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Abstract
Carotid artery disease is common and increases the risk of stroke. However, there is wide variability on the severity of clinical manifestations of carotid disease, ranging from asymptomatic to fatal stroke. The collateral circulation has been recognized as an important aspect of cerebral circulation affecting the risk of stroke as well as other features of stroke presentation, such as stroke patterns in patients with carotid artery disease. The cerebral circulation attempts to maintain constant cerebral perfusion despite changes in systemic conditions, due to its ability to autoregulate blood flow. In case that one of the major cerebral arteries is compromised by occlusive disease, the cerebral collateral circulation plays an important role in preserving cerebral perfusion through enhanced recruitment of blood flow. With the advent of techniques that allow rapid evaluation of cerebral perfusion, the collateral circulation of the brain and its effectiveness may also be evaluated, allowing for prompt assessment of patients with acute stroke due to involvement of the carotid artery, and risk stratification of patients with carotid stenosis in chronic stages. Understanding the cerebral collateral circulation provides a basis for the future development of new diagnostic tools, risk stratification, predictive models and new therapeutic modalities. In the present review we discuss basic aspects of the cerebral collateral circulation, diagnostic methods to assess collateral circulation, and implications in occlusive carotid artery disease.
Highlights
Atherosclerosis affecting the carotid artery is common in the general population and has been related to stroke [1], cognitive impairment [2] and dementia [3]
Previous studies have found an association of impaired vasomotor reactivity (VMR) with higher risk of stroke/TIA in patients with carotid stenosis [23, 24], and with increase in ischemic lesion volume in the borderzone ipsilateral to an internal carotid occlusion, supporting that brain areas extending between superficial and deep middle cerebral artery (MCA) branches are susceptible to hemodynamic compromise and impaired washout of microemboli [25]
Patients with good collateral flow had less hypoperfused tissue and less infarct growth within the penumbra zone than those with poor collaterals. These findings provide further support to the notion that poor collateral flow is an important determinant of brain tissue fate, in the setting of poor/ partial recanalization after treatment and the presence of an extensive diffusion-weighted imaging (DWI)/perfusion-weighted imaging (PWI) mismatch on pretreatment MRI [38]
Summary
Atherosclerosis affecting the carotid artery is common in the general population and has been related to stroke [1], cognitive impairment [2] and dementia [3]. The anatomical features of collateral circulation may determine patterns of stroke in occlusive carotid artery disease (i.e. borderzone vs territorial) [12]. The protective role of the collateral circulation depends on several factors including anatomical variations, systemic arterial pressure, age and the rate of development of occlusive disease. The circle of Willis constitutes the main network of collateral circulation and is immediately available to maintain perfusion in case of acute large artery occlusion. A rapid drop in intraluminal pressure results in relaxation of smooth muscle cells, vasodilation, and a drop of vascular resistance This facilitates blood flow to ischemic tissue, as long as systolic blood pressure is maintained above 50 mm/Hg, otherwise the collateral circulation fails. Angiogenesis or development of effective new conductive arterial vessels in the brain has not been conclusively proven in the human brain, with the exception of selected conditions such as moyamoya disease [20]
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