2011 ASA/ACCF/AHA/AANN/AANS/ACR/ASNR/CNS/SAIP/SCAI/SIR/SNIS/SVM/SVS guideline on the management of patients with extracranial carotid and vertebral artery disease: Executive summary - A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines and the American Stroke Association, American Association of Neuroscience Nurses