PO:37:268 | Antineutrophil cytoplasmic antibodies-associated vasculitis mimicking giant cell arteritis: a diagnostic dilemma

Bruno Biasi¹, Elena Vanni¹, Marco De Pinto², Giulia Cassone², Dilia Giuggioli^1|2. | ¹Rheumatology Unit, School of Medicine, University of Modena and Reggio Emilia, Modena, Modena, Italy; ²Rheumatology Unit, Azienda Ospedaliera-Universitaria Policlinico di Modena, Modena, Italy, Modena, Italy.

Background. Here we present a diagnostically challenging case of ANCA-associated vasculitis (AAV) that initially presented with clinical features highly suggestive of giant cell arteritis (GCA).

Case Report. An 80-year-old woman, with a medical history notable only for prior COVID-19 pneumonitis and a diagnosis of polymyalgia rheumatica (PMR) in 2024, presented to the emergency department in February 2025 with a three-week history of worsening pelvic and scapular girdle pain. Routine laboratory tests revealed elevated inflammatory markers (ESR 76 mm/h, CRP 16.8 mg/dL) and findings consistent with a urinary tract infection (UTI), with preserved renal function (GFR >60 mL/min). A presumptive diagnosis of a PMR flare triggered by the UTI was made, and antibiotic therapy was initiated. Due to the persistence of symptoms, she was referred to the rheumatology unit. A positive history of recent unintentional weight loss and new-onset jaw claudication emerged. Given the high clinical suspicion of giant cell arteritis (GCA), corticosteroid therapy was promptly initiated. Subsequent vascular ultrasound revealed intima-media complex thickening and a positive compression sign in the left frontal artery, along with moderate intima-media thickening of the right facial artery. While suggestive of GCA, these findings were insufficient for a definitive diagnosis. Following an initial symptom remission, corticosteroid tapering led to a relapse of jaw claudication and the new onset of widespread livedo reticularis. Additionally, previously absent signs of acute kidney injury (AKI) emerged, prompting hospital admission for further diagnostic workup. Laboratory testing revealed persistently elevated ESR and CRP levels, significant renal impairment (creatinine 1.67 mg/dL, GFR 29 mL/min), and positive anti-MPO antibodies (37.0 UI/mL). Over the following days, renal function deteriorated rapidly (creatinine rising to 2.2 mg/dL). Given the progressive AKI and ANCA-MPO positivity, a diagnosis of ANCA-associated vasculitis (AAV) with glomerulonephritis was established. Treatment included three intravenous methylprednisolone pulses followed by two 1 g rituximab infusions administered two weeks apart. A PET-CT scan performed shortly after steroid initiation showed no evidence of large vessel involvement. Furthermore, chest X-ray and electromyography revealed no abnormalities suggestive of other organ manifestations of AAV. At one-month follow-up, renal function had sligthly improved (creatinine 1.5 mg/dL, GFR 33 mL/min), while inflammatory markers and anti-MPO titers (5 UI/mL) had significantly decreased. The coexistence or sequential development of GCA and AAV, though rare, has been documented [Tab 1]. In our case, the absence of a temporal artery biopsy prevents the definitive exclusion of GCA and raises the possibility of an overlap syndrome, rather than an atypical manifestation of AAV mimicking cranial vasculitis.

Conclusions. This case highlights the importance of considering alternative diagnoses in patients with atypical GCA presentations, particularly when temporal artery biopsy is unavailable.