頭蓋形成術の術前評価における側臥位CTの有用性

Usefulness of CT in the Lateral Decubitus Position for Preoperative Evaluation of Cranioplasty

Ⅰ．はじめに

　重症頭部外傷や脳内出血などの開頭術後に頭蓋内圧亢進が予想される場合，過剰な頭蓋内圧を軽減するために減圧開頭術を施行することがある．テント上の疾患ではそのような状態が軽減すると頭蓋形成術を行うが，一定の時期に施行するのではなく，頭部CTで脳腫脹の軽快を確認することで手術の施行を決定することが多い．脳腫脹が退縮した時期でも，仰臥位で撮影した通常のCT画像において，開頭部後方で脳組織が膨隆している所見をしばしば経験する．

　今回われわれは，6症例において患側を上にした側臥位で頭部CTを施行し，①仰臥位でみられた脳の局所的な膨隆が是正されるか，②側臥位CT画像が頭蓋形成術の可否を判断することに役立つかを検討した．

　Patients with massive brain swelling undergo external decompressive craniectomy to manage intracranial pressure. Following supratentorial craniectomy, and after the brain swelling is relieved, cranioplasty is performed. Although feasibility of the surgery is usually assessed by CT scanning in a supine position, it is sometimes difficult to determine whether the surgery can be performed safely.

　Although nine patients underwent a decompressive craniectomy during the study period, only six patients could undergo brain CT-first in a supine position and next in a lateral decubitus position with the surgical side upward-before cranioplasty. On CT images, the distance from the midline to the brain surface was measured on the image where brain bulging was maximal, and the bulging was calculated by comparing the image with the distance measured on the contralateral side. In all cases, brain bulging decreased with this change in position. The decrease ranged from 5.5-9.2mm（mean 7.1mm). Patients with brain bulging of 2.8-3.6mm in the lateral decubitus position needed no additional procedure, or only required drainage of a very small amount of cerebrospinal fluid（CSF）from the brain surface. Those with brain bulging of 5.1-12mm showed ventricular dilatation on CT images, and required ventricular puncture or spinal CSF drainage to decrease brain bulging for cranioplasty.

　We believe that the lateral decubitus position, with the surgical side upward, ameliorates the local brain shift induced by gravity. A lateral position during CT simulates the surgical head position for cranioplasty and can help to assess whether cranioplasty is feasible.