[1] Thomas Jefferson Univ, Dept Neurosurg, Philadelphia, PA 19107 USA
[2] Yellowstone Neurosurg Associates, Yellowstone Med Ctr E, Billings, MT USA
[3] Rocky Mt Neurosurg Alliance, Aurora, CO USA
[4] Johns Hopkins Univ, Sch Med, Dept Neurosurg Anesthesiol & Crit Care Med, Baltimore, MD USA