Catastrophic APS — SCE Dermatology MCQ

Correct answer: E — Catastrophic antiphospholipid syndrome (CAPS)

The correct answer is E, catastrophic antiphospholipid syndrome (CAPS). This woman has known SLE and antiphospholipid syndrome and develops rapidly progressive retiform purpura with skin necrosis despite therapeutic anticoagulation, with biopsy showing non-inflammatory thrombotic vasculopathy across multiple vessel calibres, the hallmark histology of CAPS. CAPS is a rare, life-threatening acceleration of APS driven by widespread microvascular and small to medium vessel thrombosis rather than vasculitis, typically triggered in a patient with established aPL positivity or connective tissue disease such as SLE. Progression on adequate anticoagulation reflects the intense thrombotic microangiopathy of CAPS, which requires escalation to combined anticoagulation, high dose corticosteroids and plasma exchange or IVIG rather than anticoagulation alone. The absence of inflammatory infiltrate on biopsy excludes a vasculitic process and supports a thrombotic microangiopathic mechanism. Why the other options are wrong: D. TEN: Toxic epidermal necrolysis is a drug induced, full thickness epidermal necrolysis with mucosal involvement and a positive Nikolsky sign, driven by keratinocyte apoptosis, not vascular thrombosis, and biopsy shows epidermal necrosis rather than thrombotic vasculopathy. A. Warfarin necrosis: This occurs within days of starting warfarin due to transient protein C deficiency, typically in fatty areas (breasts, buttocks, thighs); this patient is already anticoagulated and worsening, not newly started on warfarin. C. Calciphylaxis: This is calcium deposition causing vascular occlusion, almost always in the context of end stage renal disease and deranged calcium phosphate metabolism, which is not described here. B. DIC alone: DIC produces consumptive coagulopathy with abnormal clotting screen and bleeding as well as thrombosis, but does not explain the underlying multi-calibre thrombotic vasculopathy specific to aPL mediated CAPS, and CAPS can occur with or without overt DIC. Key point: Progressive retiform purpura with non-inflammatory thrombotic vasculopathy in a known APS or SLE patient, unresponsive to anticoagulation, defines CAPS until proven otherwise.

Reference: British Society for Haematology, Arachchillage DJ et al, Guidelines on the investigation and management of antiphospholipid syndrome, Br J Haematol 2024;205(3):855-880