What is the best approach to diagnose and treat a patient presenting with symptoms of scalp psoriasis versus seborrheic dermatitis?

Distinguishing and Managing Scalp Psoriasis vs Seborrheic Dermatitis

For diagnosis, look for sharply demarcated, thick, silvery-white scaled plaques extending beyond the hairline in psoriasis, versus poorly demarcated, greasy, yellowish scales confined within the hairline in seborrheic dermatitis. 1

Diagnostic Approach

Key Clinical Features to Differentiate

Scalp Psoriasis:

• Well-demarcated, indurated plaques with thick silvery-white scale that extend across the natural hairline 2, 1
• Plaques are sharply defined and thicker than seborrheic dermatitis lesions 2
• Personal or family history of psoriasis strongly favors this diagnosis 3, 2
• Hair loss is rare and may be related to therapeutic agents rather than the disease itself 1
• Inspect the entire body for psoriatic plaques on elbows, knees, trunk, and nails 3

Seborrheic Dermatitis:

• Blurred, dark-red erythema with yellowish, greasy scales that remain within the hairline 1
• Greasy, yellow scales are the hallmark feature 4, 5
• Diffuse alopecia may frequently occur 1
• Associated with Malassezia yeast overgrowth 5, 6
• May present with hyperpigmentation or hypopigmentation in darker skin types, with minimal erythema or scaling 6

Critical Examination Steps

• Examine the scalp for plaque thickness, scale character (silvery vs greasy), and border definition (sharp vs blurred) 1
• Check if lesions cross the hairline (psoriasis) or remain confined within it (seborrheic dermatitis) 1
• Inspect elbows, knees, trunk, nails, and intertriginous areas for additional psoriatic lesions 3
• Look for secondary bacterial infection (crusting, weeping) or herpes simplex superinfection (grouped, punched-out erosions) which require specific antimicrobial treatment 2

Treatment Algorithm

For Scalp Psoriasis

First-Line Treatment:

• Use shampoos, gels, solutions, or foams rather than ointments or creams, as hair makes traditional formulations messy and difficult to use 2
• Apply clobetasol propionate 0.05% shampoo twice weekly for significant inflammation 2, 7
• Clobetasol is a class 1 (superpotent) topical corticosteroid with anti-inflammatory, antipruritic, and vasoconstrictive actions 7
• Limit treatment to 2-week courses to avoid HPA axis suppression, which is transient and reversible 7
• Common adverse events include burning/stinging (29/294 patients), scalp pustules (3/294), and folliculitis (2/294) 7

Alternative Topical Options:

• Coal tar solution 1% applied not more than twice daily has potent efficacy against pruritus 2, 8
• Vitamin D3 analogues (calcipotriol, tacalcitol) inhibit epidermal proliferation and enhance cornification 8
• Dithranol 0.1-3% cream formulations, starting at low concentration and increasing stepwise 8

Systemic Therapy Indications:

• Consider for patients with >5% body surface area involvement, or <5% BSA with scalp disease unresponsive to topical therapy causing major quality-of-life issues 3
• Options include methotrexate, cyclosporine, acitretin, or biologic agents 3, 8

For Seborrheic Dermatitis

First-Line Treatment:

• Ketoconazole 2% shampoo is the first-line treatment with an 88% response rate after initial treatment 2
• The treatment combines topical antifungal medications to reduce Malassezia yeast with topical anti-inflammatory agents 2
• Apply ketoconazole 2% cream or shampoo not more than twice daily 2

For Significant Inflammation:

• Add low-potency topical corticosteroids (hydrocortisone 1% or prednicarbate 0.02%) for limited periods only, not exceeding 2-4 weeks, especially on the face 2
• Avoid long-term corticosteroid use on the face due to risk of skin atrophy, telangiectasia, and tachyphylaxis 2
• For enhanced efficacy, combine clobetasol propionate 0.05% shampoo twice weekly with ketoconazole for sustained effect during maintenance 2

Alternative Antifungal Options:

• Selenium sulfide 1% shampoo has demonstrated efficacy alongside ketoconazole 2% 2
• Coal tar preparations can reduce inflammation and scaling 2

Supportive Measures:

• Use mild, pH-neutral (pH 5) non-soap cleansers with tepid water 2
• Apply fragrance-free moisturizers containing petrolatum or mineral oil immediately after bathing to damp skin 2
• Avoid alcohol-containing preparations as they worsen facial dryness 2, 4
• Avoid greasy or occlusive products that can promote folliculitis 2

When to Refer to Dermatology

• Diagnostic uncertainty or atypical presentation 2
• Failure to respond after 4 weeks of appropriate first-line therapy 2
• Recurrent severe flares despite optimal maintenance therapy 2
• Need for second-line treatments or systemic therapy 2
• Suspected contact dermatitis, cutaneous T-cell lymphoma, or other differential diagnoses requiring patch testing 2

Common Pitfalls to Avoid

• Do not use neomycin-containing products due to high sensitization risk (13-30% in chronic cases) 2, 4
• Avoid non-sedating antihistamines in seborrheic dermatitis as they provide no benefit 2
• Do not undertreate due to fear of corticosteroid side effects; use appropriate potency for adequate duration, then taper 2
• Avoid confusing persistent itching after treatment with treatment failure, as mild burning or itching from inflammation can persist for days after yeast is eliminated 2
• Do not use hot water; use tepid water instead to prevent worsening symptoms 2
• Avoid harsh soaps and detergents that remove natural lipids from the skin surface 2

Overlap Syndrome (Sebopsoriasis)

• Some patients present with features of both conditions, making differentiation difficult 1
• In these cases, treat with combination therapy: antifungal agents (ketoconazole 2%) plus vitamin D3 analogues or low-potency corticosteroids 9, 1
• Environmental factors such as stress, systemic infections, and medications can trigger exacerbations 9