PG is characterized by the rapid appearance of sterile pustules, tender erythematous nodules, or rarely, bullae. These primary lesions degrade into a painful ulcer with characteristic undermined, violaceous borders (Fig. 64-1), which may heal with cribriform scarring.^1,2 Multiple ulcers can occur, especially in chronic cases.³ It most commonly occurs on the lower extremities or trunk, but involvement of the hands, feet, genitalia, and peristomal skin has been reported.^3,4 Adults are much more commonly affected than children, and in certain studies, females are more commonly affected than males.³ PG is clinically classified into four different subtypes, including ulcerative, pustular, bullous, and vegetative.⁵

Approximately 1 in 100,000 persons are affected with PG each year in the United States. All ages and both sexes might be affected, but the peak of disease incidence is in the fourth and fifth decades of life and there is a slight female predominance. Only 3% to 4% of cases occur in children.

The exact cause of PG has not been determined. It is likely that PG results from a dysregulation of the immune system and an abnormal wound healing response. In contrast to normal healing wounds, matrix metalloproteinases (MMPs)-9 and -10 as well as tumor necrosis factor-alpha (TNF-α are upregulated in the stroma of PG lesions. Additionally, MMP-1 and -26, which are present in normal healing epithelium, are greatly diminished in PG.⁶ Interleukin-8 (IL-8), a chemotactic and activating agent for neutrophils, is also produced at abnormal levels by lesional fibroblasts in PG patients, who also have elevated levels of serum IL-8. It has been proposed that PG is a clinical reaction to excess, inappropriately secreted circulating IL-8, particularly in the setting of hematologic malignancy.⁷ However, abnormalities in chemokines such as TNF-α and IL-8 are not specific to PG, and can be seen in other inflammatory conditions.^8,9

The histologic features of PG are nonspecific and can vary depending on the stage of the lesion as well as the clinical subtype. In ulcerative PG, there is typically central neutrophilic abscess formation (Fig. 64-2A) as well as a variable distal angiocentric lymphocytic inflammatory infiltrate. Pustular PG is characterized by a subcorneal pustule, with a perifollicular neutrophilic infiltrate or a dense dermal neutrophilic infiltrate with subepidermal edema. In bullous lesions, there is subepidermal bulla formation, intraepidermal vesiculation, and a variable dermal neutrophilic infiltrate. The vegetative form of PG demonstrates pseudoepitheliomatous hyperplasia, dermal neutrophilic abscess formation, sinus tracts, and a palisading granulomatous reaction.⁵ Varying degrees of leukocytoclastic vasculitis may be seen (Fig. 64-2B).¹⁰ While a biopsy of a PG lesion cannot give a definitive diagnosis, inspection for features consistent with PG, as well as negative stains for microorganisms can help lead to the diagnosis. Generally, in spite of the risk of pathergy, a skin biopsy for histology and a tissue culture are recommended to diagnose PG.⁵