Intraluminal thrombus distribution pattern is associated with abdominal aortic aneurysm rupture at presentation: a single-centre retrospective study

Intraluminal thrombus (ILT) is present in the majority of abdominal aortic aneurysms (AAA), yet its role in rupture remains incompletely defined. This study examined whether the circumferential distribution pattern of ILT—classified as circumferential, eccentric, or absent/minimal on axial computed tomography angiography (CTA) at presentation—is associated with AAA rupture status. This single-centre retrospective study included 93 consecutive patients who underwent CTA for infrarenal AAA between January 2015 and December 2024. ILT distribution was classified on the axial CTA slice showing the largest aneurysm cross-section as circumferential (involving ≥ 75% of the aortic circumference), eccentric (< 75%), or absent/minimal (maximal thickness < 2 mm). The outcome was rupture status at presentation (ruptured, n = 19; non-ruptured, n = 74). Because CTA in ruptured cases was acquired after the rupture event, all analyses were framed as cross-sectional associations at presentation rather than prediction of future rupture. Baseline characteristics were compared between groups. Univariate logistic regression (standard maximum likelihood) and multivariate Firth penalised logistic regression were performed to identify factors independently associated with rupture. Model discrimination was assessed using receiver operating characteristic (ROC) curves, with the DeLong test, net reclassification improvement (NRI), integrated discrimination improvement (IDI), and calibration analyses. Of 93 patients (mean age 70.3 ± 11.0 years, 81.7% male), 19 (20.4%) presented with rupture. ILT distribution differed significantly between groups (χ² = 13.27, P = 0.001): circumferential ILT was present in 68.4% (13/19) of ruptured versus 24.3% (18/74) of non-ruptured AAA. In multivariate Firth logistic regression, circumferential ILT (aOR 3.37, 95% CI 1.10–10.31, P = 0.033) and C-reactive protein > 5 mg/L (aOR 4.40, 95% CI 1.31–14.80, P = 0.017) were independently associated with rupture. The association persisted in a model excluding CRP, which may rise as a consequence of rupture (circumferential ILT aOR 4.43, 95% CI 1.54–12.78, P = 0.006). Adding ILT distribution to a clinical model (CRP + maximum transverse diameter + morphology) increased the area under the ROC curve from 0.849 to 0.880; however, this improvement was not statistically significant (DeLong P = 0.26) and is regarded as hypothesis-generating. In a parallel analysis excluding CRP, adding ILT distribution to MTD and morphology increased the AUC from 0.763 to 0.828 (DeLong P = 0.157). In the ≥ 55 mm diameter subgroup, circumferential ILT was associated with higher rupture odds (Fisher’s exact OR 6.07, P = 0.045). Sensitivity analyses excluding patients with 30-day mortality and those with concomitant aortic dissection yielded directionally consistent results. Circumferential ILT distribution on presentation CTA was independently associated with AAA rupture status in this single-centre, rupture-enriched cohort. Because imaging in ruptured cases was obtained after rupture, these findings are associational and hypothesis-generating; they do not establish ILT distribution as a predictor of future rupture and should not currently alter surveillance intervals or surgical thresholds. Prospective, multicentre validation is required.