Surgery for Occlusive Atherosclerotic Disease
Extracranial-intracranial (EC-IC) bypass for revascularisation in the setting of atherosclerotic occlusive disease has remained a topic of intense interest and scrutiny over the last four decades. The underlying premise of EC-IC bypass in this setting is to provide blood flow augmentation in the setting of cerebral ischaemia, aiming to improve blood flow to oligemic brain tissue.
Evidence for Extracranial-Intracranial Bypass
In evaluating the efficacy of EC-IC bypass for ischaemia, the primary focus has been on anterior circulation disease. Flow augmentation bypass in the anterior circulation primarily entails a bypass between the superficial temporal artery and the middle cerebral artery (STA-MCA). The EC-IC bypass study published in 1985 failed to demonstrate the efficacy of STA-MCA bypass over medical management in over 1,300 patients with occlusive disease of the anterior circulation.1 Patients with transient ischaemic attack (TIA) or stroke in the setting of occlusive disease not amenable to carotid endarterectomy (primarily carotid occlusion) were randomised to best medical therapy or best medical therapy in addition to EC-IC bypass. Although surgery was performed successfully, with a reported bypass patency rate of 96 %, EC-IC bypass conferred no benefit in terms of stroke risk over the average follow-up period of 55.8 months.
Subsequent analysis of the study methodology identified shortcomings in study design and implementation that suggested universal abandonment of EC-IC bypass for ischaemic was premature.2,3 The trial was criticised for potential selection bias in enrolment, with concerns that a large number of patients, more than enrolled within the study, actually underwent surgery outside the trial. This would suggest that patients may have been selectively offered surgery versus enrolment in the trial based on perceived benefit of intervention, thus diluting any beneficial effect of bypass in those within the study cohort. Concern was also raised regarding the adequacy of flow augmentation provided by the STA-MCA bypass, with the notion that higher flows through larger calibre bypass could be more effective. The STA-MCA bypass has been the primary type of bypass performed in the setting of ischaemia, due to its high technical success rate and low morbidity given that anastomosis is performed to a distal cortical MCA branch with little risk of ischaemia during crossclamping. 4 However, the degree of flow augmentation can be relatively modest compared with larger conduit bypasses such as vein or radial artery interposition grafts from the cervical carotid to the more proximal MCA. Larger conduits do carry several disadvantages, however, including higher morbidity and lower patency rates,5 and the risk of ischaemia during cross-clamping of the more proximal MCA. Additionally, there is concern for hyperperfusion haemorrhage when using high flow grafts to revascularise ischaemic brain,6 presumed to be due to the inability of the vasodilated ischaemic vascular bed to compensate acutely to a large increase in perfusion created by a high flow graft. Favourable preliminary results with higher flow grafts in small series have been reported with the use of a non-occlusive bypass technique, the excimer laser-assisted anastomosis (ELANA), which allows vein grafts to be placed onto large proximal vessels, such as the supraclinoid carotid artery or proximal MCA, without clamping the vessel during anastomosis.7,8 This technique is still under evaluation, but the lack of temporary vessel occlusion during surgery, or the more physiological placement of the bypass proximally in the intracranial circulation, appears to reduce the risk of hyperperfusion.7
The most prominent drawback of the EC-IC bypass trial, which ultimately generated the most subsequent analysis and investigation, was the lack of haemodynamic evaluation in selecting patients for enrolment. In the trial, no objective physiological criteria were used to assess cerebral blood flow, and thus patients may have presented with ischaemic symptoms secondary to aetiologies unlikely to benefit from bypass, such as embolic phenomena or small vessel disease. Subsequent to the ECIC bypass trial, the role of haemodynamic assessment in evaluation of carotid occlusive disease has been extensively evaluated, demonstrating that haemodynamic evaluation can identify a subgroup at higher risk of recurrent stroke,9–15 and who would, therefore, be more appropriate candidates for revascularisation. In the last two decades, two trials have attempted to address the efficacy of bypass in such patients with haemodynamic compromise. The Japanese EC-IC bypass trial (JET) randomised patients with symptomatic internal carotid artery (ICA) or MCA occlusive and haemodynamic impairment to undergo STA-MCA bypass versus medical therapy. Interim results based on analysis of 196 patients published in a Japanese language journal reported a favourable result for bypass at two-year follow-up,16 with a reduced risk of the primary endpoint of death and stroke (23.1 % in the medical arm and 15.2 % in the surgical arm; p=0.046). The Carotid Occlusion Surgery Study (COSS) based in the US, however, did not support the efficacy of bypass for stroke-risk reduction.17