 100 (Cassell et al., 2007).

Treatment ladder. A stepwise approach is recommended, guided by the number of nails involved and severity (Rigopoulos et al., 2019):

1. Topical therapy (first-line for mild disease or few nails affected): Potent topical corticosteroids (e.g., clobetasol propionate 0.05%) applied under occlusion, topical calcipotriol (more effective for nail bed signs than matrix disease), and tazarotene 0.1% gel. Evidence quality for topicals is low per the Cochrane review, but clinical experience supports their use in mild cases (de Vries et al., 2013).
2. Intralesional corticosteroid injection (first-line for 3 or fewer nails, especially matrix-predominant disease): Triamcinolone acetonide 2.5-10 mg/mL (diluted with 1% lidocaine), 0.1-0.2 mL per injection into the proximal nail fold, every 4-5 weeks for fingernails. Recent evidence suggests lower concentrations (2.5 mg/mL) are equally effective as higher doses. Pain is the main limiting factor.
3. Systemic biologic therapy (for moderate-to-severe disease or multiple nails): Biologics targeting the IL-17 pathway show the strongest nail-specific evidence. In the TRANSFIGURE trial, secukinumab 300 mg achieved a 45% NAPSI reduction at week 16 (vs. 11% placebo) and 73% improvement at 2.5 years (Reich et al., 2019). Ixekizumab achieved complete nail clearance (NAPSI = 0) in 67% of patients at week 60 in UNCOVER-3. In the head-to-head IXORA-S trial, ixekizumab was superior to ustekinumab for nail clearance (62% vs. 29% at week 52). Bimekizumab, targeting both IL-17A and IL-17F, achieved the highest rates of simultaneous PASI 100 + mNAPSI 0 in head-to-head trials: 46% vs. 18% (adalimumab), 51% vs. 27% (ustekinumab), and 63% vs. 36% (secukinumab). IL-23 inhibitors (risankizumab, guselkumab) are also effective for nails, though real-world data suggest IL-17 pathway agents may achieve faster and more complete nail clearance.

Key clinical points. Nail psoriasis responds more slowly to treatment than skin disease. If you’re on a biologic for your nails, expect that visible improvement typically takes 3-6 months due to the slow rate of nail growth (fingernails grow approximately 3 mm per month). Complete regrowth of a fingernail takes 4-6 months; toenails may take 12-18 months.

Research stage: Established entity. Evidence strength: High. Phase 3 RCTs with nail-specific endpoints; Cochrane review available; expert consensus guidelines published.

11.2 Palmoplantar Psoriasis

Psoriasis affecting the palms and soles is recognised as a distinct clinical entity with unique characteristics that set it apart from generalised plaque psoriasis. It affects approximately 3-4% of psoriasis patients but causes disproportionate disability: even a small affected area on the hands or feet can impair grasping, walking, and occupational function, with DLQI scores comparable to those of patients with much larger BSA involvement (Brunasso et al., 2025).

Distinct immune profile. Palmoplantar psoriasis involves both the IL-23/Th17 axis and the IL-36 pathway (Section 7.3), with higher expression of IL-36 cytokines in palmoplantar skin compared with other body sites. Genetics also differ: palmoplantar forms show higher prevalence in East Asian populations and may have distinct HLA associations.

Clinical presentation. Two main patterns are recognised: (1) hyperkeratotic plaques with painful fissures on palms and soles, and (2) palmoplantar pustulosis (PPP), characterised by sterile pustules on an erythematous base, primarily affecting the thenar and hypothenar eminences and the instep. PPP is increasingly classified as a related but distinct condition from plaque psoriasis, with stronger IL-36 involvement and weaker response to IL-17 inhibitors.

Treatment. First-line options include potent topical corticosteroids (often under occlusion to enhance penetration through the thick palmoplantar stratum corneum), topical vitamin D analogues, and acitretin. Phototherapy with localised NB-UVB or PUVA hand/foot units is effective. Among biologics, guselkumab (an IL-23 inhibitor) has been approved specifically for palmoplantar pustulosis in Japan. JAK inhibitors show particular promise: tofacitinib achieved near-complete clearance in all 16 patients in one open-label study, and larger studies report 40-53% achieving complete resolution (Brunasso et al., 2025). Apremilast demonstrated efficacy in a Phase 3 trial in Japan for palmoplantar pustulosis (Terui et al., 2024).

Research stage: Established entity, emerging treatments. Evidence strength: Moderate-High. Phase 3 RCT data for multiple agents; distinct pathophysiology well-characterised.

11.3 Scalp Psoriasis

Scalp involvement occurs in over 50% of psoriasis patients at any given time (and up to 80% at some point during the disease course), making it one of the most common manifestations (Griffiths et al., 2021). The scalp is now recognised as a high-impact site, a designation that may qualify patients for systemic or biologic therapy even when total BSA is low, given its outsized effect on quality of life (NICE CG153).

