Pyoderma Gangrenosum and Neutrophilic Dermatoses
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Direct answer
Pyoderma gangrenosum is a painful, rapidly enlarging, sterile ulcer with a violaceous and undermined border, and it belongs to the group of neutrophilic dermatoses. It is strongly linked with inflammatory bowel disease, rheumatoid arthritis and myeloid haematological disorders. The diagnosis rests on excluding infection and recognising the characteristic ulcer and pathergy, and systemic corticosteroids are the usual first-line treatment.
What you must remember
- Pyoderma gangrenosum: a painful ulcer with a violaceous, undermined edge; it is a diagnosis of exclusion, made only after infection and other causes of ulceration are ruled out.
- Pathergy: new skin breakdown after minor trauma, such as a needle prick; a key clue in pyoderma gangrenosum and Behçet disease.
- Sweet syndrome: an acute febrile neutrophilic dermatosis with tender erythematous plaques, fever and neutrophilia, linked with acute myeloid leukaemia, infection and drugs such as G-CSF.
- Systemic associations: inflammatory bowel disease, rheumatoid arthritis and myeloid haematological disorders are the classic links for pyoderma gangrenosum.
- Histology: a dense dermal neutrophilic infiltrate without vasculitis; the biopsy is taken from the active edge, with cultures to exclude infection.
- First-line treatment: systemic corticosteroids; second-line options include ciclosporin and biologics such as infliximab.
- Surgical caution: debridement or surgery can trigger pathergy and enlargement, so it is avoided during active disease.
Comparing the two classic neutrophilic dermatoses
Neutrophilic dermatoses are sterile skin conditions in which neutrophils infiltrate the dermis without infection being the cause. Pyoderma gangrenosum and Sweet syndrome are the two members most often compared in clinical practice and in exams, and their key differences are set out below.
| Feature | Pyoderma gangrenosum | Sweet syndrome |
|---|---|---|
| Lesion | Painful ulcer with violaceous, undermined edge | Tender erythematous plaques or papules |
| Typical site | Lower legs, often at a site of trauma | Face, neck, upper limbs and trunk |
| Systemic features | Usually absent | Fever and neutrophilia |
| Pathergy | Present in a minority (roughly a quarter to a third) | Occasionally present |
| Common association | Inflammatory bowel disease, arthritis | Acute myeloid leukaemia, infection, drugs |
| Course | Progressive, can take months to heal | Often settles within weeks with treatment |
| First-line treatment | Systemic corticosteroids | Systemic corticosteroids |
Both conditions respond to corticosteroids, so treatment does not separate them, and the morphology of the lesion is the first clue to the diagnosis. The presence or absence of fever and neutrophilia then directs the work-up toward bowel disease or a haematological malignancy, which is where the diagnosis is most often missed in practice and in exam answers.
Common slips in management
Exam questions usually test a management error or a missed association.
- Trap: debriding a pyoderma gangrenosum ulcer to speed healing Correct: surgical trauma provokes pathergy and enlargement, so systemic immunosuppression is used instead.
- Trap: treating an enlarging ulcer with antibiotics alone Correct: pyoderma gangrenosum is sterile and does not resolve with antibiotics, so infection is excluded and corticosteroids are started.
- Trap: treating Sweet syndrome as an infection because of the fever Correct: fever and neutrophilia are part of Sweet syndrome, and corticosteroids, not antibiotics, are the treatment.
- Trap: skipping a haematological review in a patient with Sweet syndrome Correct: acute myeloid leukaemia is the classic malignancy link, so blood counts and a search for malignancy are needed.
Frequently asked questions
What is pathergy and which diseases show it?
Pathergy is the development of a new pustule or ulcer at a site of minor skin trauma, such as a needle prick or a biopsy site. It is a classic feature of pyoderma gangrenosum and is also seen in Behçet disease. Its presence supports the diagnosis and warns against surgical debridement.
Which systemic disease most often accompanies pyoderma gangrenosum?
Inflammatory bowel disease, especially ulcerative colitis and Crohn's disease, is the most common systemic association of pyoderma gangrenosum. Rheumatoid arthritis and myeloid haematological disorders are also linked with it. The skin ulcer may appear before or after the bowel disease is diagnosed.
How does Sweet syndrome differ from pyoderma gangrenosum?
Sweet syndrome is an acute febrile illness with tender erythematous plaques and neutrophilia, whereas pyoderma gangrenosum is a painful ulcer without systemic features. Sweet syndrome is strongly linked with acute myeloid leukaemia, infection and drugs such as G-CSF. Both respond to corticosteroids, but the clinical pattern guides the rest of the work-up.
Why is the biopsy for pyoderma gangrenosum taken from the edge?
The active, advancing edge of pyoderma gangrenosum shows the most typical neutrophilic inflammation, so a biopsy taken there has the greatest diagnostic value. Cultures from the same site help exclude infection. Clinicians should warn the patient that the wound may enlarge after any procedure because of pathergy.