What is the differential diagnosis of papular purple lesions on the anterior face of the shins and feet?

Differential Diagnosis of Papular Purple Lesions on Anterior Shins and Feet

The most likely diagnosis is erythema nodosum, which classically presents as tender, erythematous to purplish nodules on the anterior lower extremities, particularly the shins, and represents a septal panniculitis without vasculitis. 1, 2, 3

Primary Differential Diagnoses

Erythema Nodosum (Most Common)

• Presents as tender, raised, erythematous nodules that evolve through color changes from bright red to livid red or purplish, and finally to yellow or greenish (resembling deep bruises) over days to weeks 2
• Lesions are typically bilateral and symmetrically distributed on the anterior shins, ankles, and knees 2, 3
• Never ulcerates and heals without scarring or atrophy, which is a key distinguishing feature 2, 3
• Represents a septal panniculitis without vasculitis, confirmed by deep incisional or excisional biopsy showing thickened septa with inflammatory infiltrate and characteristic Miescher's radial granulomas 2, 3
• Most commonly associated with streptococcal pharyngitis, sarcoidosis, inflammatory bowel disease, tuberculosis, medications (oral contraceptives, antibiotics), pregnancy, and malignancy 3, 4

Cutaneous Lymphoproliferative Disorders

• Lymphomatoid papulosis presents as recurrent grouped or disseminated papulonodular lesions that spontaneously regress within weeks or months, leaving scars and hyperpigmentation 5, 1
• Primary cutaneous diffuse large B-cell lymphoma, leg type, manifests as red to bluish-red tumors specifically on the legs in elderly patients 5
• These lesions are typically non-tender (unlike erythema nodosum) and require biopsy showing CD30+ or CD20+ cells depending on subtype 5

Pyoderma Gangrenosum

• Initially presents as erythematous papules or pustules that rapidly progress to deep excavating ulcerations with sterile purulent material 6, 1
• Ulceration is the hallmark feature that distinguishes it from erythema nodosum 6
• Often preceded by trauma (pathergy phenomenon) 1

Cutaneous Sarcoidosis

• Specific granulomatous lesions include maculopapules, subcutaneous nodules, plaques, and lupus pernio 7
• When associated with erythema nodosum and bilateral hilar lymphadenopathy, constitutes Löfgren's syndrome with good prognosis 1, 7
• Biopsy reveals non-caseating granulomas 7

Sweet's Syndrome

• Characterized by tender, red inflammatory nodules or papules, with strong predilection for upper limbs, face, or neck (less commonly lower extremities) 1
• Associated with systemic symptoms and responds to corticosteroids 1

Diagnostic Approach

Initial Clinical Assessment

• Document lesion characteristics: tenderness, color evolution, presence or absence of ulceration, symmetry, and exact anatomic distribution 2, 3
• Assess for systemic symptoms: fever, malaise, upper respiratory symptoms, joint pain, gastrointestinal symptoms 3, 4
• Obtain medication history: oral contraceptives, antibiotics, and other potential drug triggers 3

Laboratory Evaluation

• Complete blood count with differential, erythrocyte sedimentation rate, and C-reactive protein 3
• Streptococcal testing: throat culture, rapid antigen test, or anti-streptolysin-O titer 3
• Stratify tuberculosis risk and perform purified protein derivative test with chest radiography if indicated 3
• Consider stool cultures if inflammatory bowel disease suspected 3

Tissue Diagnosis

• Deep incisional or excisional biopsy is essential for definitive diagnosis, particularly to distinguish erythema nodosum (septal panniculitis without vasculitis) from other panniculitides and lymphoproliferative disorders 2, 3
• Biopsy from lesion periphery helps exclude other disorders when pyoderma gangrenosum is considered 6
• Skin scraping with KOH preparation to exclude fungal infections 1

Common Pitfalls

• Avoid surgical debridement if pyoderma gangrenosum is suspected, as pathergy can worsen lesions 6
• Do not assume all anterior shin lesions are erythema nodosum—the presence of ulceration, lack of tenderness, or failure to resolve within 3-6 weeks should prompt consideration of alternative diagnoses 2, 4
• Erythema migrans from Lyme disease can be partially purpuric on lower extremities, but typically has expanding borders and occurs at tick bite sites 5
• Misdiagnosis occurs frequently—biopsy confirmation is critical when clinical presentation is atypical 6