EC-IC Bypass Evidence

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16 EC-IC Bypass Evidence

The first description of an operation intended to provide alternative cerebral blood flow (CBF) was given by German and Taffel in 1939, in which a temporalis muscle pedicle was placed over the cerebral cortex in monkeys (Figure 16–1).1 They later subjected both treated and untreated controls to carotid and vertebral occlusion, and reported the monkeys with temporalis pedicles overlying the cortex survived, whereas untreated controls did not. In 1942, Kredel first performed the procedure in three human patients who had suffered ischemic stroke, reporting postoperative clinical improvement in all three patients.2 However, the technique was largely abandoned over the next decade when postoperative angiograms failed to provide evidence of collateral flow through the graft.

C. Miller Fisher shed new light on the concept of re-establishing cerebral perfusion in 1951 when he postulated that extracranial to intracranial (EC-IC) arterial bypass might be performed as a treatment for occlusive vascular disease.3 That same year, Pool and Potts created a superficial temporal artery (STA) to anterior cerebral artery (ACA) shunt during treatment of a giant ACA aneurysm. Although the shunt was successful intraoperatively, postoperative angiogram confirmed the shunt had become occluded.4

The first EC-IC arterial bypass was not described until 1966 when Yasargil and Donaghy successfully performed an STA to middle cerebral artery (MCA) anastomosis in a dog. Shortly thereafter, Yasargil successfully performed the same procedure in a human, providing the neurosurgical community with what was deemed a breakthrough in the treatment of intracranial atherosclerosis. Following Yasargil’s description of the EC-IC arterial bypass, neurosurgeons around the world utilized the procedure with the intent of re-establishing CBF in a variety of cerebrovascular diseases. Surgeons subsequently described bypasses from the occipital and middle meningeal arteries to the MCA, from the occipital artery to the posterior inferior cerebellar artery,5 from the occipital artery to the anterior inferior cerebellar artery (AICA), and from the STA to the superior cerebellar artery.5,6

Over time surgical technique improved, and the most frequently employed bypass procedure became the STA to MCA bypass for the indication of symptomatic internal carotid artery (ICA) or MCA stenosis. However, as was the case with most other surgically treated diseases at the time, there were no extant data to demonstrate that performing the procedure provided patients with any significant short- or long-term benefit. The only early report came in the form of a small retrospective series published by Sundt et al. in 1977, the utility of which was limited by multiple biases. The group reported on a retrospective series of 56 patients who underwent STA-MCA bypass for a wide variety of indications, including TIAs, “orthostatic ischemia,” progressive stroke, and aneurysmal bypass. With regards to graft patency, those who had grafts prior to 1973 had low patency (25%) whereas those operated after 1974 had high patency (95%). The other outcome described was major ischemic stroke or death from the surgery, of which they reported none. The report served more to describe the acceptable safety of the procedure, rather than to examine its efficacy.7

As evidenced by this first series, the greatest impediments to a meaningful conglomerate of data were the lack of consistency in patient selection and evaluation, the lack of an appropriate medically treated control population, and the lack of consistent outcomes follow-up. As evidence-based medicine emerged and the Food and Drug Administration’s requirements for proving pharmacologic safety and efficacy became increasingly stringent, neurologists and neurosurgeons too recognized the imperative of defining the “efficacy” of risk-containing surgical procedures. In a 1977 Stroke editorial introducing the initiation of the first ever EC-IC bypass study, Fletcher H. McDowell wrote,

Pending the results of the EC-IC Bypass Study, neurosurgeons published a handful of case reports, series, and commentaries on the appropriate indications, safety, and utility of EC-IC bypass.915 In 1983, Chater reported on 400 patients who underwent EC-IC bypass for TIAs and hemodynamic intracranial lesions, with a permanent neurological morbidity rate of approximately 2% and operative mortality of 2.5%, and an ipsilateral postoperative stroke incidence of 0.9% per year after a 55-month follow-up.16 Chater et al. then reported on 105 patients with intracranial ICA stenosis (60% to 98%) and ischemia 1 to 3 months prior to EC-IC bypass. Of these, surgical mortality was 1%, permanent surgical morbidity was 2%, and after 54 months of follow-up, 22 patients had died, of which three were related to stroke; thus the overall late death rate was 4% per year. Although Chater’s reports had begun to examine a more uniform patient population, both of his retrospective series contained no medically treated control group against which the success of surgery could be measured.17

International cooperative study of ec-ic arterial anastomosis

In 1985, nearly 8 years after McDowell’s introduction of the International Cooperative Study of Extracranial-Intracranial Arterial Anastomosis (termed the EC-IC Bypass Trial), the group reported the results of this randomized, prospective trial. Of the 1377 patients enrolled in the trial, 714 (52%) were assigned to medical treatment alone (daily aspirin and aggressive hypertension control) and 663 (48%) were assigned to receive EC-IC bypass in addition to medical therapy. Inclusion and exclusion criteria are presented in Table 16–1.

