P191 Beyond the obvious in giant cell arteritis clinic: three diagnostic mimics

Abstract Background/Aims Suspected giant cell arteritis (GCA) is a frequent referral reason to Rheumatology services. Diagnosis is primarily clinical, supported by inflammatory markers, imaging, and histology, although none are definitive. Dedicated GCA clinic slots improve efficiency but may introduce diagnostic bias and limit consideration of alternative conditions. Methods We report three cases initially suspected as GCA that were subsequently identified as rarer mimics, with distinctive imaging features and atypical clinical features. Results Case 1 An 82-year-old male presented with five months of fatigue, weight loss, fevers and sweats. He developed right-sided headache, hearing loss, jaw claudication and scalp tenderness, without mastoid tenderness. He had quiescent rheumatoid arthritis on methotrexate. Blood tests revealed raised C-reactive protein (CRP) 41mg/L and normocytic anaemia. An extensive infection screen, including magnetic resonance imaging (MRI) head, was reassuring and temporal artery biopsy (TAB) was negative. Symptoms and inflammatory markers improved on high dose prednisolone. On reducing steroids, he developed left tongue deviation and rising CRP. Further MRI skull base and cervical spine demonstrated nasopharyngeal fluid collection and skull base osteomyelitis. Transnasal biopsy revealed necrotic tissue and staphylococcus aureus infection. Despite treatment with broad spectrum antibiotics, the patient deteriorated and died. Mimic 1: Skull base osteomyelitis Case 2 A 72-year-old female was admitted to hospital with right-sided headache, and right eye vision loss. Ophthalmic examination revealed right optic disc pallor. High dose prednisolone was commenced. CRP improved from 16mg/L to 5mg/L. Temporal artery ultrasound and biopsy were subsequently normal, MRI brain and computed tomography (CT) angiogram were reported as normal. Her headache improved. Owing to lack of alternate diagnosis, and a fitting history, tapering steroid treatment continued. A subsequent whole-body CT demonstrated extensive lymph node, bone and lung metastases. Biopsy confirmed metastatic breast cancer and re-review of the initial MRI highlighted subtle pituitary fossa enhancement, likely involving optic chiasm. Palliative care was commenced. Mimic 2: Metastatic cancer Case 3 A 55-year-old male with type 2 diabetes mellitus presented with a 7-day history of right-sided temporal headache and jaw claudication, with right scalp tenderness and lower facial numbness. CRP was elevated at 52mg/L. Prednisolone 40mg daily resulted in partial resolution of symptoms. TAB was normal, however. He subsequently presented in distress with headache, black nasal discharge and hypertensive urgency and was admitted to hospital. MRI head demonstrated invasive sinusitis, with right sphenoid mass. Histology confirmed necrosis and fungal hyphae, with a Rhizopus organism isolated, requiring long term antifungals. Mimic 3: Invasive fungal sinusitis Conclusion These cases highlight the importance of comprehensive systematic evaluation in GCA clinic, including early consideration of alternative diagnoses when atypical features are present, when there is incomplete response to corticosteroids or when supportive positive test results are lacking. Disclosure J. Day: None. N. Mir: None. R. Waller: None. E. Price: None.