Abstract
Objectives: To study the feasibility of approaches in addressing the mastoid segment of facial nerve through transcanal route exclusively under endoscopic guidance and to verify the usefulness of the pyramidal eminence as a landmark.
 Study design: The prospective experimental study
 Setting: “Transcanal endoscopic temporal bone dissection lab” situated in the Surgiscope hospital, Chittagong, Bangladesh.
 Materials: 4 (Four) cadaveric temporal bones.
 Interventions: Two pure transcanal endoscopic approaches were applied to excavate the mastoid segment of facial nerve. In anterior-medial approach, the dissection was progressed from medial to the lateral direction through the retrotympanum focusing the anterior-medial side of the mastoid segment of facial nerve. Whereas in anterior-lateral approach, the dissection was progressed from lateral to the medial direction through the posterior canal wall focusing the anterior-lateral side of the mastoid segment of facial nerve. In both approaches, the pyramidal eminence was considered as an anatomical landmark to navigate the dissection for the excavation of the mastoid segment of facial nerve.
 Main outcome measures: The efficacy of each approach in respect of the complete excavation of the mastoid segment of facial nerve and the worthiness of the pyramidal eminence as the landmark for navigating the dissection.
 Results: In both transcanal endoscopic approaches, the entire mastoid segment of facial nerve could be exposed successfully. But in anterior-medial approach, the chorda tympani nerve was needed to be sacrificed. The search of the mastoid segment of facial nerve with the guidance of the pyramidal eminence was observed as effective and safe.
 Conclusion: The entire mastoid segment of facial nerve could be successfully addressed through transcanal route under pure endoscopic guidance. The pyramidal eminence has appeared as an important landmark for such endeavor.
 Bangladesh J Otorhinolaryngol; October 2019; 25(2): 108-115
Highlights
Bangladesh J Otorhinolaryngol endoscopic guidance.Recently practised transcanal endoscopic surgery for the lesions of the tympanic part of the facial nerve is a good example of such attempt.[1,2]
Switching from microscope to endoscope for approaching to the tympanic part of facial nerve has beenproved to be least morbid.,attempt to the lesions of other territories of the fallopian canal such as geniculate ganglion and the proximal part of mastoid segment of facial nervehad been done with success through transcanal route exclusively under endoscopic guidance.[3,4]
The fallopian canal of the mastoid segment of facial nerve extends from the second genu down to stylomastoid foramen of the temporal bone having almost a vertical course of 15.4 ± 2.14 mm long.[5]
Summary
The approach to the entire mastoid segment of the facial nerve has still remained under the domain of microscopic mastoid surgery. Unlike the tympanic segment of facial nerve, the mastoid segment is not readily approachable as it remains entirely hidden. The fallopian canal of the mastoid segment of facial nerve extends from the second genu down to stylomastoid foramen of the temporal bone having almost a vertical course of 15.4 ± 2.14 mm long.[5] The second genu and the adjacent small part of the mastoid segment are confined in the posterior wall of the retrotympanum. After leaving the retrotympanum in between the posterior wall of the external auditory canal and the horizontal semicircular canal, it goes down along the anterior wall of the mastoid process towards the stylomastoid foramen. The distance between the ampular end of the horizontal semicircular canal and the
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