Table 16–1 Inclusion and Exclusion Criteria for the EC-IC Bypass Trial.

Inclusion Criteria

Exclusion Criteria

Notably, patients were allowed to enroll in the trial if they had tandem stenosis (two separate stenosis lesions) of the proximal and distal ICA, the proximal lesion being amenable to carotid endarterectomy18 and the distal lesion qualifying for EC-IC bypass. Which surgery was performed first was left to the discretion of the operating surgeons, with the caveat that CEA, if performed first, was to occur at least 30 days prior to randomization in the EC-IC Bypass Trial.

Patients randomized to surgery underwent end-to-side STA or occipital artery anastomosis to a cortical MCA branch. Both surgical and medical patients received aspirin 325 mg qid and optimized hypertensive control for the duration of the trial. The surgical and medical arms were matched well for age, race/ethnicity, and risk factors. Over 70% of patients had abnormal neurologic findings at entry, but preoperative functional status testing determined the majority (over 90%) had little or no functional impairment. Regarding the type of vascular disease, the majority of patients (59% in medical arm, 58% in surgical arm) had isolated ICA occlusion, and the next most common were tandem lesions followed by isolated ICA stenosis. Only 8% in each arm had MCA stenosis and 8% had MCA occlusion.

The results of the trial demonstrated that EC-IC bypass provided no significant reduction in major and fatal strokes, ipsilateral strokes, major ipsilateral strokes, or all strokes and deaths combined. Separate analyses in patients with different angiographic lesions did not identify a subgroup with any benefit from surgery. They also reported 30-day surgical mortality and major stroke morbidity rates of 0.6% and 2.5%, respectively. The graft patency rate was reported as 96%, roughly similar to previous series.19

The completion of such a well-run, large-scale trial was thought a major achievement in surgical literature, but its negative results served as a major blow to the perceived utility of the EC-IC bypass procedure. A sharp decline in the number of procedures performed annually followed its publication; however, neurologists and neurosurgeons remained mindful of the study’s limitations.

Study Criticism

The major criticism of the EC-IC Bypass Trial was that patients were not selected for surgery based on the preoperative physiology of their occlusive disease, but rather were selected based on the anatomy of their intracranial occlusive disease. The way that intracranial occlusive disease influences flow dynamics and cerebral perfusion, and thus at-risk tissue that may benefit from bypass, was increasingly recognized as physiology far more complex than simply identifying a stenotic lesion on angiogram. Some argued that patients with a high oxygen extraction ratio (OER) and a critically under-perfused cerebral hemisphere may have been positioned to benefit in a significant way from a bypass procedure. In order to determine OER, however, advanced technology using positron emission tomography (PET) scanning was required, which was not available at the start of the trial, and upon publication of the trial was only available at a few specialized centers.

Furthermore, the bypass study contained no description of patients’ collateral circulation sources into the clinically affected hemisphere. This information was considered critical with the advent of carotid endarterectomy data showing that intraoperative determination of collateral flow by radioactive xenon washout was predictive of subsequent stroke: no infarctions occurred in patients with collateral CBF greater than 40 ml/100 g/min, whereas strokes incidence was 3.4% per year when CBF was less than 25 ml/100 g/min.20 The concept applies to EC-IC bypass in that the ideal candidate would be one with naturally available but insufficient collateral channels, for which the bypass would provide improved blood flow.

Some argued that the study was designed to select patients who were not in physiologic need of bypass—those with large numbers of low-risk or non-hemodynamic obstructions, a population that was presumed to gain little benefit from bypass surgery. The other patient population that raised concerns were those with tandem stenosis. These separate stenoses were not managed in a uniform way, or even clearly defined in the bypass trial’s results.

The outcome after completion of the trial was that neurosurgeons and neurologists still believed that there was a role for EC-IC bypass in treating cerebrovascular disease, but that it would be critical to identify the small subset of patients whose preoperative flow physiology would position them best to benefit from what was now understood to be a relatively low-risk procedure with excellent postoperative patency rates.

Moyamoya Disease: EC-IC Bypass and EDAS

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