A 35-year-old woman with no significant medical history presented with a painful ulcer on the right pretibial area. It started as a small pustule that subsequently evolved into a rapidly enlarging ulcer over the course of several weeks. She may have injured the area after falling from her bicycle just prior to the onset of the lesion. A comprehensive review of systems is notable for intermittent crampy abdominal pain and diarrhea for the past year, which had been attributed to irritable bowel syndrome. Laboratory evaluation shows only a mild anemia, but biopsy samples from the terminal ileum are consistent with Crohn disease. A skin biopsy from the edge of the ulcer itself is nondiagnostic. The ulcer improves after initiation of therapy with infliximab for her Crohn disease. Pyoderma gangrenosum is suspected.

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  The incidence of the disease is 1 patient in 100,000 people per year.¹

  
  
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  While pyoderma gangrenosum (PG) can occur at any age, it is most common in women aged 20 to 50 years old, and children represent only about 4% of cases.^2,3

  
  
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  In about half of patients, PG is idiopathic.²

  
  
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  Roughly half of patients have an underlying disease association, most commonly inflammatory bowel disease, followed by inflammatory arthritis and hematologic disorders, such as acute myelogenous leukemia, chronic myelogenous leukemia, or myelodysplasia.²

  
  
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  Other less common disease associations include human immunodeficiency virus (HIV), hepatitis C, systemic lupus erythematosus, and Takayasu arteritis.^4,5

  
  
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  The condition may occur in several syndromes, including pyogenic arthritis, pyoderma gangrenosum, acne (PAPA) syndrome; synovitis, acne, pustulosis, hyperostosis, osteitis (SAPHO) syndrome, and chronic recurrent multifocal osteomyelitis.³

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  An alteration in neutrophil chemotaxis is most likely involved, but the pathophysiology is not well characterized.¹

  
  
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  Defects in cell-mediated immunity, humoral immunity, and monocyte function have also been reported; however, no consistent association has been determined.⁴

  
  
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  Cutaneous lesions are initiated and aggravated by trauma (eg, needle sticks or other minor injuries), a phenomenon known as pathergy (Figure 94-1).⁴

Clinical Features

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  The classic presentation is a painful ulcer with a purulent base and irregular edges (Figures 94-2,94-3,94-4,94-5,94-6,94-7,94-8, and 94-9).^1,2

  
  
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  The border of the ulcer classically appears undermined with a gunmetal gray color.²

  
  
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  Lesions may start as pustules, bullae, or nodules that subsequently undergo necrosis leading to the classic ulcerated appearance.²

  
  
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  Ulcers often expand rapidly and coalesce with one another but may present with a more chronic, slowly enlarging course.²

  
  
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  Particularly in peristomal lesions, crater-like holes representing the openings of fistulous tracts that may drain pus can be seen.⁶

  
  
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  After healing, the lesions leave atrophic, cribriform, hyperpigmented scars (Figures 94-10 and 94-11).^2,6