Chronic Spinal Cord Injury Reduces Aortic Wall Shear Stress and Increases Diameter

Janice J. Yeung, MD, Stanford University, General Surgery, University of Rochester, Rochester, NY; Thomas A. Abbruzzesse, MD, Stanford, CA; Hyun Jin Kim, Stanford, CA; Irene E. Vignon, Stanford, CA; Mary T. Draney, PhD, Stanford, CA; Kay K. Yeung, MD, Stanford, CA; John K. Karwowski, MD, Stanford, CA; Inder Perkash, MD, Stanford, CA; Charles A. Taylor, PhD, Stanford, CA; Ronald L. Dalman, MD, Stanford, CA;

Purpose: Resistive aortic hemodynamic conditions and reduced antegrade wall shear stress (WSS) upregulate pro-inflammatory gene expression and proteolysis, potentially promoting abdominal aortic aneurysm (AAA) disease. We used abdominal ultrasound, magnetic resonance (MR) imaging and computational flow modeling to determine the relationship between reduced infrarenal aortic blood flow and AAA risk in chronic spinal cord injury (SCI) patients.
Methods: Aortic diameter was prospectively measured in 1) chronic SCI (n=123) and 2) age and risk factor matched ambulatory control patients (n=129) via ultrasound (US). In 5 SCI and 6 control patients all with aortic diameter < 3cm , MR angiography (MRA) and phase-contrast MR imaging (PC-MRI) were used to measure aortic shape and blood flow, respectively. These data sets were used to generate fractal tree bifurcation models extending from the common iliac arteries to pre-capillary arterioles (10um) for comparisons between control and non-aneurysmal SCI aorta and a representative small AAA model (Figure 1 - top row: anatomic models, bottom row: calculated resting WSS between groups).
Results: Age, gender, and smoking history were comparable between SCI and control patients (p=NS). Hypertension, increased BMI and family history of AAA disease were more prevalent in control subjects (all p<.03). Time since spinal injury was 26±12 (mean ± SD) years. Despite lower blood pressure and smaller body habitus, aortic diameter was larger (p<.01) and the prevalence of large (≥2.5 cm, p<.01) or aneurysmal (≥3.0 cm, p<.05) aortas was greater in SCI patients. Paradoxically, SCI common iliac artery diameter was smaller (<1.0 cm = 48% SCI vs. 26% control, p<.0001). Focal, pre-aneurysmal enlargement was noted in 4/5 SCI patients without AAA (Figure 2, representative SCI patient, vs. Figure 3, control), and although aortic pressure waveforms and pulse pressures were similar between groups, computational modeling demonstrated that resting infrarenal aortic WSS was 2x lower in aneurysm-free SCI patients and 5x lower in the small AAA model as compared to hemodynamic conditions in control patients (Figure 1, bottom row: color scale for WSS noted to the right of each representative image).
Conclusion: Despite diminished iliac artery diameter, chronic SCI is associated with increased aortic diameter and AAA prevalence. Aortic degeneration may be accelerated by reduced antegrade shear exposure present in this high risk patient group, both at rest and in the absence of ambulation. Increased antegrade shear forces induced by exercise may reduce AAA risk in sedentary individuals.