DEFINITION

A neutrophilic dermatosis characterized by rapidly progressive, painful, necrotic ulcers with undermined violaceous borders, often associated with systemic disease.

ETIOPATHOGENESIS

• Primary mechanism: Dysregulated innate immunity with abnormal neutrophil chemotaxis and activation

• Not infectious despite the name

• Key drivers:

  • ↑ neutrophil recruitment (IL-8, TNF-α)

  • Abnormal inflammasome activity

  • Immune complex–mediated component in some cases

• Pathergy phenomenon: Minor trauma → exaggerated neutrophilic response → ulcer formation

Associations (high-yield):

• Inflammatory bowel disease (especially ulcerative colitis)

• Rheumatoid arthritis

• Hematological disorders (especially monoclonal gammopathy, leukemia)

• Seronegative arthritis

CLINICAL FEATURES

• Begins as pustule, papule, or nodule → rapidly breaks down into ulcer

• Ulcer characteristics:

  • Painful

  • Necrotic base

  • Undermined, violaceous borders (key sign)

• Common sites: legs (especially pretibial)

• Variants:

  • Ulcerative (classic)

  • Bullous (associated with hematologic malignancy)

  • Pustular (IBD-associated)

  • Vegetative (superficial, less aggressive)

HISTOPATHOLOGY

1. FOUNDATIONS

• Epidermis: Barrier layer (keratinocytes) protecting against trauma and microbes

• Dermis: Contains vasculature, collagen, immune cells

• Neutrophils: First-line responders in acute inflammation; phagocytosis and enzyme release

• Vessels: Maintain tissue perfusion; susceptible to immune-mediated injury

2. INITIATING EVENT

• Trigger: Trauma, systemic inflammation, or unknown

• Early event: Exaggerated neutrophilic recruitment into skin

• Driven by cytokines (IL-1, IL-8, TNF-α)

3. PATHOGENESIS

1. Dysregulated immune signaling → excessive neutrophil chemotaxis

2. Neutrophils infiltrate dermis → release proteolytic enzymes (elastase, MPO)

3. Tissue destruction → dermal necrosis

4. Secondary vascular damage → ischemia worsens necrosis

5. Epidermal breakdown → ulcer formation

4. HISTOPATHOLOGY EXPLAINED

Early lesion:

• Dense neutrophilic infiltrate in dermis

  • Normally neutrophils clear infection; here they are excessive and unregulated

• Sterile pustules (no organisms)

Established lesion:

• Dermal necrosis

• Mixed inflammatory infiltrate (neutrophils ± lymphocytes, histiocytes)

• Secondary vasculitis-like changes

  • Not primary vasculitis; vessel damage is secondary to neutrophilic attack

Ulcer stage:

• Epidermal ulceration

• Undermined edges correspond to lateral spread of neutrophilic destruction beneath epidermis

5. TEMPORAL EVOLUTION

• Early: Neutrophilic dermal infiltrate

• Intermediate: Tissue necrosis, abscess formation

• Late: Ulceration + granulation tissue + fibrosis

6. NAMING LOGIC & TERMINOLOGY

• “Pyoderma” → misleading (no infection)

• “Gangrenosum” → reflects necrotic ulceration resembling gangrene

• Pathergy: exaggerated response to minor trauma

7. STAINING & MARKERS

• H&E:

  • Neutrophils → multilobed nuclei, pale cytoplasm

  • Necrosis → eosinophilic debris

• Special stains:

  • Gram, PAS, Ziehl–Neelsen → negative (important to exclude infection)

• No specific immunohistochemical marker (diagnosis is clinical + exclusion)

8. PATTERN RECOGNITION & DIAGNOSTIC LOGIC

• Key pattern: Sterile neutrophilic dermatosis with ulceration

Diagnostic approach:

• Painful ulcer + violaceous undermined edge → suspect PG

• Exclude:

  • Infection (bacterial, fungal, mycobacterial)

  • Vasculitis

  • Malignancy (e.g., cutaneous lymphoma)

If neutrophils + no organisms + systemic association → think PG

9. CLINICO-PATHOLOGICAL CORRELATION

• Neutrophil-mediated destruction → rapid ulcer expansion

• Undermined border → lateral dermal destruction beneath intact epidermis

• Pain → inflammatory cytokines + tissue necrosis

• Pathergy → explains worsening after surgery or trauma

DIFFERENTIAL DIAGNOSIS

• Cutaneous vasculitis (true vessel wall inflammation)

• Necrotizing infections

• Venous ulcers

• Malignancy (Marjolin ulcer, lymphoma)

• Sweet syndrome (but non-ulcerative plaques)

MANAGEMENT

General principle:

Suppress neutrophilic inflammation

First-line:

• Systemic corticosteroids (rapid anti-inflammatory effect)

Steroid-sparing:

• Cyclosporine (T-cell suppression → ↓ cytokines driving neutrophils)

Others:

• TNF-α inhibitors (e.g., infliximab) — especially in IBD-associated cases

• Dapsone (neutrophil inhibition)

• Methotrexate / azathioprine

Local care:

• Gentle wound care

• Avoid trauma (due to pathergy)

Important exam point:
Debridement may worsen lesion due to pathergy

PROGNOSIS

• Chronic, relapsing course

• Improves with treatment of underlying disease

• Scarring common

EXAM-FOCUSED INSIGHTS

• Painful ulcer + violaceous undermined edge = classic clue

• Strong association with IBD

• Pathergy is key diagnostic feature

• Histology is non-specific but neutrophil-dominant

• Diagnosis = clinical + exclusion

MUST-KNOW QUESTIONS

1. What is the hallmark clinical feature of pyoderma gangrenosum?
   Painful ulcer with undermined violaceous border

2. What type of dermatosis is PG?
   Neutrophilic dermatosis

3. Is PG infectious?
   No (sterile condition)

4. What is pathergy?
   Exaggerated lesion formation after minor trauma

5. Most common systemic association?
   Inflammatory bowel disease

6. Key histological feature?
   Dense neutrophilic infiltrate without organisms

7. Is vasculitis primary in PG?
   No, vascular damage is secondary

8. First-line treatment?
   Systemic corticosteroids

9. Why should surgical debridement be avoided?
   It may worsen lesions due to pathergy

10. Bullous PG is associated with which condition?
    Hematologic malignancy

11. Main cytokines involved?
    IL-8, TNF-α

12. What must always be excluded before diagnosing PG?
    Infection