Extracranialintracranial Arterial Bypass

EC-IC arterial bypass involves the use of general anesthesia, open craniotomy, and end-to-side anastomosis of the superficial temporal artery to a branch of the middle cerebral artery (MCA) (Fig. 6.1). Currently, this technique is being used primarily in the setting of intracranial aneurysm therapy, moyamoya disease, and

FIGURE 6.1 CT angiography of an EC-IC bypass, showing the new intracranial course of the right superficial temporal artery, anastamosed to the middle cerebral artery M2 segment.

preservation of vascular flow during tumor removal.20 EC-IC arterial bypass was previously advocated in patients with ICA stenosis or occlusion, carotid siphon stenosis, MCA occlusion, and peripheral branch MCA occlusion. Moreover, early reports and clinical studies supported its use in patients with TIAs, prolonged reversible ischemic neurological deficits, and completed infarcts.21 However, in 1985 an international randomized study of 1377 patients by the EC-IC Bypass Study Group subsequently demonstrated a lack of benefit in patients with atherosclerotic disease of the carotid and middle cerebral arteries.22

Failure of the trial to demonstrate a benefit for EC-IC bypass was unexpected, since many who performed the procedure had already concluded it to be useful and effective.23 The study itself was well designed, involved a large number of patients, had a perfect follow-up record, and a high bypass patency rate. No distinction was made, however, between the hemodynamic or embolic origin of cerebral and retinal ischemic events. More importantly, physiological imaging was not used in the selection of patients for the procedure. Given that the study was not specifically designed to measure a benefit in neurologic function, which may occur if blood flow to the ischemic penumbra was restored, patients selected using physiological parameters could still demonstrate a clinical benefit from EC-IC bypass.

In a study by Powers et al.24 on 29 EC-IC bypass patients compared to 23 non-surgical patients, selection criteria on the basis of reduced cerebral perfusion pressure by positron emission tomography (PET) failed to prove beneficial. However, this was a nonrandomized trial, and compared with the nonsurgical group, surgical patients had more TIAs, multiple TIAs, fewer ICA occlusions without recurrent symptoms, and symptoms within 30 days prior to entry into the study. Stronger evidence seems to support the use of physiological methods of imaging to select patients for EC-IC bypass. For instance, the St. Louis Carotid Occlusion Study demonstrated that increased oxygen extraction fraction (OEF) measured by PET predicts subsequent ipsilateral stroke in patients with symptomatic carotid occlusion.25 In other studies, postoperative improvement in regional cerebral blood flow detected by PET correlated with clinical improvement.26,27

The potential benefit of EC-IC bypass, therefore, has not been well studied in acute stroke patients carefully selected using newer physiological methods, including PET, xenon CT, single-photon emission computed tomography (SPECT), or CT or MR perfusion.5 Ongoing trials such as the Carotid Occlusion Surgery Study (COSS), the entry criteria of which include recent symptomatic occlusion of the ICA and increased OEF measured by PET, may help to clarify the benefit of emergency EC-IC bypass for selected patients (Figure 6.2). The Japanese EC-IC Bypass Trial (JET) is another ongoing randomized trial of EC-IC bypass in patients with severe hemodynamic failure measured by SPECT. Although final results are pending, JET preliminarily demonstrates a reduced incidence of major stroke or death in the 2-year period after surgery.28,29

With respect to timing of surgery, there is little evidence to either support or challenge the use of emergent EC-IC bypass in the setting of acute cerebral ischemia. The EC-IC Bypass Study results do not apply to acute atherosclerotic stroke patients, since patients within 8 weeks of an acute cerebral ischemic event were excluded.30 In a study by Engel et al.,21 patients presenting with progressive strokes who were surgically treated within 3-4 weeks following symptom onset had the least benefit. Other studies have not convincingly shown improved outcome with the use of emergency EC-IC bypass.31,32

In the emergency situation, we believe this technique can be successfully employed in selected patients with symptomatic ischemia due to dissection or atherosclerotic disease despite being on maximal medical therapy. We have also seen patients with carotid occlusion and an isolated cerebral hemisphere (poor collateral flow) benefit from EC-IC bypass. Any potential benefit, however, must

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