OBJECT: The Carotid Occlusion Surgery Study (COSS) was conducted to determine if 
superficial temporal artery-middle cerebral artery (STA-MCA) bypass, when added 
to the best medical therapy, would reduce subsequent ipsilateral stroke in 
patients with complete internal carotid artery (ICA) occlusion and an elevated 
oxygen extraction fraction (OEF) in the cerebral hemisphere distal to the 
occlusion. A recent publication documented the methodology of the COSS in detail 
and briefly outlined the major findings of the trial. The surgical results of 
the COSS are described in detail in this report.
METHODS: The COSS was a prospective, parallel-group, 1:1 randomized, open-label, 
blinded-adjudication treatment trial. Participants, who had angiographically 
demonstrated complete occlusion of the ICA causing either a transient ischemic 
attack or ischemic stroke within 120 days and hemodynamic cerebral ischemia 
indicated by an increased OEF measured by PET, were randomized to either 
surgical or medical treatment. One hundred ninety-five patients were randomized: 
97 to the surgical group and 98 to the medical group. The surgical patients 
underwent an STA-MCA cortical branch anastomosis.
RESULTS: In the intention-to-treat analysis, the 2-year rates for the primary 
end point were 21% for the surgical group and 22.7% for the medical group (p = 
0.78, log-rank test). Fourteen (15%) of the 93 patients who had undergone an 
arterial bypass had a primary end point ipsilateral hemispheric stroke in the 
30-day postoperative period, 12 within 2 days after surgery. The STA-MCA 
arterial bypass patency rate was 98% at the 30-day postoperative visit and 96% 
at the last follow-up examination. The STA-MCA arterial bypass markedly 
improved, although it did not normalize, the level of elevated OEF in the 
symptomatic cerebral hemisphere. Five surgically treated and 1 nonsurgically 
treated patients in the surgical group had a primary end point ipsilateral 
hemispheric stroke after the 30-day postoperative period. No baseline 
characteristics or intraoperative variables revealed those who would experience 
a procedure-related stroke.
CONCLUSIONS: Despite excellent bypass graft patency and improved cerebral 
hemodynamics, STA-MCA anastomosis did not provide an overall benefit regarding 
ipsilateral 2-year stroke recurrence, mainly because of a much better than 
expected stroke recurrence rate (22.7%) in the medical group, but also because 
of a significant postoperative stroke rate (15%). Clinical trial registration 
no.: NCT00029146.