Predominance of Nonatherosclerotic Internal Elastic Lamina Calcification in the Intracranial Internal Carotid Artery
Background and Purpose—Calcification of the intracranial internal carotid artery (iICA) is an independent risk factor for stroke. These calcifications are generally seen as manifestation of atherosclerosis, but histological investigations are limited. The aim of this study is to determine whether calcifications in the iICA are present in atherosclerotic plaques, or in other parts of the arterial wall.
Methods—Thirty-nine iICAs were histologically assessed, using digital microscopy to quantify the amount of calcification in the different layers of the arterial wall.
Results—Calcifications were found in the intima, around the internal elastic lamina and in the medial layer of the arterial wall. In 71% of the arteries, internal elastic lamina calcification contributed most to the total calcified cross-sectional surface area. Internal elastic lamina calcification was unrelated to the occurrence of atherosclerotic intimal lesions. Intimal calcifications were most often associated with atherosclerotic lesions, but also many noncalcified atherosclerotic lesions were found.
Conclusions—In the iICA, calcifications are predominantly present around the internal elastic lamina, suggesting that this nonatherosclerotic type of calcification contributes to the previously observed increased risk of stroke in patients with iICA calcifications.
Calcifications of the intracranial internal carotid artery (iICA), diagnosed using computed tomographic (CT) scans, are an independent risk factor for stroke that contributes to the occurrence of 75% of all strokes.1
The presence of calcified atherosclerotic plaques seems to be a logical explanation for the observed association between calcification and stroke. However, older literature describes the occurrence of both intimal and medial calcification in the siphon of the ICA,2 the latter being composed of calcification of both media and internal elastic lamina (IEL) of the arterial wall.3
If medial/IEL calcification is regularly present in the iICA, the calcification on imaging may not be a proxy for atherosclerosis, and nonatherosclerotic calcification might also contribute to the previously described risk of stroke. The aim of this study is to describe the amount, location, and morphology of calcifications in the iICA.
The iICA was examined in 20 consecutive patients (39 arteries; 20 left and 19 right) administered for cerebral autopsy in our hospital. Material was handled according to the code of proper use of human tissue, used in The Netherlands. Collection of the material was approved by the local biobank review committee (protocol 15–252).
The ICA was dissected close to the circle of Willis and the petrous bone. The arteries were fixed in 4% formaldehyde, and subsequently decalcified using EDTA. Decalcification was necessary to maintain the morphology, and interpretability, of the arterial wall layers. Because histological evaluation of calcification is based on visualization of matrix previously altered by the calcification process, and not calcium ions itself, decalcification does not influence analysis4 (Methods section in the online-only Data Supplement). To study atherosclerosis that affects arteries heterogeneously, the arteries were divided in a proximal and distal segment. Microscopic slides of rings cut perpendicularly to the lumen were stained with hematoxylin and eosin and elastin van Giesson.
Digitalized slides (Scan-Scope XT scanner; Aperio Technologies)5 were analyzed using Aperio Image Scope software. Calcifications were identified as sharply demarcated, acellular spots, and areas (Figure I in the online-only Data Supplement). Intimal calcification was, arbitrarily, considered substantial when ≥1% of the intimal surface was calcified.
Because of right-skewed distributions, χ2 tests were performed. Missing data were only because of technical issues. Available case analyses were performed. Data are presented as median and interquartile range, unless specified otherwise.
Median age of the patients was 64 years (range, 41–90 years). In 16 patients, a recent cerebral CT scan was available. The iICA calcifications present in 15 of 16 patients were comparable with those presented in literature. Histologically, calcifications were observed at 3 locations: in the intima, around the IEL, and in the media.
Substantial intimal calcification was found in 27 of 78 arterial segments. In these 27 segments, atherosclerotic lesions were more often present (85%) than nonatherosclerotic lesions (15%; P<0.0005). The other way around, however, in atherosclerotic lesions (37/78 arterial segments), substantial calcification was found in only 62% (P=0.139; Table I in the online-only Data Supplement).
Thirty-eight of thirty-nine arteries showed calcification of (part of) the circumference of the IEL (Figure 1). The median percentage of the circumference that was calcified was 36% (26% to 45%). Sixty-four percent of the arteries had a maximal calcification of >50% of the circumference, which was not related to the occurrence of atherosclerotic lesions or intimal calcification. In 22 of 78 arterial segments, >50% of the IEL circumference was calcified, with in 10/22 coexistence with an atherosclerotic intimal lesion (P=0.670) and in 7 of 22 with substantial intimal calcification (P=0.09).
Calcification in the medial layer was only found in small amounts (Figure II in the online-only Data Supplement).
Total Arterial Wall Calcification
Thirty-eight of thirty-nine arteries revealed substantial calcification of intima (≥1% intimal surface area calcified) or IEL (≥1% of circumference calcified) or media (≥1% medial surface area calcified). IEL calcification contributed most to the total calcified cross-sectional surface area (79%; 47%–95%), followed by intimal (14%; 0%–51%) and medial calcification (0%; 0%–4%; Figure 2; Table II in the online-only Data Supplement). Furthermore, in 71% of the arteries with substantial calcification of intima or IEL or media, the absolute surface of IEL calcification was biggest, followed by intimal (26%) and medial (3%) calcification.
Calcification of the iICA on CT scan was recently identified as an independent risk factor for stroke.1 The present study has 2 important results. First, we found that iICA calcifications are present not only in the intimal layer of the arterial wall but also around the IEL and in the media. More importantly, in the majority of arteries (71%) the contribution of IEL calcification to the total calcified cross-sectional surface area in the vascular wall is biggest. These results suggest that also IEL calcification is detected when iICA calcification is measured by CT scanning and IEL calcifications might, therefore, be at least partly responsible for the previously observed association between iICA calcification and stroke. Second, although intimal calcification is related to the presence of atherosclerotic plaques, IEL calcification is not.
IEL calcifications might be a plausible risk factor for stroke because consequent stiffening of the arterial wall will lead to increased pulse pressure,6 which leads to microvasculature and tissue damage.7 Furthermore, a relation between increased pulse pressure and cerebrovascular events has been suggested.8 It would also be possible that reduced capacity to expand the vascular wall of stiffened arteries could lead to increased mechanical stress on atherosclerotic plaques, increasing the risk of plaque rupture and stroke.
Medial and IEL calcifications are potentially reversible lesions, as evidenced by drug-induced (and even spontaneous) regression observed in several other vascular territories.9 Thus, confirmation of a contribution of IEL/medial calcification to the occurrence of stroke might imply important novel options for stroke risk management.
This study has some limitations. We studied a small cohort of random cerebral autopsy patients, lacking the regular stroke patients. Therefore, these results have to be confirmed in a group of patients with stroke. Furthermore, we only studied the iICA in these patients and, therefore, we cannot relate our findings to atherosclerosis elsewhere in the intracranial or extracranial vasculature.
In conclusion, in the iICA calcifications are predominantly located around the IEL, indicating that these nonatherosclerotic calcifications might contribute to the previously observed increased risk of stroke in patients with iICA calcifications on CT scan.
Sources of Funding
The research was supported by a grant of the Netherlands Organization for Scientific Research/Foundation for Technological Sciences (Project 12726).
Dr Isgum reports grants from Pie Medical Imaging and The Netherlands Organization for Health Research and Development outside the work described in the article. The other authors report no conflicts.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.115.011196/-/DC1.
- Received August 27, 2015.
- Revision received September 25, 2015.
- Accepted September 29, 2015.
- © 2015 American Heart Association, Inc.
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