Towards the development of guidelines for the surgical treatment of carotid artery disease: a cranial perfusion approach

The development of carotid artery disease in the form of atherosclerotic plaque and the progressive narrowing of the carotid artery lumen is the leading cause of stroke, the third most prevalent cause of death in the world today. Surgical interventions in the form of carotid endarterectomy (CEA) surgery and carotid angioplasty and stenting (CAS) have been shown to be effective in treating high grade (>50% diameter reduction) stenosis. The advent of the CAS procedure was heralded as the modern minimally invasive alternative to the invasive CEA procedure. However, uptake of the procedure has been slow due to associated peri-operative complications, with the results of the recently published CREST trial showing a higher peri-procedural stroke rate for CAS compared to CEA at 4.1% and 2.3%, respectively. During both procedures blood flow is required to be interrupted in the treated ICA for a period of time. However, the ability of the cerebrovascular system to compensate for this ICA occlusion and maintain cerebral perfusion without adverse ischemic events occurring, is yet unknown. The main collateral pathway for the redistribution of blood flow within the brain is a ring link network of arteries at the base of the brain, known as the Circle of Willis (CoW). However, studies have shown that less than 20% of the population have a complete CoW, with the remainder having missing or hypoplastic (under-developed) arteries. It has been suggested that an incomplete CoW can predispose 1 in 6 patients to hemodynamic compromise due to ICA occlusion. The present research focusses on examining how these variations in the CoW anatomy effect cerebral perfusion during ICA occlusion and determine their contribution to peri-procedural complications. In this study, a generic representative model of the CoW was created and the artery segments varied to represent 20 of the 23 CoW variations highlighted in literature. A computational flow study was then conducted on all 20 variations to determine their effect on cerebral perfusion. Also incorporated in the model was both a static and dynamic autoregulation model. Autoregulation is a mechanism by which the body attempts to maintain a relatively constant cerebral blood flow rate over a range of cerebral perfusion pressure from 70-170 mmHg. However, it has been shown that autoregulation is impaired in elderly patient and in the presence of carotid artery disease. By incorporating the autoregulation model in this study the effect variations in autoregulation functionality have on cerebral perfusion was also investigated. The computational analysis has identified specific CoW variations that are high risk for brain ischemia during ICA occlusion. These high risk variations correspond to 25.9% of the population. It was also found that the autoregulation mechanism had severely limited (<2%) recovery effects on cerebral blood flow in high risk patient due to the lack of sufficient collateral pathways through which to redistribute blood supply from the patent inlet arteries. These findings were validated experimentally using a 3-scale flow phantom of the generic CoW geometry. In conclusion, specific, identifiable variations in anatomy of the CoW can limit its ability to operate effectively as a collateral pathway during ICA occlusion and therefore make a specific cohort of patient high risk for peri-procedural complication during ICA occlusion.