title: A hypothesis of epiarachnoidal growth of vestibular schwannoma at the cerebello-pontine angle: surgical importance.
abstract: OBJECTIVE
  The purpose of this study is to clarify the rearrangement of the arachnoid membrane on the vestibular schwannoma during its growth in relation to adjacent neurovascular structures for a better understanding of dissecting plane of arachnoid during surgery.
  
  
  METHODS
  Arachnoid membrane over the tumour was investigated during surgery with suboccipital transmeatal approach in twenty-six tumours. All microsurgical procedures were recorded with a video and reviewed. The tumour growth was classified into five stages depending upon the tumour diameter in the cerebello-pontine (CP) angle: Stage 1; purely intracanalicular (2 cases), Stage 2; less than 5 mm (2 cases), Stage 3; > or = 5 and <15 mm (8 cases), Stage 4; > or = 15 and <25 mm (9 cases) and Stage 5; > or = 25 mm (5 cases). Rearrangement of the arachnoid on the tumour was conceptualised throughout all stages.
  
  
  RESULTS
  All tumours of Stage 1 and 2 were entirely located in the subarachnoid space of the cerebello-pontine cistern without arachnoidal rearrangement, while all tumours of Stages 3 to 5 were enveloped, in the CP angle, with invaginated arachnoid membrane consisting of cerebello-pontine cistern except two surfaces; the medial pole and the tumour surface under the facial and cochlear nerves near the porus.
  
  
  CONCLUSIONS
  The tumour originates subarachnoidally within the internal auditory meatus (IAM) and grows epiarachnoidally in the CP angle. Rearrangement of the arachnoid begins with its adhesion on the medial pole of the tumour along the porus, resulting in the arachnoidal invagination into the cerebello-pontine cistern with further growing of the tumour.
authors.affiliation: Department of Neurosurgery, Osaka City University Graduate School of Medicine, Osaka 545-8585, Japan. kohata@med.osaka-cu.ac.jp
authors.forename: K
authors.initials: K
authors.lastname: Ohata
authors.forename: N
authors.initials: N
authors.lastname: Tsuyuguchi
authors.forename: M
authors.initials: M
authors.lastname: Morino
authors.forename: T
authors.initials: T
authors.lastname: Takami
authors.forename: T
authors.initials: T
authors.lastname: Goto
authors.forename: A
authors.initials: A
authors.lastname: Hakuba
authors.forename: M
authors.initials: M
authors.lastname: Hara
mesh_terms: D000328:Adult; D000368:Aged; D002530:Cerebellopontine Angle / Q000473:pathology*; D005260:Female; D006801:Humans; D008297:Male; D008578:Meninges / Q000473:pathology*; D008875:Middle Aged; D009464:Neuroma, Acoustic / Q000473:pathology* / Q000601:surgery