Reduced ICA Diameter and Reduced Stroke Risk
To the Editor:
I read with interest the report from Rothwell and Warlow1 describing a reduced risk of stroke with reduced internal carotid artery (ICA) diameter distal to severe symptomatic carotid stenosis. In postulating a reason, the authors state that poststenotic narrowing must indicate low intraluminal pressure due to reduced flow across the stenosis, and then conclude that the low flow is less likely to dislodge emboli.
An alternative hypothesis is that the narrow ICA is a result of remodeling in response to a reduced blood velocity. Labropoulos et al2 have found evidence for compensatory arterial enlargement in areas of small hemodynamically insignificant plaque in carotid, coronary, and lower-limb arteries. These plaques, although small, would cause localized increase in blood velocity that may be triggering the enlargement process, bringing the blood velocity closer to its original value. If the same mechanism is causing the decrease in ICA diameter, this would imply that the reduced blood velocity occurs prior to the diameter change and that the low velocity has been present long enough for remodeling to occur.
The reduction in blood velocity can occur either because of reduced supply through a proximal tight, hemodynamically significant stenosis or because of reduced demand in the distal vascular bed. In my experience of lower-limb duplex scanning, the arterial diameter is not normally reduced distal to a tight stenosis even though the blood pressure is reduced, as shown by a low ankle brachial pressure index. Indeed, a reduction in diameter would increase the vascular resistance and could further reduce flow to an ischemic region. I feel it is more likely that the reduced ICA diameter occurs in response to a reduced demand in the distal vascular bed caused by previous embolic occlusions.
Rothwell and Warlow1 state that patients with poststenotic narrowing of the ICA have a low risk of stroke on follow-up but a high frequency of major stroke in the past and a high frequency of infarction in the ipsilateral cerebral hemisphere compared with patients without poststenotic narrowing. These past episodes would have reduced the blood flow in the affected hemisphere and would be consistent with the remodeling hypothesis.
The lower risk of subsequent stroke may arise in 2 ways. First, the reduced blood flow means that the blood velocity through a given stenosis is reduced, the shear stress is less, and therefore the risk of shedding emboli is reduced. Second, the fact that remodeling takes time means that patients with narrow ICAs are more likely to have stable plaque which has already passed through the higher-risk acute stages.
Supporting evidence for the remodeling hypothesis may be available from below-knee unilateral amputees. Superficial femoral diameters could be measured bilaterally and the ratio in amputees compared with normal values. If there is evidence for remodeling, the time course could be studied by serial measurements after amputation.
- Copyright © 2000 by American Heart Association