What is the diagnosis and treatment for an adult patient with a skin disease on the knees and elbows, with no significant medical history, suspected to be psoriasis or eczema?

Diagnosis and Management of Skin Disease on Knees and Elbows

The most likely diagnosis is plaque psoriasis, and first-line treatment should be high-potency topical corticosteroids combined with vitamin D analogues (calcipotriene). 1

Clinical Diagnosis

Psoriasis is the primary consideration when evaluating skin lesions on the knees and elbows in an adult patient. The American Academy of Dermatology identifies salmon-colored (erythematous) plaques with white (silvery) scales on the elbows as pathognomonic for psoriasis vulgaris. 1 The extensor surfaces (elbows and knees) represent the classic distribution pattern for plaque psoriasis. 2, 1

Key Diagnostic Features to Assess:

• Lesion morphology: Look for well-demarcated erythematous plaques with overlying silvery scale 2
• Distribution: Bilateral and symmetrical involvement of extensor surfaces 2
• Associated findings: Check for nail changes (pitting, onycholysis), scalp involvement, and intertriginous areas 2
• Pruritus severity: Mild itching is common with psoriasis, though severity varies 1
• Joint symptoms: Screen for morning stiffness lasting >30 minutes, joint pain, swelling, or "sausage digits" (dactylitis) to rule out psoriatic arthritis 2

Differential Diagnosis Considerations:

Atopic eczema typically affects flexural surfaces (not extensors) in adults and presents with intense itching, dry skin, and a personal or family history of atopic disease. 2 The flexural distribution pattern distinguishes eczema from psoriasis. 2

Dermatitis herpetiformis presents with intensely itchy, blistering symmetrical rash on elbows, knees, and buttocks, but is associated with celiac disease and confirmed by granular IgA deposits on biopsy. 3

Disease Severity Assessment

Determine body surface area (BSA) involvement to guide treatment selection:

• Mild disease: <3% BSA (e.g., only elbows and knees) 1
• Moderate disease: 3-10% BSA 1
• Severe disease: >10% BSA 1

Screen for psoriatic arthritis, as 25-30% of psoriasis patients develop joint involvement, which mandates systemic therapy regardless of skin BSA. 1 Ask specifically about early morning stiffness duration, joint swelling, and heel/Achilles tendon pain (enthesitis). 2

Treatment Algorithm

For Mild Disease (Limited to Elbows/Knees):

First-line therapy: High-potency topical corticosteroids combined with vitamin D analogues (calcipotriene). 1 This combination enhances efficacy compared to monotherapy. 1

• Apply high-potency corticosteroid (e.g., clobetasol propionate) once or twice daily 4
• Combine with calcipotriene to improve outcomes 1
• Use occlusive dressings cautiously, as they significantly increase corticosteroid potency and absorption risk 2

Common pitfall: Avoid prolonged use of high-potency corticosteroids on large BSA due to skin atrophy risk and systemic absorption. 4 Limit treatment duration and monitor for adverse effects including burning/stinging (occurs in ~10% of patients), folliculitis, and HPA axis suppression. 4

For Moderate-to-Severe Disease or Treatment Failure:

Escalate to phototherapy (UVB or PUVA) or systemic therapy if:

• BSA exceeds 3-10% 1
• Topical therapy fails after adequate trial 2
• Quality of life is significantly impaired despite limited BSA 2
• Psoriatic arthritis develops 2

Systemic therapy options for moderate-to-severe disease:

• Methotrexate: First-line oral systemic agent due to lower cost, though evidence base is limited 2
• TNF-alpha inhibitors: First-line biologic therapy for moderate-to-severe psoriasis and psoriatic arthritis 2
• Combination therapy: MTX plus cyclosporine for refractory cases 2

Warning: Methotrexate carries significant risks including hepatotoxicity, pulmonary toxicity (interstitial pneumonitis in 1% of RA patients), bone marrow suppression, and teratogenicity. 5 Monitor liver function tests, complete blood counts, and ensure reliable contraception in women of childbearing age. 5

Management of Psoriatic Arthritis:

If joint symptoms are present, refer to rheumatology for comprehensive joint assessment. 2 Mild psoriatic arthritis may be managed with NSAIDs alone, but if unresponsive after 2-3 months, initiate methotrexate or TNF-alpha blockade. 2 Left untreated, psoriatic arthritis causes progressive joint damage, severe physical limitations, and disability. 2

Monitoring and Comorbidities

Screen for associated conditions at diagnosis and follow-up:

• Cardiovascular disease, metabolic syndrome, and diabetes 1
• Inflammatory bowel disease 1
• Depression and quality of life impairment 1

Follow-up assessment should include treatment response, adverse effects from topical corticosteroids (skin atrophy, telangiectasia), and development of new joint symptoms. 2, 4