Vertebrobasilar insufficiency, manifesting with symptoms from dizziness to unconsciousness and in some cases death, has been attributed to a reduction in blood flow in the vertebrobasilar vessels. The aim of this study was to investigate whether the degree of atherosclerosis in the intracranial vertebral artery can be regarded as a contributory factor in vertebrobasilar insufficiency. Atherosclerosis develops as an aggregation of lipid-laden macrophages, smooth muscle cells, and collagen and elastic fibres in the subendothelium of the vessel. The fibres form a dense fibrous cap overlying a core of lipid and cell debris that protrudes into the vessel lumen, decreasing the vessel radius, thereby compromising normal blood flow.
A sample of 162 intracranial vertebral arteries was obtained from 81 embalmed cadavers of white and black, female and male South African adults, aged between 20 and 80 years. The arteries were routinely processed, transversely sectioned, stained with haematoxylin and eosin, and Weigert’s resorcin-fuchsin elastic stain, and examined under the light microscope at magnifications of Ω40 to Ω400. Atherosclerosis was classified as Grades 0 to 5 according to the degree of luminal occlusion, where Grade 0 represented no atherosclerosis and Grade 5 an atherosclerotic plaque occluding more than 75 % of the luminal cross-sectional area. From measurements of 108 of these vertebral arteries, the estimated degree of vessel luminal occlusion and, therefore, the estimated decrease in luminal radius and cross-sectional area by the developing plaque was calculated.
As there was no significant difference in the incidence of atherosclerosis between population groups or genders, the data were pooled. In the total sample, 5 % of the vertebral arteries had no atherosclerosis (Grade 0), 10 % Grade 1, 31 % Grade 2, 35 % Grade 3, 18 % Grade 4 and 1 % Grade 5 atherosclerosis. Compared with 0 % with Grades 0 and 1, an estimated decrease in arterial luminal cross-sectional area of about 20 % in Grade 2, 30 % in Grade 3, 75 % in Grade 4 and 95 % in Grade 5 atherosclerosis was found. That is, there was a decrease in arterial luminal radius of 0 % in Grades 0 and 1, 8 % in Grade 2, 16 % in Grade 3, 50 % in Grade 4 and 76 % in Grade 5 atherosclerosis. As blood flow is directly related to the fourth power of the radius of the vessel, these decreases in arterial luminal radius correspond to a blood flow decrease in the atherosclerotic vertebral arteries of 0, 28, 50, 94 and 100 %, respectively.
In conclusion, because at least 50 % of the intracranial vertebral arteries showed marked (Grade 3+) atherosclerotic changes, corresponding to significant reductions in luminal radius, and thus cross-sectional area, large decreases in blood flow are likely to occur in these vessels. Therefore, the data suggest that atherosclerosis of the intracranial vertebral artery is a risk factor for vertebrobasilar insufficiency in this population.
This work was carried out in the Department of Anatomical Sciences, University of the Witwatersrand, Johannesburg, South Africa. The assistance of Mrs A. Mortimer, Mrs L. York, Mrs S. Rogers and Mr A. Webb is gratefully acknowledged. The research was funded by a grant from the Medical Faculty Research Endowment Fund.