Abdominal aortic aneurysms’ histomorphology differs on the individual patient level and is not associated with classic risk factors – the HistAAA study

Objective Abdominal aortic aneurysm (AAA) treatment is upon a diameter threshold by open (OAR) or endovascular aortic repair. So far, attempts for medical growth abrogation have failed. This study aims to elucidate the heterogeneity of AAA based on histomorphology in correlation to individual patient data and aneurysm metrics. Patients and Methods Aneurysm samples from the left anterior wall from four university center biobanks underwent histologic analysis including angiogenesis, calcification, fibrosis, type and grade of inflammation in adventitia and media. Clinical information included age, comorbidities, etc., type of aneurysm (intact, symptomatic, ruptured, inflammatory) and growth. Aneurysm morphology included diameter and semi-automated geometric analysis using Endosize © (Therenva) and finite element methods (A4Clinics © Research Edition, Vacops GmbH). Results 364 patients’ samples (85.4% male, median age 69 years) were evaluated and scored for acute (mixed/granulocytes) or chronic (mononuclear/plasma cells) inflammation, which was not associated with rupture (52x), symptomatic (37x; p = 0.51) or diameter (57 [52–69] mm; p = 0.87). The degree of fibrosis and the presence of angiogenesis were significantly higher (both p < 0.001) with increasing inflammation, which in turn significantly decreased with patient age (est = −0.015/year, p = 0.017). No significant differences in were seen for ruptured (vs. intact), acute (vs. elective), male (vs. female) or diabetic patients. Current smoking was associated with chronic inflammation (p = 0.007) and a higher degree of fibrosis (p = 0.03). Aneurysm geometric morphology (n=252) or annual growth rate (n=142) were not associated with histologic characteristics. Yet, local luminal thrombus formation was significantly higher with increasing inflammation (p = 0.04). Conclusion Type and degree of inflammation are the most distinguishable histologic characteristics in the AAA wall between individual patients, yet not associated with diameter or rupture. Local luminal thrombus formation is associated with inflammatory features and suggests a vivid bio-physical compartment with intra-individual differences.