Acquired Perforating Dermatosis (APD) Masquerading as Calciphylaxis in a Patient with ESKD and Uncontrolled Secondary Hyperparathyroidism

Introduction: APDs are uncommon pruritic disorders characterized by trans epidermal elimination of dermal material and occur mainly in patients with chronic kidney disease or diabetes. Their hyperkeratotic papules can mimic calciphylaxis, resulting in a diagnostic delay. Case Description: 45-y-o African-American woman with lupus nephritis ESKD on thrice-weekly HD (4 h) ×4 y, missed sessions for 4–5 months (spKt/V < 1.2) and nonadherent to diet and phosphate binders. She developed worsening leg pain and papulonodular lesions below the knees. Labs in Fig1; afebrile, no systemic symptoms/allergies. Skin exam: multiple umbilicated hyperkeratotic papules, absent livedo/retiform purpura (Fig2). Calciphylaxis suspected, but punch biopsy showed vertically oriented collagen crossing ulcerated epidermis—diagnostic of APD without vascular calcification (Fig3). Management and Outcome: Dialysis frequency was restored to thrice weekly with a goal Kt/V ≥ 1.4, a non-calcium phosphate binder, and topical clobetasol plus oral antihistamines relieved pruritus. Serum phosphate fell to 5.6 mg/dL and PTH to 680 pg/mL by week 12; skin pain improved and lesions flattened, leaving post-inflammatory hyperpigmentation (Fig4). Discussion: APD, seen in 4–11 % of end-stage kidney disease cases, should be suspected when papules appear in haemodialysis patients with poorly controlled chronic kidney disease–mineral-bone disorder; severe hyperphosphataemia, a high calcium–phosphate product and secondary hyperparathyroidism trigger collagen degeneration, intractable pruritus and transepidermal elimination that can masquerade as calciphylaxis. Prompt skin biopsy differentiates the two entities, sparing patients unnecessary calciphylaxis therapy and redirecting management to improving dialysis adequacy, normalising mineral metabolism and treating itch.