Development of Pyoderma Gangrenosum in a Patient with Acquired Reactive Perforating Collagenosis.

Introduction: Pyoderma gangrenosum (PG) is a rare neutrophilic dermatosis associated with systemic diseases, but its pathogenesis remains unclear. Acquired reactive perforating collagenosis (ARPC) is characterized by transepidermal elimination of degenerated collagen and is usually linked to diabetes mellitus or renal dysfunction. ARPC is not recognized as a precursor to PG. We report a rare case of PG occurring in a patient with ARPC, successfully treated with adalimumab. Case Presentation: An 80-year-old woman with diabetes presented with generalized erythema and later developed multiple papules and nodules with central crusting. Histology confirmed ARPC. Despite topical corticosteroids, lesions ulcerated and progressed into painful necrotic ulcers on her extremities. Laboratory findings showed neutrophilia and elevated C-reactive protein, but no infection or autoimmune markers. Histopathology of the ulcers revealed dense neutrophilic infiltration with transepidermal elimination of collagen, consistent with PG. Diagnostic criteria (Delphi, Su, Paracelsus) supported PG secondary to ARPC. Topical corticosteroids were ineffective. Due to comorbid diabetes, high-dose systemic corticosteroids were avoided; instead, low-dose prednisolone with adalimumab was initiated. The patient received adalimumab induction (160 mg at week 0, 80 mg at week 2) followed by 40 mg weekly. The ulcers rapidly improved, and by 12 weeks, near-complete epithelialization was achieved without recurrence during steroid tapering. Conclusion: This case represents PG occurring in a patient with ARPC, suggesting that persistent epidermal injury in ARPC may trigger neutrophilic dermatosis. Adalimumab proved effective, offering a safe therapeutic option in PG patients with comorbidities limiting corticosteroid use. Recognition of this association may aid earlier diagnosis and optimize treatment strategies.