Carolinas Research Day 2021

THE LONG-RANGE EFFECT OF THE ANATOMICAL VARIATION OF THE LEFT COMMON CAROTID ARTERY: SEVERE LEFT INTERNAL CAROTID ARTERY ATHEROSCLEROSIS Eshaan Zaveri*, OMS-II; Hunter Peden*, MA, OMS-II; Zachary Kline, OMS-III; Michael Pavy, OMS-III; Artem Boyev, DO; Zoltan Hajdu, MD VCOM Carolinas Campus, Spartanburg, SC * equal contribution

Abstract

Results

Conclusions

Figure 1: Left common carotid artery originating from the brachiocephalic trunk (BT) and the carotid bifurcations

Background: One of six deaths in the United States from cardiovascular diseases is due to stroke. With more than 795,000 people suffering from a stroke each year, this disease is the leading cause of long term disability with a huge economic impact. The prevalence of stroke attributed to extracranial internal carotid artery (ICA) atherosclerosis is about 13.4 per 100,000 persons. We have previously reported that there is a strong correlation between the anatomical variations of the left common carotid artery (LCCA) origin and intimal thickening in this vascular segment. Hypothesis: Based on palpatory findings during anatomical dissections, we hypothesize that anatomical variations of the LCCA origin have a direct effect on the left ICA atherosclerosis. Methods: The carotid arteries of 34 anatomic cadavers were investigated in VCOM anatomy labs between 2018-2020. Of these, 17 cases presented the anatomic variation of the LCCA originating from the brachiocephalic trunk (BT); the other 17 were arbitrarily selected controls where the LCCA originated from the aortic arch (AA). Measurements of the AA and the primary branches were taken in situ and these were used for 3D reconstruction of the arterial trees using the SimVascular software. Histological samples of ICAs were used for internal measurements and structural evaluation of the walls. Computational fluid dynamics (CFD) calculations were applied to evaluate the flow and shear stress in the ICAs. Results: The CFD calculations on the 3D-reconstructed arterial trees predicted that, beside the previously reported LCCA intimal thickening, serious flow disturbances exist at the level of the left ICA when the LCCA originates from the BT. Initial palpatory assessment indicated hardening of the left ICA walls in these cases. Sectioning of the paraffin-embedded samples of ICA further indicated severe atherosclerosis on the left side. Internal measurements of the samples from the cases with the anatomical variation revealed a 37% reduction of the arterial lumen compared to the control group. Von Kossa staining showed evidence of severe calcification in all 17 cases with the anatomical variation, while only six ICAs showed low or moderate levels of calcium in the control group. Conclusion: We extended our previous observation regarding the anatomical variations and intimal thickening of the LCCA. An LCCA originating from the BT instead of the AA will have a long-range effect in the left ICA in the form of severe atherosclerosis. Early screening for anatomical variations in the carotid artery could identify a risk factor for ICA atherosclerosis and potentially reduce the incidence of strokes with timely intervention.

➢ In the VCOM Carolinas anatomy lab, 78 total cadavers were evaluated and 17 cases of the anatomical variation were found, where the left common carotid artery originates from the brachiocephalic trunk. This means that between the academic years 2018 – 2020 21.79% of cases presented with the variations, which is more than double of that described in the literature. ➢ In addition to the intimal thickening of the proximal LCCAs originating from the BT, massive atherosclerosis in the LICAs was also found in these cases. ➢ It seems that the mild fluid dynamics perturbation (intimal thickening) due to the anatomic variations of the LCCA are enhanced along the length of the LICA, resulting in calcium deposits in the arterial wall. ➢ Computational fluid dynamics calculations show that the changes in flow and shear stress caused by the anatomical variation of the LCCA origin are enhanced along the carotid tree, having a more severe outcome in the LICA. Overall, we believe that anatomical variation of the aortic arch arteries has much more impact on the hemodynamics than previously thought. Our results demonstrate the long-range negative effect of the anatomical variation of the left common carotid artery origin on the left internal carotid artery. Clinical importance: Based on our findings, around 20% of the US population could have an anatomic variation which enhances the intimal thickening of the LCCA and atherosclerosis of the LICA; atherosclerosis of the carotid arteries is responsible for about half of the cerebrovascular accidents is the US, and therefore, developing a screening method to detect the anatomical variation, and closely monitor the patients who screen positive, would have a huge socio-economic impact on our health care system. Challenges of the study: Our project was based on cadaveric material, meaning the lack of the blood flow with individual characteristics; every patient is different, making it very hard to model their individual cardiovascular parameters; our computational fluid dynamics model suggests that little changes in the flow pattern could significantly alter the changes caused by abnormal branching in both positive and negative direction. Future studies: ➢ we will keep collecting cadaveric samples from the VCOM anatomy labs, and keep improving our model; ➢ we will be performing an in vivo, ultrasonic study to detect potential differences in the flow pattern between the left and right common carotid arteries; ➢ we will investigate existing 3D computerized tomography data complemented by the patient’s history.

