A Case of Pneumococcal Periaortitis: Periaortitis is not Always Autoimmune.

Introduction: Periaortitis is a rare inflammatory condition with both infectious and non-infectious aetiologies, often presenting a diagnostic challenge. A comprehensive evaluation-including history, physical examination, imaging, and a broad infectious work-up-is essential, as negative blood cultures do not rule out infection, and premature use of glucocorticoids may be harmful in cases of infectious periaortitis.

Case description: We report the case of a 70-year-old man presenting with flank pain and periaortitis, in whom positron emission tomography/computed tomography (PET/CT) scan revealed moderate-to-severe ¹⁸F-fluorodeoxyglucose (FDG) uptake. Despite negative cultures, a Karius test and urine antigen confirmed Streptococcus pneumoniae as the causative agent. The patient underwent successful open surgical repair of a mycotic pararenal abdominal aortic aneurysm, followed by targeted antibiotic therapy.

Discussion: Pneumococcal periaortitis is a rare condition with few reported cases, though S. pneumoniae has been implicated in various vascular infections including mycotic aneurysms, and diagnostic challenges persist, especially when blood cultures are negative. The Karius Spectrum^™, despite its cost and limited guidance for use, and FDG-PET/CT imaging have emerged as helpful tools in identifying infectious aetiologies, with high maximum standardized uptake value values favouring infection over autoimmune causes, as seen in this case.

Conclusion: Infectious periaortitis should be suspected when there is a high uptake of FDG on PET/CT prompting an infectious disease work-up which may include a Karius test.

Learning points: Periaortitis may have an autoimmune aetiology; however, an infectious cause should always be considered, particularly when there is high ¹⁸F-fluorodeoxyglucose uptake on positron emission tomography/computed tomography imaging.Streptococcus pneumoniae is a rare cause of periaortitis and has been more commonly reported in association with mycotic aneurysms.Microbial cell-free deoxyribonucleic acid testing, such as the Karius test, may be a useful tool for identifying specific bacteria, especially when blood cultures are negative.