Clinical features. Scalp psoriasis ranges from mild, fine scaling to thick, adherent plaques extending beyond the hairline onto the forehead, temples, and posterior neck. Unlike seborrhoeic dermatitis (which is a key differential diagnosis), scalp psoriasis typically features well-demarcated, silvery-white plaques with more pronounced induration. Trichoscopy (dermoscopy of the scalp) reveals characteristic twisted red loops and white scaling.

Management algorithm. Treatment follows a stepwise approach:

1. Descaling phase: Keratolytic agents (salicylic acid 2-5%, coconut oil, urea) soften and remove thick scale to allow penetration of active treatments. This step is essential and often overlooked.
2. Active treatment: First-line is a combined calcipotriol/betamethasone gel or foam. The solution/foam vehicle is preferred over ointments for scalp use because of better cosmetic acceptability and hair-friendly application. Medicated shampoos containing coal tar, ketoconazole, or clobetasol provide adjunctive benefit.
3. Systemic therapy: For moderate-to-severe scalp disease resistant to topicals, systemic agents are indicated. A network meta-analysis ranked secukinumab 300 mg Q4W and bimekizumab 320 mg Q4W as the most effective biologics for scalp psoriasis (PubMed, 2025).

Novel topicals. Two recently approved agents represent significant advances for scalp psoriasis:

• Roflumilast foam 0.3% (approved May 2025, ages 12+): A topical PDE4 inhibitor in a foam vehicle specifically designed for scalp application. Steroid-free with no duration-of-use restrictions (Dermatology Times, 2025).
• Tapinarof (Section 23): The aryl hydrocarbon receptor agonist, also steroid-free, shows efficacy for scalp psoriasis with its characteristic “remittive effect.”

Oral therapies in development. Icotrokinra (oral IL-23R peptide, Section 27.6) is being tested in Phase 3 for scalp-specific endpoints.

Research stage: Established entity, new treatments emerging. Evidence strength: High. Network meta-analyses, Phase 3 data, and newly approved agents.

11.4 Inverse and Genital Psoriasis

Inverse psoriasis (also called flexural psoriasis) affects skin folds: axillae, groin, inframammary folds, intergluteal cleft, and genital skin. Genital psoriasis specifically affects the vulva, penis, or perianal area and may occur with or without flexural involvement elsewhere. Up to 63% of psoriasis patients experience genital involvement at some point in their lives, yet it remains under-reported, under-diagnosed, and undertreated (Wiley, 2024).

Clinical features. Inverse psoriasis presents as smooth, shiny, erythematous plaques, typically lacking the thick silvery scale of classical plaque psoriasis because the moist, occluded environment of skin folds causes maceration and scale loss. In darker skin, plaques may appear purple, hyperpigmented, or grey rather than erythematous (see Section 14.3). Genital psoriasis is frequently misdiagnosed as fungal infection, contact dermatitis, or sexually transmitted infection, leading to inappropriate treatment and diagnostic delay.

Treatment challenges. The thin, sensitive skin of intertriginous areas poses unique treatment difficulties:

• Increased absorption: Topical medications are absorbed more readily in occluded areas, increasing the risk of local side effects (atrophy, striae, telangiectasia) from corticosteroids.
• Irritation: Many topical agents (vitamin D analogues, dithranol, tazarotene) cause significant irritation in skin folds.

First-line treatment is low-to-mid potency topical corticosteroids for short courses (1-2 weeks), followed by steroid-sparing maintenance with calcineurin inhibitors (tacrolimus 0.1% ointment or pimecrolimus 1% cream), which lack the atrophy risk of corticosteroids and are well-tolerated on sensitive skin (Section 23).

Systemic therapy. For moderate-to-severe cases, dedicated clinical trials now demonstrate efficacy of systemic agents specifically for genital and inverse psoriasis:

• Ixekizumab (IL-17A inhibitor) was studied in the IXORA-Q trial, a genital psoriasis-specific RCT, showing rapid and significant improvement in genital DLQI and sexual health indices.
• Apremilast has been studied for genital-specific outcomes, showing modest but significant improvement.
• IL-23 inhibitors: A 52-week multicentre study (2024) confirmed efficacy for moderate-to-severe inverse psoriasis (Springer, 2025).
• Delgocitinib 0.25% ointment, a topical pan-JAK inhibitor, is in Phase II for inverse psoriasis, potentially offering a novel steroid-free topical option (PubMed, 2024).

Sexual health impact. Genital psoriasis significantly affects sexual function and intimate relationships. Section 17.2 discusses sexual dysfunction in detail. Clinicians should proactively ask about genital involvement and sexual health, as patients rarely volunteer this information. A brief, normalising approach (“Psoriasis commonly affects genital skin. Is this something you’ve experienced?”) can open the conversation.

Research stage: Established entity. Evidence strength: Moderate-High. Dedicated RCTs now available for genital and inverse psoriasis; novel topical agents in Phase II.