Navigating clinical overlap: A case of polyarteritis nodosa in the setting of antiphospholipid syndrome

Polyarteritis nodosa (PAN) is a systemic necrotising vasculitis of medium-sized arteries, often challenging to differentiate from other autoimmune and thrombotic disorders due to overlapping clinical features. This case-based literature review describes a 56-year-old male with a prior diagnosis of catastrophic antiphospholipid syndrome who presented with digital ischemia, myalgias, and a progressive cutaneous rash. Despite initial treatment with anticoagulation, symptoms persisted, and a skin biopsy ultimately revealed medium-vessel vasculitis consistent with PAN. Muscle biopsy was inconclusive, underscoring the difficulty of diagnosis in the setting of coexisting vasculopathies. This case illustrates the diagnostic complexity at the intersection of inflammatory and thrombotic syndromes and highlights the potential for concurrent or misclassified disease. A review of literature reveals rare but documented associations between PAN and antiphospholipid syndrome, yet no diagnostic tool for differentiation exists. Early recognition and accurate differentiation are critical to optimise outcomes in such overlapping syndromes. Future studies should evaluate a possible differentiation score or explore further diagnostics to help differentiate these inflammatory and thrombotic syndromes to better recognise and treat these conditions.