内耳道内伸展を示さず小脳橋角部脳槽内に限局した聴神経鞘腫の1例:“脳槽内”聴神経鞘腫

Acoustic neurinoma located exclusively in cerebellopontine angle cistern (“cisternal” acoustic neurinoma):a case report

I．はじめに

　聴神経鞘腫は一般に，内耳孔より外側の内耳道内に存在する前庭神経のglial-Schwann cell junction付近から発生するため^6），その初期から内耳道の変化を示し，耳鳴や聴力低下をきたすことが特徴とされている^5）．一方，内耳道の変化を示さない聴神経鞘腫が存在することも古くから知られており^4），聴神経鞘腫はまれに内耳孔よりも脳幹側の前庭神経から発生することがあると考えられている^2）．しかし，術前検査にて内耳道の変化を認めなかった症例も，手術所見では腫瘍の一部は内耳道内に伸展していたと報告されており^1,4），腫瘍がまったく内耳道内に伸展せず，小脳橋角部脳槽内に限局していることが，手術により確認された聴神経鞘腫の報告は認められない．われわれは腫瘍が大きいにもかかわらず，腫瘍の内耳道内伸展をまったく認めず，小脳橋角部脳槽内に限局した聴神経鞘腫（以下“脳槽内”聴神経鞘腫と呼ぶ）の1例を経験した．この症例は一般的な聴神経鞘腫とは異なった臨床的特徴を示していたことから，“脳槽内”聴神経鞘腫の症候学的特徴や治療上の問題点などについて考察を加え報告する．

This is a case report of acoustic neurinoma which was located exclusively in the cerebellopontine angle (CPA) cistern and which did not extend into the inter-nal auditory meatus (“cisternal” acoustic neurinoma) The 43-year-old female patient had signs of the left trigeminal nerve impairment including left face neural-gia. However, she did not have any neurootological symptom. No abnormal bony changes in the internal auditory meatus (IAM) were found by high-resolution bone-window CT. MRI showed a left CPA tumor of 25mm not extending into the IAM. The tumor was totally removed by the lateral suboccipital approach. Itoriginated from the vestibular nerve medial to the porus acusticus and was located exclusively in the CPA cistern. No tumor extension into the IAM was con-firmed. The cochlear nerve was involved in the tumor capsule and could not be preserved. The pathological diagnosis was that it was a neurilemmoma. Early dia-gnosis of “cisternal” acoustic neurinoma is difficult be-cause it does not show neurootological symptoms in the early stage. The lateral suboccipital approach is appropriate for the removal of a “cisternal” acoustic neurinoma. However, in spite of the good preoperative hearing, the preservation of hearing is difficult because of the large tumor size.