Mitral Bioprosthetic Valve Thrombosis Mimicking Prosthetic Valve Endocarditis: A Diagnostic Challenge Highlighting the Role of Multimodality Imaging

Aims: Bioprosthetic valve thrombosis (BPVT) is an under-recognised cause of prosthetic valve dysfunction and may mimic prosthetic valve endocarditis. This case highlights the diagnostic value of integrating multimodality imaging with clinical, microbiological, and therapeutic findings. Presentation of Case: A 63-year-old woman with a 33-mm MOSAIC® mitral bioprosthesis implanted six years earlier was admitted after two episodes of fever. Transthoracic echocardiography showed an increase in the mean transmitral gradient from 7.4 mmHg before admission to 12 mmHg, with leaflet thickening and reduced mobility. Transoesophageal echocardiography demonstrated adherent echogenic material but could not reliably differentiate thrombus from vegetation. ^18F-FDG PET/CT showed intense diffuse circumferential uptake around the mitral bioprosthesis (SUVmax 7.2), initially raising suspicion of prosthetic valve endocarditis. However, repeated blood cultures remained negative and the inflammatory syndrome resolved spontaneously without antibiotic therapy. ECG-gated cardiac CT demonstrated circumferential leaflet thickening with hypoattenuating lesions (<90 HU), strongly supporting bioprosthetic valve thrombosis. Anticoagulation resulted in progressive haemodynamic improvement, with the mean transmitral gradient decreasing to 8 mmHg and subsequently to 6 mmHg. Discussion: This case illustrates the potential for BPVT to mimic prosthetic valve endocarditis, including increased ^18F-FDG uptake. PET/CT findings should therefore be interpreted in conjunction with clinical, microbiological, echocardiographic, and anatomical imaging data. Conclusion: In cases of discordant findings, multimodality imaging, particularly cardiac CT, can be decisive in distinguishing bioprosthetic valve thrombosis from infective endocarditis and guiding appropriate therapy.