1. Department of Pediatrics, University of Tsukuba Hospital, 2-1-1 Amakubo, Tsukuba, Ibaraki, Japan
2. Department of Child Health, Institute of Medicine, University of Tsukuba, 2-1-1 Amakubo, Tsukuba, Ibaraki, Japan
3. Department of Neurosurgery, Institute of Medicine, University of Tsukuba, 2-1-1 Amakubo, Tsukuba, Ibaraki, Japan
4. Center for Medical Sciences, Ibaraki Prefectural University of Health Sciences, 4669-2 Ami, Inashiki-gun, Ami-machi, Ibaraki, Japan