Localized Pruritic Papular Eruption on Forearm
The most likely diagnosis is contact dermatitis (irritant or allergic), and initial management should be mid- to high-potency topical corticosteroid (triamcinolone 0.1% or clobetasol 0.05%) applied twice daily for 2 weeks, combined with identification and avoidance of the causative agent. 1
Diagnostic Approach
The localized 5×4 cm cluster of itchy papules on the forearm suggests a focal dermatologic process rather than systemic disease. The key diagnostic features to assess include:
- Distribution pattern: Localized lesions with visible borders strongly suggest contact dermatitis rather than systemic causes 1
- Occupational/environmental exposures: Recent contact with chemicals, plants (poison ivy), metals (nickel), fragrances, or new products 1, 2
- Timing: Acute onset (hours to days) suggests irritant contact dermatitis; delayed onset (48-72 hours after exposure) suggests allergic contact dermatitis 1
- Lesion morphology: Erythema with scaling and defined borders indicates contact dermatitis; central punctum with surrounding wheal suggests arthropod bite reaction 3
Primary Differential Diagnoses
Contact Dermatitis (Most Likely)
Contact dermatitis presents as erythematous, pruritic, scaly lesions with visible borders in a localized distribution matching the area of contact with an offending substance 1. The forearm is a common site due to frequent environmental exposures 4. This can be either:
- Irritant contact dermatitis: Non-immune-mediated skin irritation from direct chemical injury 1, 2
- Allergic contact dermatitis: Delayed type IV hypersensitivity reaction requiring prior sensitization 1, 2
Arthropod Bite Reaction (Papular Urticaria)
Insect bites can cause clustered pruritic papules, often with central punctum and surrounding wheal 3. Thysanoptera (thrips) bites specifically cause small pink itchy papules on trunk and arms 5. However, the localized 5×4 cm distribution is less typical than scattered lesions 3.
Prurigo Nodularis
Prurigo presents as intensely pruritic nodular lesions, but typically develops chronically and may be associated with atopy, internal disease, or compulsive scratching 6. The acute presentation makes this less likely.
Initial Management Algorithm
Step 1: Immediate Treatment (First 2 Weeks)
For localized lesions (<20% body surface area):
- Apply triamcinolone 0.1% cream or clobetasol 0.05% cream twice daily to affected area 1, 7
- Triamcinolone acetonide is FDA-indicated for inflammatory and pruritic manifestations of corticosteroid-responsive dermatoses 7
- Mid- to high-potency topical steroids provide relief within 12-24 hours for acute allergic contact dermatitis 1
Adjunctive measures:
- Cold water compresses to reduce inflammation and pruritus 2
- Oral antihistamines (loratadine 10mg daily) for symptomatic itch relief, particularly at night 4
- Avoid scratching to prevent secondary infection and lichenification 2
Step 2: Identify and Eliminate Causative Agent
- Take detailed exposure history: new soaps, detergents, jewelry, plants, occupational chemicals, topical medications 1, 2
- Determine if problem resolves with avoidance of suspected substance 1
- The forearm location suggests contact with environmental allergens or irritants during daily activities 4
Step 3: Reassess at 2 Weeks
If improving:
- Continue topical corticosteroid taper over 1-2 additional weeks 1
- Maintain avoidance of identified trigger 1
If not improving or worsening:
- Consider patch testing to identify specific allergen (approximately 3000 potential sensitizers exist) 2
- Rule out secondary bacterial infection requiring antibiotic therapy 2
- Consider alternative diagnoses including prurigo, arthropod reaction, or drug-induced eruption 6, 5
Critical Pitfalls to Avoid
Do not use systemic steroids for localized disease: Oral prednisone is reserved for extensive involvement (>20% body surface area) 1. For localized forearm lesions, topical therapy is sufficient and avoids systemic side effects.
Do not discontinue steroids abruptly: If systemic steroids become necessary for severe widespread disease, taper over 2-3 weeks to prevent rebound dermatitis 1.
Do not ignore persistent lesions: Failure to respond to appropriate topical therapy after 2-4 weeks warrants further evaluation, including possible biopsy to exclude other diagnoses 4.
Do not overlook secondary infection: Monitor for signs of bacterial superinfection (increased warmth, purulence, spreading erythema) which requires antibiotic therapy in addition to topical corticosteroids 2.