Diagnosis and Initial Management of Multiple Small Papules on the Forearm
The most likely diagnosis is lymphomatoid papulosis (LYP), and initial management should include obtaining a biopsy (at least 4 mm punch or spindle-shaped incisional biopsy) for histologic confirmation with immunophenotyping, followed by observation given the self-healing nature of this condition. 1
Clinical Reasoning
Why Lymphomatoid Papulosis is Most Likely
The presentation of multiple small papules clustered in a 5×4 cm area on the forearm strongly suggests LYP based on several key features:
LYP characteristically presents as recurrent grouped or disseminated papulonodular skin lesions that spontaneously regress within weeks or months, with each individual lesion self-healing even as new lesions may appear 1, 2
The grouped distribution pattern in a localized area is typical for this lymphoproliferative disorder, which can present with papules ranging from small to nodular 1
The forearm is a common site for LYP lesions, which can occur anywhere on the body 1
Differential Diagnoses to Consider
While LYP is most likely, other conditions in the differential include:
Contact dermatitis typically presents with erythematous, pruritic papules with visible borders and a clear history of exposure to irritants or allergens 3, 4
Cutaneous fungal infections usually begin as erythematous papules that may become pustular with central ulceration and elevated borders 2
Actinic keratoses on the forearm are often multiple and hyperkeratotic, but typically occur in older patients with significant sun exposure 1
Diagnostic Workup
Essential First Step: Biopsy
Obtain a punch biopsy of at least 4 mm diameter or a spindle-shaped incisional biopsy of adequate length and depth to allow appropriate histologic workup with immunophenotyping 1
The biopsy will reveal:
Wedge-shaped infiltrate with scattered or clustered CD30+ tumor cells intermingled with inflammatory cells (Type A, most common) 1
CD30+ tumor cells typically express CD4, though CD8+ phenotypes can occur, with variable loss of pan-T-cell antigens 1
This histologic pattern distinguishes LYP from other papular eruptions and confirms the diagnosis 1
Additional Diagnostic Considerations
If the biopsy does not confirm LYP:
Perform skin scraping with KOH preparation to rule out fungal infection if the clinical picture suggests this diagnosis 2
Consider patch testing if contact dermatitis remains in the differential and the causative substance is unclear 4, 5
Initial Management Strategy
For Confirmed Lymphomatoid Papulosis
Observation is the primary management approach given the self-healing nature of LYP:
Individual lesions spontaneously regress within weeks to months without intervention 1, 2
No immediate treatment is required for localized, asymptomatic lesions 1
Document the appearance, size, and location at baseline with photographs if possible 6
Important Monitoring Considerations
Long-term follow-up is essential because:
LYP may manifest concurrently with mycosis fungoides (MF), which presents with patches, plaques, or tumors 1
Patients require ongoing surveillance for development of associated lymphoproliferative disorders 1
Instruct the patient to report any non-healing lesions, rapidly growing nodules, or systemic symptoms 6
When to Consider Active Treatment
While observation is standard, treatment may be warranted if:
Lesions are cosmetically disturbing or symptomatic (though this is not specified in the current presentation) 1
New lesions continue to develop rapidly or fail to follow the expected self-healing pattern 1
Critical Pitfalls to Avoid
Do not mistake LYP for an infectious process requiring antibiotics, as the CD30+ lymphoproliferative nature requires different management 1
Do not perform superficial shave biopsies, as adequate depth is essential for proper histologic evaluation and immunophenotyping 1
Do not assume all grouped papules are benign without histologic confirmation, as early cutaneous lymphomas can present similarly 1
Ensure complete staging evaluation once LYP is confirmed to rule out extracutaneous involvement, though this is rare in true LYP 1