聴神経腫瘍における腫瘍の形態的性状と内耳道拡大に関する検討:骨破壊のメカニズムに関する考察

Morphological and clinical study of acoustic tumor with respect to enlargement of internal auditory canal:mechanism of bone destruction

I．はじめに

　聴神経腫瘍に伴う内耳道の拡大は画像診断学的に極めて重要な所見であるが，そのメカニズムは今日でもまだよく分かっていない．腫瘍の緩徐な増大に伴う慢性の物理的圧迫が内耳道壁破壊の最大要素と考えられるが，腫瘍のサイズが大きくても必ずしも拡大の程度が著しいとは限らず，様々な要因が関与している可能性がある．

　今回われわれは，聴神経腫瘍36例の形態的性状および臨床症状と内耳道拡大との関連について検討を行い，内耳道の拡大のメカニズムについて考察を行った．

The mechanism of widening of the internal auditory canal (IAC) by an acoustic tumor is still unknown. De-gree of IAC enlargement was expressed as 'IAC ratio' calculated from the measurement of bilateral horizontal diameters from hone-imaging CT.

Comparing IAC ratio to radiological findings of tumors, such as size, solidity, mode of attachment to pyramidal bone and the presence of ventricular dilata-tion, we found a positive correlation between the ratio and the volume of solid tumors (especially of smaller ones) and more marked destruction of IAC in associa-tion with ventricular dilatation. Preoperative degrees of hearing ability or facial nerve function didn't correlate with that of IAC enlargement.

It is our hypothesis that the mechanism involved in the destruction of IAC is as follows:(1) The repeated strokes of the tumor caused by brain pulsation gradual-ly erode the bony wall of IAC (especially, that of the posterior wall). The apparently less destruction of IAC in cases with mostly cystic large tumors might he due to the restriction of this pulsatile movement.(2) In-creased CSF pressure on the site has some influence on the mechanism of bone erosion as also does solidity of the tumor.