How to differentiate and manage bullous impetigo vs vasculitis vs pyoderma gangrenosum in the foot?

Differentiating Bullous Impetigo, Vasculitis, and Pyoderma Gangrenosum in the Foot

Pyoderma gangrenosum is a diagnosis of exclusion requiring you to rule out ecthyma gangrenosum (bacterial vasculitis requiring antibiotics) and bullous impetigo (bacterial infection), as misdiagnosis occurs in a substantial percentage of cases and wrong treatment—particularly surgical debridement or antibiotics alone for PG—will worsen the condition. 1, 2, 3

Diagnostic Algorithm

Step 1: Assess Pain and Progression Timeline

Bullous Impetigo:

• Painless or mildly tender vesicles/bullae that rupture easily, leaving honey-colored crusts 1
• Gradual onset over days
• Superficial, not deeply excavating 3

Ecthyma Gangrenosum (Bacterial Vasculitis):

• Initially painless erythematous papules progressing to painful necrotic lesions within 24 hours 1
• Extremely rapid progression (hours to 1 day)
• Occurs in immunocompromised/neutropenic patients 1

Pyoderma Gangrenosum:

• Severely painful from onset 4, 5
• Rapidly developing over days to weeks 3
• Deep excavating ulcerations with purulent material 3
• Violaceous, undermined borders 4

Step 2: Obtain Cultures and Biopsy

Critical distinction: Culture the lesion and perform biopsy from the periphery (not the center) 2, 3

• Positive bacterial culture = bullous impetigo or ecthyma gangrenosum (requires antibiotics, not immunosuppression) 1
• Sterile culture + non-specific neutrophilic infiltrate on biopsy = likely pyoderma gangrenosum 3, 6
• Biopsy helps exclude vasculitis (which shows vessel wall inflammation/necrosis), malignancy, and infection 2, 3

Step 3: Screen for Associated Systemic Disease

For pyoderma gangrenosum specifically:

• 50-70% have underlying systemic disorders 3
• Most common: inflammatory bowel disease (classic PG) or hematologic malignancy (bullous variant) 7
• Screen with colonoscopy, complete blood count, and comprehensive metabolic panel 3, 4

Management Based on Diagnosis

If Bullous Impetigo (Bacterial Infection):

• Topical mupirocin for localized disease
• Oral antibiotics (cephalexin, dicloxacillin) for extensive involvement
• Never use immunosuppression

If Ecthyma Gangrenosum (Bacterial Vasculitis):

• Urgent broad-spectrum antibiotics covering Pseudomonas (antipseudomonal beta-lactam + aminoglycoside) 1
• Immunosuppression is contraindicated and potentially fatal 1
• Requires infectious disease consultation

If Pyoderma Gangrenosum (Sterile Inflammatory Process):

First-line treatment:

• Systemic corticosteroids (prednisone 0.5-1 mg/kg/day) are the established first-line therapy 1, 2
• For localized disease (<5% body surface area, ≤3 lesions), topical tacrolimus 0.1% twice daily can be used as monotherapy, achieving complete remission in mean 6 weeks 1, 6

Second-line for steroid-refractory disease:

• Infliximab 5 mg/kg should be initiated if rapid response to corticosteroids is not achieved within 2-4 weeks 1, 2
• Response rates exceed 90% for disease duration <12 weeks but drop below 50% for chronic cases (>3 months) 2
• Adalimumab is an alternative anti-TNF option with demonstrated efficacy 1, 8

Critical pitfall to avoid:

• Never perform surgical debridement during active pyoderma gangrenosum—pathergy (trauma-induced worsening) is a defining feature and surgery will worsen the condition 1, 2
• Surgery should only be considered after complete remission or in neutropenic patients after marrow recovery 1

Monitoring and Prognosis

• Pyoderma gangrenosum requires clinical follow-up at least every 2 weeks during active treatment 2
• Lesions typically heal within 4 weeks after treatment discontinuation 2
• Recurrence occurs in >25% of cases, often at the same anatomical location 2, 3
• Mean treatment duration: 11.5 months for classic PG, 9 months for bullous variant 7