MRIによる脳幹周囲の解剖学的検討が有用であった脳幹損傷の1例

The Usefulness of MRI Around the Tentorial Incisura for Determining the Mechanism of Brain Stem Imjury : A Case Report

　Ⅰ．はじめに

外傷性一次性脳幹損傷には，diffuse brain injury（DBI）の重症例に伴うものではなく，比較的軽度な外力により発生し，受傷直後からの意識障害も遷延せずに転帰良好な例が存在することが指摘されている^3,4,9-12)．このタイプの脳幹損傷はテント切痕縁による脳幹の局所性損傷とされ，側方からの外力によって打撲部と同側で，テント切痕縁と脳幹の距離が最も近い脳幹側方部に病変が生じると考えられている^10,11)．

　今回，われわれは右頭部を打撲した頭部外傷で対側の局所性中脳損傷例を経験し，magnetic resonance imaging（MRI）によって中脳とテント切痕縁の距離の左右差が，損傷の機序に関与したと示唆された1例を経験したので，文献的考察を加え報告する．

　We report a case of isolated brain stem injury caused by the tentorium cerebelli. A 17-year-old male was admitted to our hospital. Thirty minutes before admission,he was struck by another motorcycle while driving his own motorcycle. The Glasgow Coma Scale (GCS) was 10. He had no extremity weakness and his pupils were normal. CT scan showed no evidence of intracranial lesions except for facial bone fractures. His consciousness level improved to GCS 13 at 6 hours after admission. Follow-up CT scan was normal,however MRI 3 days after admission showed a contusion at the left lateral midbrain. He was discharged without any neurological deficits on 6 days after admission. Analysis of the distance between the brain stem and the tentorial margin using MR cisternography showed that the left side was shorter than the right side.

　We presumed that an isolated lateral brain stem injury was caused by the direct impact of the tentorium cerebelli. Typically the location of this type of lesion is present in the same side as that of impact. However in this case the orientation was opposite to that. We considered that the distance between the brain stem and the tentorial margin affected the mechanism of this lesion.