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Seventeen cases of the left common carotid artery (B, green arrow) originating from the brachio- cephalic trunk were identified and 17 matching cases of origination from the aortic arch (A, green arrow) were arbitrarily selected. A’ and B’ are magnifications of the highlighted areas of A and B. ICA – internal carotid artery; ECA – external carotid artery; CCA – common carotid artery. Dotted red lines show the level of the histological samples.

Figure 2: Histological examination of the left internal carotid artery and measurements

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Introduction and Methods

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Introduction Usually, in humans the left common carotid artery (LCCA) originates from the aortic arch (AA). However, variations of the LCCA have been described – most interestingly for us, in about 9% of the cases, the LCCA is branching off the brachiocephalic trunk (BT).

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References

Hematoxylin-eosin (HE) staining was used for general evaluation of the normal (A) and variation (B) samples. For detection of the calcium deposits, von Kossa staining was used (C-normal; D-variation), black is showing calcification. Measurements of the left carotid tree was performed before and after sample collection (E). No statistical significance is detected in BT circumference, left CCA circumference, left CCA length and left ICA internal circumference between the normal (n=17) and variation samples (n=17). However, the left ICA circumference and left ICA wall thickness in the variation samples shows significant (p≤0.05) increase and decrease, respectively, compared to the normal samples.

1. Layton KF, Kallmes DF, Cloft HJ, Lindell EP, Cox VS. Bovine aortic arch variants in humans: clarification of a common misnomer . Am j Neuroradiol . 2006. 27:1541-1542. 2. Benjamin EJ et al. Heart disease and stroke statistics 2018 update: a report from the American Heart Association. Circulation. 2018; 137: e67-e492. 3. Marulanda-Londone, E and Chaturvedi S. Stroke due to large vessel atherosclerosis. Neurology Clinical Practice . 2016; 6: 252-258. 4. Flaherty ML et al. Carotid Artery Stenosis as a Cause of Stroke. Neuroepidemiology . 2013; 40: 36-41. 5. Ross R. Atherosclerosis – an inflammatory disease. The New England Journal of Medicine . 1999; 340 (2): 115- 126. 6. Nakashima Y et al . Early atherosclerosis in humans: role of diffuse intimal thickening and extracellular matrix proteoglycans. Cardiovascular Research . 2008; 79: 14-23. 7. Gimbrone MA et al . Vascular endothelium, hemodynamics, and the pathobiology of atherosclerosis. Cardiovascular Pathology . 2013; 22: 9-15 8. Jakanani GC and Adair W . Frequency in variation of aortic arch anatomy as depicted on multidetector CT. Clinical Radiology . 2010; 65: 481-487

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70% Layton et al., Am J Neuroradiol, 2006

The goal of this work was to evaluate the LICA from normal and variation cases by histology and computed fluid dynamics. Material and Methods Based on literature data, intimal thickening is the first step in atherosclerosis and happens upon endothelial activation. It is known that blood vessel endothelium is sensitive to shear stress; this is actually one of the major endothelial activators. Previously we reported that we found a histological and computed fluid dynamics correlation between the intimal thickening and the LCCA originating from the BT. During these dissections, we also observed that the left internal carotid arteries (LICAs) branching off the LCCAs originating from the BTs felt firmer on palpation than those from LCCAs originating from the AA. We hypothesized that the LCCA origin has direct effect on the LICA atherosclerosis. ➢ The aortic arches with the left carotid tree were measured after curricular dissections; ➢ Then, 0.5 mm ring samples were collected 1 cm above the origin of the LICA; these were processed for paraffin embedding and 5 µm sectioning; ➢ After staining the samples with hematoxylin-eosin, ImageJ was used to measure the outer and inner perimeters (circumferences) and measurements were adjusted to counteract the shrinkage during histological processing; ➢ Using the pre- and post-processing measurements, the carotid trees were 3D-reconstructed in SimVascular; the shear stress was calculated by the software at 1 cm above the origin of the LICA; ➢ von Kossa staining was used to detect calcium deposits (as sign of atherosclerosis) in the wall of the LICAs.

Figure 3: Computational fluid dynamics calculations show decreased shear stress in the left internal carotid arteries when the left CCA is originating from the brachiocephalic trunk

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Acknowledgements

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This project was funded by the VCOM Research Eureka Accelerator Program awarded to ZH. We would like to thank our student and faculty colleagues for the initial dissections in the anatomy labs.

The average shear stress (measured in dynes/cm 2 ) at the beginning of the left ICA (along the endothelium, 1 cm above the origin) was recorded in both normal (n=17) and variation (n=17) cases. In the variation cases (where the left CCA is originating from the BT), the shear stress is significantly lower (p≤0.05) than in the normal cases (left CCA is originating from the aortic arch).

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