Pathology of Self-Expanding Transcatheter Aortic Bioprostheses and Hypoattenuated Leaflet Thickening

BACKGROUND: Hypoattenuated leaflet thickening (HALT) is believed to reflect leaflet thrombosis; however, no systematic histological examination of HALT has ever been performed. The aim of this study was to evaluate histological findings of explanted self-expanding transcatheter aortic bioprosthetic valves from clinical trials and to compare microCT findings of suspected HALT with histology findings of valve thrombosis and its characterization over time. METHODS: A total of 123 self-expanding transcatheter aortic valves were collected through autopsy (n=89) or surgical explant (n=34) from 11 CoreValve/Evolut clinical trials. Histological findings in transcatheter aortic valve leaflets were evaluated. MicroCT imaging was used to evaluate HALT in histology. Cases with infective endocarditis (10/123) or transcatheter aortic valve-in-surgical aortic valve procedures (3/123) were excluded. RESULTS: A total of 110 cases were divided into 3 groups based on implant duration: <30 days (n=42), 30 to 365 days (n=35), and >365 days (n=33). Thrombus and inflammation scores were consistent across groups, while scores for pannus, calcification, and structural change increased over time. The analysis of leaflet thickening by histology was performed on 320 leaflets and any degree of leaflet thickening was observed in 46.5% (149/320) of leaflets. Histologically, leaflet thickening was confirmed as an acute, organizing, and organized thrombus (ie, pannus). In the <30 days group, all leaflet thickening was due to acute thrombus, while most thrombi were organized >30 days. The types of thrombi could not be differentiated by microCT imaging. CONCLUSIONS: HALT represents the presence of a thrombus and its progression. Our data suggest that treatment of HALT would likely be most effective in the early stages before the thrombus becomes organized and emphasizes the need for early detection.