Polyangiitis overlap syndrome is a high-risk clinical phenotype for relapse: Case series from the KEIO-vasculitis cohort.

Polyangiitis overlap syndrome, characterized by the coexistence of giant cell arteritis (GCA) and anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV), is a rare and poorly understood condition. Clinical features, treatment responses, and long-term prognosis of this entity remain unclear. This study aimed to elucidate clinical characteristics, relapse patterns, and potential therapeutic strategies for polyangiitis overlap syndrome. We retrospectively analysed consecutive patients with GCA or AAV at our institution between January 2012 and September 2024 and identified cases of polyangiitis overlap syndrome. Clinical data, including symptoms, laboratory findings, treatment regimens, and outcomes, were extracted from medical records and analysed descriptively. Among 60 GCA and 151 AAV cases, we identified six cases of polyangiitis overlap syndrome. The median age at onset was 68 years (range: 54-75), and 33% were female. Microscopic polyangiitis was the most common AAV subtype (50%). Fever was the predominant symptom (83%), and kidney involvement was observed in 83%, followed by pulmonary involvement (67%). All patients received high-dose glucocorticoids; four received cyclophosphamide, and two received tocilizumab as induction therapy. Over a median follow-up of 32 months (range: 5-122), four of six patients (67%) experienced relapse, all due to AAV-related activity. Notably, two patients who received tocilizumab as maintenance therapy remained in remission with minimal glucocorticoid exposure. Polyangiitis overlap syndrome represents a high-risk phenotype for relapse, driven by AAV activity. Our findings suggest that IL-6 receptor blockade is a potential therapeutic option, highlighting the need for further research to establish optimal maintenance strategies in this complex disease entity.