Cranial Giant Cell Arteritis Presenting as Pyrexia of Unknown Origin: A Diagnostic Challenge With Negative PET-CT but a Positive Halo Sign.

Giant cell arteritis (GCA) classically presents with cranial symptoms, but systemic features may predominate and obscure the diagnosis. We report the case of a woman in her early 70s who presented with a six-week history of nausea, vomiting, anorexia, weight loss, and intermittent confusion, followed by persistent pyrexia during a prolonged inpatient admission. Extensive investigations for infection, malignancy, and systemic autoimmune disease were unrevealing, including computed tomography (CT) imaging, echocardiography, microbiological testing, autoimmune panels, and fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT). She received five courses of broad-spectrum intravenous antimicrobial therapy without a clinical or biochemical response. Her C-reactive protein remained above 120 mg/L for several weeks. Despite the absence of new headache, jaw claudication, scalp tenderness, or visual symptoms, temporal artery ultrasound was requested because of concern for atypical GCA. This demonstrated bilateral incompressible echolucent halos, consistent with cranial GCA. Prednisolone 40 mg daily was commenced, after which the fever resolved within 48 hours, cognition normalised, and oral intake improved substantially. She was discharged after 51 days on a tapering corticosteroid regimen. This case emphasises that GCA should remain in the differential diagnosis of pyrexia of unknown origin in older adults with sustained systemic inflammation and a poor response to antimicrobial therapy, even when classical cranial symptoms are absent and FDG-PET/CT does not demonstrate large-vessel vasculitis. Early temporal artery ultrasound may reduce diagnostic delay, unnecessary antimicrobial exposure, and prolonged hospitalisation.