Beyond the Mitral Valve: Thoracic Antero-Posterior Diameter Identifies Distinct Risk Phenotypes in Mitral Valve Prolapse

Background: Risk stratification in mitral valve prolapse (MVP) remains challenging, particularly in symptomatic patients without severe mitral regurgitation (MR) or left ventricular (LV) dysfunction. Thoracic geometry is frequently altered in MVP, but the clinical and prognostic significance of directly measured antero-posterior (A-P) thoracic diameter remains poorly defined. Methods: In this single-center retro-prospective observational study, 116 symptomatic adults with echocardiographically confirmed MVP, preserved LV ejection fraction (≥55%), and non-severe primary MR underwent clinical assessment, thoracic morphometry, echocardiography, speckle-tracking analysis, carotid ultrasonography, and exercise stress echocardiography (ESE). Patients were categorized using an a priori A-P diameter threshold of <13 or ≥13 cm (n = 58 each), after age- and sex-matching. Cardiovascular events were assessed separately during a retrospectively reconstructed pre-index period and prospective post-index follow-up. Cox regression, receiver operating characteristic (ROC), and Kaplan–Meier analyses were performed. Results: The <13-cm phenotype had smaller cardiac chambers, lower LV mass, less pronounced MR and mitral annular disjunction, but lower exercise capacity, more symptoms, and greater exercise-induced ST-segment abnormalities, without inducible regional wall-motion abnormalities on ESE. During the pre-index period (4.58 ± 4.37 years), 25 non-fatal cardiovascular events were identified: 4/58 (6.9%) versus 21/58 (36.2%) in the <13- and ≥13-cm groups, respectively. A-P diameter (adjusted HR per 1-cm increase, 1.773; 95% CI, 1.178–2.667; p = 0.006) and more-than-mild peak-exercise MR (adjusted HR, 3.486; 95% CI, 1.273–9.544; p = 0.015) were independently associated with pre-index events. During prospective follow-up (7.32 ± 1.14 months), 14 non-fatal cardiovascular events occurred, with no fatal cardiovascular events: 1/58 (1.7%) versus 13/58 (22.4%), respectively. A-P diameter (adjusted HR per 1-cm increase, 3.234; 95% CI, 1.821–5.745; p < 0.001) and more-than-mild peak-exercise MR (adjusted HR, 5.053; 95% CI, 1.104–23.125; p = 0.037) remained associated with subsequent events. A-P diameter showed high discrimination for post-index events (AUC, 0.952; 95% CI, 0.886–1.000; p < 0.001), and prospective event-free survival differed by the prespecified 13-cm threshold (log-rank p = 0.001). Conclusions: A-P thoracic diameter identifies distinct MVP phenotypes and was associated with both previous and subsequent cardiovascular events. These exploratory prospective findings support thoracic depth as a simple, radiation-free marker potentially complementary to conventional MVP risk assessment, requiring external validation.