Tympanic facial nerve segment surgery has been traditionally performed using microscopic approaches, but currently, exclusive endoscopic approaches have been performed for traumatic, neoplastic, or inflammatory diseases, specially located at the geniculate ganglion, greater petrosal nerve, and second tract of the facial nerve, until the second genu. The tympanic segment of the facial nerve can be reached and visualized using an exclusive transcanal endoscopic approach, even in poorly accessible regions such as the second genu and geniculate ganglion, avoiding mastoidectomy, bony demolition, and meningeal or cerebral lobe tractions, with low complication rates using a minimally invasive surgical route.
Key points
- •
Endoscopic magnification of the tympanic cavity anatomy and good knowledge of middle ear structures allow a minimally invasive approach for the removal of disease.
- •
The transpromontorial approach allows eradication of lesions involving fundus of internal auditory canal and petrous apex, with limited extension to the intracochlear, intravestibular, and pericarotid regions.
- •
The suprageniculate approach allows eradication of pathologic conditions involving the triangular area between geniculate ganglion inferiorly, middle cranial fossa dura superiorly, and labyrinthic bloc posteriorly.
- •
Endoscopic transcanal facial nerve decompression is applicable in cases of posttraumatic facial palsy, in particular, when there is an involvement of the geniculate ganglion region.
- •
The endoscopic approach allows a complete exposure of the facial nerve course, from the labyrinthic tract to the second genu, with low morbidity.
Introduction
The dissemination of endoscopic ear surgery in the otological community in the last decade expanded the use of the external auditory canal (EAC) as a natural surgical corridor to access diseases located in the tympanic cavity. Several investigators have described the use of exclusive endoscopic transcanal approaches to tympanic cavity cholesteatomas.
These approaches allowed also an improvement of the tympanic cavity anatomic knowledge, especially over the last 5 years. In fact, several studies in literature were focused on the endoscopic anatomy of the retrotympanum and epitympanum.
Recently, the endoscopic anatomy from the EAC to the internal auditory canal (IAC) was described in detail, and this allowed also the identification of the facial nerve pathway from the second genu until the geniculate ganglion (GG), and from the GG to the intralabyrinthine segment of the facial nerve into the IAC.
Moreover, using the EAC as a natural surgical corridor, the investigators described the possibility of reaching the tympanic segment of the facial nerve, studying the anatomic conformation and its relationships with the surrounding anatomic structures.
The continued progress of the facial nerve endoscopic anatomy knowledge permitted an advancement in surgery, performing facial nerve transcanal exclusive endoscopic surgery for the treatment of facial nerve diseases located on its tympanic portion, GG, and suprageniculate fossa.
Introduction
The dissemination of endoscopic ear surgery in the otological community in the last decade expanded the use of the external auditory canal (EAC) as a natural surgical corridor to access diseases located in the tympanic cavity. Several investigators have described the use of exclusive endoscopic transcanal approaches to tympanic cavity cholesteatomas.
These approaches allowed also an improvement of the tympanic cavity anatomic knowledge, especially over the last 5 years. In fact, several studies in literature were focused on the endoscopic anatomy of the retrotympanum and epitympanum.
Recently, the endoscopic anatomy from the EAC to the internal auditory canal (IAC) was described in detail, and this allowed also the identification of the facial nerve pathway from the second genu until the geniculate ganglion (GG), and from the GG to the intralabyrinthine segment of the facial nerve into the IAC.
Moreover, using the EAC as a natural surgical corridor, the investigators described the possibility of reaching the tympanic segment of the facial nerve, studying the anatomic conformation and its relationships with the surrounding anatomic structures.
The continued progress of the facial nerve endoscopic anatomy knowledge permitted an advancement in surgery, performing facial nerve transcanal exclusive endoscopic surgery for the treatment of facial nerve diseases located on its tympanic portion, GG, and suprageniculate fossa.
Anatomical considerations
From an endoscopic point of view, it is possible to consider the tympanic facial nerve into 2 portions regarding the orientation to the cochleariform process (CP): precochleariform and postcochleariform segments.
Precochleariform Segment of Tympanic Portion of Facial Nerve
Precochleariform segment is the portion of the tympanic facial nerve lying superiorly and anteriorly to the posterior bony limit of the CP.
This segment of the facial nerve is composed by the GG and the greater petrosal nerve (GPN). It is necessary to remove the malleus head to obtain good visualization of the precochleariform segment and the GG area.
The precochleariform segment has a parallel orientation with respect to semicanal of tensor tendon of the malleus, lying superiorly to this semicanal. Microscopic access to the anterior epitympanum should be made by a mastoidectomy, posterior atticotomy, and removal of the incus and head of the malleus.
On the other hand, after ossicular chain removal, which can be considered mandatory, the GG can be easily accessed using an exclusive endoscopic transcanal route, with the advantages of sparing mastoid tissues and avoiding more extended approaches.
