title: Inter- and intrapatient variability of facial nerve response areas in the floor of the fourth ventricle.
abstract: BACKGROUND
  Surgical exposure of intrinsic brainstem lesions through the floor of the 4th ventricle requires precise identification of facial nerve (CN VII) fibers to avoid damage.
  
  
  OBJECTIVE
  To assess the shape, size, and variability of the area where the facial nerve can be stimulated electrophysiologically on the surface of the rhomboid fossa.
  
  
  METHODS
  Over a period of 18 months, 20 patients were operated on for various brainstem and/or cerebellar lesions. Facial nerve fibers were stimulated to yield compound muscle action potentials (CMAP) in the target muscles. Using the sites of CMAP yield, a detailed functional map of the rhomboid fossa was constructed for each patient.
  
  
  RESULTS
  Lesions resected included 14 gliomas, 5 cavernomas, and 1 epidermoid cyst. Of 40 response areas mapped, 19 reached the median sulcus. The distance from the obex to the caudal border of the response area ranged from 8 to 27 mm (median, 17 mm). The rostrocaudal length of the response area ranged from 2 to 15 mm (median, 5 mm).
  
  
  CONCLUSIONS
  Facial nerve response areas showed large variability in size and position, even in patients with significant distance between the facial colliculus and underlying pathological lesion. Lesions located close to the facial colliculus markedly distorted the response area. This is the first documentation of variability in the CN VII response area in the rhomboid fossa. Knowledge of this remarkable variability may facilitate the assessment of safe entry zones to the brainstem and may contribute to improved outcome following neurosurgical interventions within this sensitive area of the brain.
authors.affiliation: Klinik für Neurochirurgie, Universitäts-Spital Zürich, Zürich, Switzerland. bertalanffy@ini-hannover.de
authors.forename: Helmut
authors.initials: H
authors.lastname: Bertalanffy
authors.forename: Nadir
authors.initials: N
authors.lastname: Tissira
authors.forename: Niklaus
authors.initials: N
authors.lastname: Krayenbühl
authors.forename: Oliver
authors.initials: O
authors.lastname: Bozinov
authors.forename: Johannes
authors.initials: J
authors.lastname: Sarnthein
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001931:Brain Mapping; D001932:Brain Neoplasms / Q000473:pathology / Q000601:surgery; D001933:Brain Stem / Q000473:pathology; D002648:Child; D003399:Craniotomy / Q000379:methods*; D004558:Electric Stimulation / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology; D005154:Facial Nerve / Q000502:physiology*; D005260:Female; D020546:Fourth Ventricle / Q000473:pathology* / Q000601:surgery; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D055815:Young Adult