[1] Hokkaido Univ, Grad Sch Med, Dept Otolaryngol Head & Neck Surg, Kita Ku, Sapporo, Hokkaido 0608638, Japan
[2] Hokkaido Univ, Grad Sch Med, Dept Neurosurg, Sapporo, Hokkaido 0608638, Japan
[3] Hokkaido Univ, Grad Sch Med, Dept Plast & Reconstruct Surg, Sapporo, Hokkaido 0608638, Japan