Therefore, endoscopy guarantees true advantages compared with microscopy in terms of surgical maneuvering and access of extreme anterior segment of the tympanic facial nerve toward the GG ( Fig. 1 A).
Geniculate Ganglion
The CP represents an excellent landmark to identify the GG, which is located just medially and superiorly to the CP.
GG is in the floor of the anterior epitympanic space and has a horizontal orientation parallel to the semicanal of the tensor tendon of the malleus ( Fig. 1 B).
In 66.7% of cases, the GG is covered by the bone of the anterior epitympanic space cells, so in these cases the cells of the anterior epitympanic space just anteriorly and superiorly to the CP should be removed in order to expose the GG.
From literature, in 33.3% of cases a partial dehiscence of the ganglion in the anterior epitympanic space cells was found.
Another anatomic landmark for GG is the transverse crest, which is a bony ridge extending inferiorly from the tegmen tympani of the anterior epitympanic space, just anterior to the CP, also known as the “COG” ( Fig. 1 B).
The transverse crest is not always clearly described in the literature, and a frequent variability of this structure is noted. It has different conformations and relationships to the GG, tensor fold, and supratubal recess. From recent work, it can be considered different conformation of the COG.
In the authors’ series, the COG is a complete bony crest in 58.3% of cases, having a transverse inclination attaching anteriorly and superiorly to the most anterior portion of the tegmen tympani. In these cases, the transverse crest “indicates” posteriorly and inferiorly the CP, and it is a landmark for the GG during transcanal endoscopic approach. On the other hand, in 41.7% of cases, an incomplete or rudimental transverse crest was found, in close relationship to the tegmen of the anterior epitympanic space. When incomplete or rudimental, the transverse crest could not be considered a true landmark for the GG, either because it does not clearly indicate the ganglion or because it lies significantly anteriorly and laterally to the nerve.
The location of the GG is shown by these 2 anatomic structures. The GG is found lying just superiorly and anteriorly with respect to the CP, whereas the COG descends from the tegmen like a finger, indicating the location of the ganglion ( Fig. 2 ).
Greater Petrosal Nerve
Endoscopic access to the GPN is obtained by removing the head of the malleus in order to reach good access to the anterior bony wall of the anterior epitympanic space. It is necessary to remove the transverse crest and the supratubal recess (when present), anteriorly to the GG, in order to expose the GPN, following the anterior orientation of the facial nerve ( Fig. 1 C).
In 40% of cases, a dehiscence of the middle cranial fossa (MCF) dura is observed at the anterior epitympanic space, and the dura of the MCF is in a close relationship with both, the GG and the GPN ( Fig. 1 D).
On the contrary, in 60% of cases, it is necessary to drill in order to find the dura of the MCF.
Postcochleariform Segment of Tympanic Portion of Facial Nerve
The postcochleariform segment is the portion of the tympanic facial nerve lying posteriorly to the posterior bony limit of the CP. This segment represents the floor of the posterior epitympanic space.
In 75% of cases, the postcochleariform segment has a slightly oblique orientation to the semicanal of the tensor tendon of the malleus, descending above the oval window and the stapes from the CP anteriorly to the second genu.
In 25% of cases, this segment has a parallel orientation to the semicanal of the tensor tendon of the malleus. The postcochleariform segment is parallel with respect to the lateral semicircular canal. That landmark is important in order to reach the aditus ad antrum endoscopically.
In the most posterior portion, in the transition zone between the tympanic and mastoid segment, the facial nerve becomes transcanally inaccessible by microscope, whereby a mastoidectomy is required to reach this area.
On the other hand, the transcanal endoscopic approach allows a direct route to the postcochleariform segment of the facial nerve after incus and head of the malleus removal, sparing mastoid tissues and wide external incisions. The only portion of facial nerve that does not require ossicular chain removal to be exposed is the most posterior, in close relationship with the second genu and the pyramidal eminence.
Intralabyrinthine portion of facial nerve
The intralabyrinthine portion of facial nerve runs from the GG to the IAC anteriorly to posteriorly, and to the fundus of the IAC, laterally to medially.
The medial turn of the cochlea represents an important landmark for the intralabyrinthine facial nerve, because this segment of the nerve runs just superiorly to the cochlea from the GG superiorly and anteriorly, and the intrameatal facial nerve inferiorly and posteriorly ( Fig. 3 C).
The intralabyrinthine tract of the facial nerve from the IAC to the GG could be identified by drilling an anatomic triangle between the GG superiorly, the basal turn of the cochlea anteriorly, and the spherical recess IAC posteroinferiorly.
The intralabyrinthine facial nerve is covered by a compact bony wall, and the dissection of this component of the facial nerve is quite difficult and could be exposed and visualized by drilling between the GG and the cochlea anteriorly, and the medial wall of the vestibule, and the IAC posteriorly. Although the intralabyrinthine facial nerve was covered by a compact bony wall, it is extremely fragile, and at this point, the facial nerve has its smallest diameter so it could be easily damaged.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
