OBJECT: The Carotid Occlusion Surgery Study (COSS) was a large, prospective 
clinical trial that examined whether superficial temporal artery-middle cerebral 
artery (STA-MCA) bypass, in addition to best medical therapy, reduced the risk 
of ipsilateral ischemic stroke in patients with carotid artery occlusion and 
hemodynamic cerebral ischemia. Despite improved cerebral hemodynamics and 
excellent bypass graft patency rates, COSS failed to show a benefit for the 
surgical group with respect to ipsilateral stroke recurrence at 2 years after 
treatment. This was due to a lower than expected rate of recurrent ipsilateral 
stroke in the medically treated group and a high rate of perioperative 
ipsilateral strokes in the surgical group. Critics of the trial have cited 
surgeon inexperience and technical difficulties related to the performance of 
the bypass graft as a leading cause of failure of the trial.
METHODS: The authors retrospectively identified all patients from the COSS with 
an ipsilateral, perioperative (< 30 days) ischemic stroke after STA-MCA cortical 
branch anastomosis. Study records, operative notes, stroke adjudication forms, 
and imaging studies were reviewed. Ischemic strokes were characterized as bypass 
graft related or non-bypass graft related based on clinical and radiographic 
findings.
RESULTS: Fourteen of 93 surgically treated patients experienced an ipsilateral, 
perioperative ischemic stroke. Postoperatively, the mean oxygen extraction 
fraction (OEF) ratio between the symptomatic and asymptomatic cerebral 
hemisphere significantly improved in these patients (1.30 ± 0.18 preoperative vs 
1.12 ± 0.11 postoperative; p = 0.02), but did not normalize. In this cohort, 
total MCA occlusion time during the anastomosis (54.3 ± 23.5 minutes) was no 
different from the MCA occlusion time in those surgical patients who did not 
have a perioperative stroke (45.4 ± 24.2 minutes, p = 0.2). Bypass graft patency 
rates in patients with a perioperative stroke were 92% at 30 days (11 of 12 
patients with patency data) and 83% at last follow-up visit (10 of 12 patients 
with patency data). These patency rates were not significantly different from 
those achieved at 30 days (100%; 76 of 76 patients with patency data; p = 0.14) 
and at last follow-up (99%; 71 of 72 patients with patency data; p = 0.052) in 
patients without a perioperative stroke. Eighty-six percent (12 of 14 patients) 
of strokes were likely attributable to factors unrelated to the STA-MCA 
anastomosis. Only 21% of strokes (3 of 14 patients) were in the territory of the 
recipient vessel and likely related to technical performance of the anastomosis 
itself. One patient was thought to have dual stroke mechanisms.
CONCLUSIONS: Only a small minority of ipsilateral, perioperative ischemic 
strokes in the COSS could be attributed to technical problems of the bypass 
anastomosis. The majority of ischemic strokes could not be ascribed to this 
cause and were most likely due to patient hemodynamic fragility and the 
inability of patients to tolerate surgery.