Carotid Artery Diameter Correlates with Plaque Volume but not with Degree of Stenosis.

The severity of carotid atherosclerosis is expressed by degree of stenosis, despite most strokes resulting from embolisation of plaque contents and not flow reduction. An alternative descriptor of athero-embolic risk is carotid plaque volume (CPV). Evidence has accrued that CPV, alongside plaque composition and ulceration, may signify plaque vulnerability. In a series of 339 carotid endarterectomies, CPV was greater in patients with symptomatic than with asymptomatic disease (0.97 cm3 vs. 0.74 cm3). CPV did not correlate with severity of carotid stenosis (p = .77).1Ball S. Rogers S. Kanesalingam K. Taylor R. Katsogridakis E. McCollum C. Carotid plaque volume in patients undergoing carotid endarterectomy.Br J Surg. 2018; 105: 262-269Crossref PubMed Scopus (24) Google Scholar To explain this, it was hypothesised that larger volume atherosclerosis can result in carotid outward remodelling and dilatation, with the consequence that large CPVs may exist without significant luminal narrowing.2Steinke W. Els T. Hennerici M. Compensatory carotid artery dilatation in early atherosclerosis.Circulation. 1994; 89: 2578-2581Crossref PubMed Google Scholar It was investigated whether arterial outward remodelling can be demonstrated in the presence of carotid atherosclerosis > 50% stenosis (North American Symptomatic Carotid Endarterectomy Trial [NASCET] method) compared with those with < 50% stenosis. Secondly, in those with > 50% stenosis undergoing carotid surgery, whether a correlation exists between carotid diameters, stenosis, and CPV. Following ethics approval (11/NW/0308), patients undergoing carotid endarterectomy for symptomatic or asymptomatic > 50% stenosis were compared with an age (± 2 years) and sex matched asymptomatic comparator group with < 50% stenosis undergoing routine, per protocol, carotid duplex. In the surgical cohort, CPV was measured using the Archimedes technique of weighing using a fluid displacement technique in saline.3Hughes S.W. Archimedes revisited: a faster, better, cheaper method of accurately measuring the volume of small objects.Phys Educ. 2005; 40: 468-474Crossref Scopus (193) Google Scholar All imaging was with B mode ultrasound. Transverse measurements were taken from outer to outer wall in the anteroposterior dimension from mid common carotid artery (CCA), proximal internal carotid artery (pICA), site of maximum dilatation, and the distal internal carotid artery (dICA). Stenosis degree was measured following national protocol but expressed as the midpoint of the decile (e.g., 70 – 79% stenosis; 75 was used).4Oates C.P. Naylor A.R. Hartshorne T. et al.Joint Recommendations for Reporting Carotid Ultrasound Investigations in the United Kingdom.Eur J Vasc Endovasc Surg. 2009; 37: 251-261Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar A pragmatic sample of 60 individuals was collated and the cohort was analysed by sex. Following removal of patients with acoustic shadowing, 36 males and 21 females and the same number of comparators were analysed. Continuous data were normality tested using Shapiro–Wilks tests and funnel plots. Diameter differences were tested between groups using Student’s t test. Statistical significance was considered to be p < .05. Spearman rank correlation was used to assess the association between the arterial segments, CPV, and stenosis, with R values considered moderate (0.5–0.7), good (0.7–0.9), and excellent (0.9–1). In females with < 50% stenosis, mean ± standard deviation (SD) diameters (cm) for CCA, pICA, and dICA were 0.69 ± 0.1, 0.59 ± 0.08, and 0.49 ± 0.06, respectively. In females with > 50% stenosis, the mean ± SD CCA, pICA, and dICA were 0.77 ± 0.10, 0.67 ± 0.10, and 0.48 ± 0.07, respectively. Mean ± SD CCA and pICA diameter between either group was significantly different (p = .020 and p = .010 respectively) (Fig. 1A). There was no difference in dICA diameter (p = .82). In males with < 50% stenosis, mean ± SD diameters (cm) for CCA, pICA, and dICA were 0.77 ± 0.09, 0.63 ± 0.09, and 0.49 ± 0.05, respectively. In males with > 50% stenosis, the mean ± SD diameters (cm) for CCA, pICA, and dICA were 0.86 ± 0.07, 0.90 ± 0.08, and 0.53 ± 0.09 cm. All measurements between group were significantly different (p < .010, p < .010, and p < .030 respectively) (Fig. 1A). As CPV and stenosis are independent of sex; all recruits with > 50% stenosis were combined to compare CPV, stenosis, and vessel diameter. CPV was significantly associated with the CCA diameter (r = 0.7, p < .010) and pICA diameter (r = 0.71, p < .010) (Fig. 1B). There was no significant correlation between the dICA (r = 0.23, p = .087), degree of stenosis (r = 0.042, p = .76), or PSV (r = 0.14, p = .30). These data show that CCA and pICA diameters in both genders and dICA in men are larger in the presence of > 50% carotid stenosis and had a good degree of correlation with CPV. The dICA was unchanged in women but significantly different in men, suggesting that this segment may also enlarge in disease. This report may explain how large CPVs can be present in the absence of significant luminal stenosis through outward remodelling. This study indicates correlation between CPV and artery diameter but is not mechanistic, and further study is required to define the cause of carotid outward remodelling. One possibility is that dilatation is related to arteriopathy within the carotid bulb, and that this process predominantly affects the more proximal ICA and CCA. Another explanation may relate to the haemodynamics of blood flow across stenotic lesions.5Ward M. Pasterkamp G. Yeung A. Borst C. Arterial Remodeling.Circulation. 2000; 102: 1186-1191Crossref PubMed Google Scholar This study contributes to the existing literature by highlighting that carotid plaque biology and the interaction with the artery wall is more complex than can be explained by stenosis measurements. If the hypothesis that CPV is an indicator for future major adverse cardiovascular event risk is correct, then this work further demonstrates that stenosis is an imperfect surrogate marker for this.6Sillesen H. Sartori S. Sandholt B. Baber U. Mehran R. Fuster V. Carotid plaque thickness and carotid plaque burden predict future cardiovascular events in asymptomatic adult Americans.Eur Heart J Cardiovasc Imaging. 2018; 19: 1042-1050Crossref PubMed Google Scholar Additionally, practitioners should be aware of the dilated carotid artery harbouring high volumes of atherosclerosis that may not show significant stenosis but may hold high embolic potential. In conclusion, the volume of the surgically removed plaque correlates with vessel enlargement but not PSV or percentage of stenosis. Validation of the trends demonstrated herein requires a larger prospective analysis, corrected for body size and blood pressure and alongside assessment of plaque composition and haemodynamics. None.