Toshihiro Ogiwara1, Tetsuyoshi Horiuchi2, Kazuhiro Hongo1. 1. Department of Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan. 2. Department of Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan. Electronic address: tetuyosi@shinshu-u.ac.jp.