What is the differential diagnosis for a skin inflammatory process consisting of painful annular lesions, 3 to 6 cm in diameter, without central clearing, with some lesions having slight pus collection?

Differential Diagnosis of Painful Annular Lesions Without Central Clearing

The most critical consideration for painful annular lesions 3-6 cm in diameter with pustule formation and no central clearing is pyoderma gangrenosum, followed by bacterial infections (particularly in immunocompromised hosts), Sweet's syndrome, and atypical presentations of cutaneous leishmaniasis.

Primary Differential Diagnoses

Pyoderma Gangrenosum (PG)

• PG characteristically begins as erythematous papules or pustules that rapidly progress to deep excavating ulcerations containing purulent material that is sterile on culture unless secondary infection has occurred 1
• Lesions are often preceded by trauma (pathergy phenomenon) and can occur anywhere on the body, though commonly on the shins 1
• The presence of pain and pustule formation in annular lesions of this size strongly suggests PG, particularly if the patient has inflammatory bowel disease or other systemic conditions 1
• Initial lesions may appear as single or multiple erythematous papules or pustules before developing into the characteristic deep ulcerations 1

Sweet's Syndrome (Acute Febrile Neutrophilic Dermatosis)

• Presents as tender, red inflammatory nodules or papules, typically affecting upper limbs, face, or neck 1
• Part of the acute neutrophilic dermatoses group and can be distinguished from PG by distribution and histological features showing neutrophilic infiltrate 1
• Strong predilection for women and patients with colonic involvement 1

Bacterial Infections in Immunocompromised Hosts

• Ecthyma gangrenosum begins as painless erythematous macules that rapidly become painful and necrotic within 12-24 hours, increasing from 1 cm to >10 cm in <24 hours 1
• Classically associated with Pseudomonas aeruginosa but can occur with other gram-negative organisms, S. aureus, Stenotrophomonas maltophilia, and fungi 1
• Represents cutaneous vasculitis with bacterial invasion of vessel walls 1
• Dermatologic manifestations of gram-negative infections include erythematous maculopapular lesions, focal or progressive cellulitis, and cutaneous nodules 1

Cutaneous Leishmaniasis (CL)

• Can present with atypical, multifocal lesions, particularly in immunocompromised patients 1
• Purulence is not typical of CL unless secondarily infected with bacteria (suppurative staphylococcal or impetiginous streptococcal superinfection) 1
• The differential diagnosis includes cutaneous fungal and mycobacterial infections, pyoderma gangrenosum, and spider bites 1

Secondary Considerations

Erythema Annulare Centrifugum (EAC) - Atypical Presentation

• Typically spreads centrifugally with central clearing, but atypical presentations exist 2, 3
• Usually lacks the pain and pustule formation described in this case 2
• May be associated with infections, drugs, or underlying conditions 2, 4

Granuloma Annulare - Unlikely

• Characterized by nonscaly annular plaques with indurated borders 3
• Typically painless and lacks pustule formation 3
• Does not fit the clinical picture of painful lesions with pus collection 3

Tinea Corporis - Less Likely

• While annular, typically presents with raised, scaly borders and central clearing 3, 5
• The absence of central clearing and presence of pain with pustules makes this diagnosis less probable 1, 3

Critical Diagnostic Approach

Immediate Assessment Required

• Evaluate for systemic toxicity: fever >38.5°C, tachycardia >110 bpm, hypotension, or signs of SIRS 6
• Assess immune status: neutropenia, HIV status, immunosuppressive medications, chemotherapy 1
• Document precise lesion morphology, distribution, and progression rate 7

Essential Investigations

• Culture purulent material for bacteria and fungi (though sterile in PG unless secondarily infected) 1
• Skin biopsy from lesional edge for histopathology and direct immunofluorescence 1, 7
• Blood cultures if systemic signs present 1
• Complete blood count to assess for neutropenia 1

Common Pitfalls to Avoid

• Do not assume all pustular annular lesions are infectious; PG is frequently misdiagnosed as bacterial infection and inappropriately treated with antibiotics alone 1
• Avoid delaying biopsy in atypical presentations, as histopathology is essential for distinguishing between neutrophilic dermatoses and infectious etiologies 1
• Do not overlook the possibility of secondary bacterial infection in non-infectious inflammatory conditions 1
• In immunocompromised patients, the differential expands significantly to include disseminated fungal infections (Aspergillus, Mucor, Fusarium) and Nocardia, which require different management approaches 8