E166 Non-specific orbital inflammation as initial presentation of polyarteritis nodosa

Abstract Background/Aims We present a case of polyarteritis nodosa (PAN) where the initial presentation was non-specific orbital inflammation (NSOI, orbital pseudotumour). A 78-year-old man first presented to ophthalmology in March 2024 with isolated NSOI of the right eye. MRI orbits described increased signalling throughout the intraorbital fat consistent with an inflammatory aetiology, which resolved completely with high-dose steroids. In June 2024, he re-presented with bilateral hand and wrist synovitis, new left-sided proptosis, sinusitis, and a 40 kg weight loss over 6 months. Inflammatory markers were markedly raised (CRP 291). CT orbits showed left proptosis with diffuse thickening of the superior oblique muscle and intraorbital optic nerve. Extensive autoimmune and infection screens, including ANCA and hepatitis B, were negative. Methods Vasculitis was suspected, and empirical intravenous and oral steroids were given to preserve vision. Biopsy was not obtained due to rapid resolution of symptoms. Steroids were stopped after 10 days, and CT-PET performed off steroid treatment showed no abnormal activity other than an incidental thyroid nodule. In July 2024, he presented again with left eye swelling and reduced visual acuity, requiring urgent intravenous methylprednisolone with good response. Finally, in October 2024, he attended the rheumatology clinic with bilateral synovitis, sinusitis, and new erythema nodosum over his distal upper limbs. Biopsy demonstrated mixed inflammatory infiltrate without fibrinoid necrosis, and prominent focal intimal thickening, consistent with PAN or thrombophlebitis. Results In the context of his systemic symptoms and clinical history, a diagnosis of PAN was made. He was commenced on methotrexate 15 mg once weekly, with no further flares reported since. Conclusion PAN is a rare necrotising arteritis of the medium or small arteries, not associated with ANCA. There are limited studies on non-HBV-associated PAN, and the diagnosis is increasingly rare. Definitive diagnosis is made when consistent histology is seen on biopsy, usually prompted by clinical suspicion of abdominal pain, mononeuritis multiplex, peripheral neuropathy, cutaneous, or renal manifestations. NSOI is a common ophthalmology presentation with a broad differential. It is often associated with autoimmune disease, but not with PAN. A literature search looking for cases of PAN presenting with NSOI identified 2 cases, and 2 through article reference search.* In all cases, initial presentation was NSOI, with diagnosis confirmed by tissue biopsy following later typical manifestations of PAN. Meticulous exclusion of alternate causes of NSOI should be performed, and tissue biopsy obtained, before orbital inflammation is deemed to be caused by PAN. *Strategy and references available on request. Disclosure Y. Kapoor: None. B. Jones: None. T. Nasrin: None. S. Bartram: None. M. Clynes: None.