|Year : 2021 | Volume
| Issue : 2 | Page : 87-89
Intratemporal variations of facial nerve course
Mohammed Albokashy1, Saleh Alabood2, Saleh Alamry1
1 Department of Otorhinolaryngology-Head and Neck Surgery, Prince Sultan Military Medical City, Riyadh, Saudi Arabia
2 Department of Otolaryngology, Security Forces Hospital, Riyadh, Saudi Arabia
|Date of Submission||11-Nov-2020|
|Date of Decision||20-Jan-2021|
|Date of Acceptance||31-Jan-2021|
|Date of Web Publication||30-Jun-2021|
Dr. Mohammed Albokashy
Department of Otorhinolaryngology-Head and Neck Surgery, Prince Sultan Military Medical City, Riyadh
Source of Support: None, Conflict of Interest: None
Facial nerve (FN) is the seventh cranial nerve with a unique complex anatomical course. Knowledge about the normal predictable course within the temporal bone with its possible variations is crucial to avoid accidental injuries to the FN, preventing its devastating subsequences. In ear surgeries, certain landmarks and relations to each other are key for safe drilling. In this report, we describe a case with computed tomography findings during routine preoperative assessment for cochlear implant with anomalous FN course being displaced posterolaterally on the tympanic and mastoid segments. This highlights the importance of imagining before such surgeries.
Keywords: Cochlear implant, facial nerve, mastoidectomy
|How to cite this article:|
Albokashy M, Alabood S, Alamry S. Intratemporal variations of facial nerve course. Saudi J Otorhinolaryngol Head Neck Surg 2021;23:87-9
|How to cite this URL:|
Albokashy M, Alabood S, Alamry S. Intratemporal variations of facial nerve course. Saudi J Otorhinolaryngol Head Neck Surg [serial online] 2021 [cited 2021 Jul 23];23:87-9. Available from: https://www.sjohns.org/text.asp?2021/23/2/87/320334
| Introduction|| |
Facial nerve (FN) is the seventh cranial nerve with a unique complex anatomical course. Surgical identification of FN is crucial for safe tympanomastoid surgeries. Iatrogenic FN injury is one of the most concerning complications. When such injury happens, it will result in devastating consequences with severe implications both physically and emotionally with variable degree of facial asymmetry such as mouth deviation and inability to close the eye that might lead to eye dryness., Iatrogenic FN injury was reported to be between 0.6% and 3.7% in primary tympanomastoidectomy surgeries, with a higher risk on revision surgeries. With experienced surgeons, the use of FN monitors, and the fact that the FN has a predictable course within the temporal bone relaying on certain surgical landmarks in ear surgery, accidental injuries are considerably rare. It becomes challenging when there are dehiscence or unexpected variations in its anatomical course. Therefore, otolaryngologists must have a thorough knowledge of the normal anatomy and its possible variations. Temporal bone computed tomography (CT) scan can detect any deviation in the course of the FN, which can provide key information in surgical planning for otological surgeries.
| Case Report|| |
A 13-year-old girl presented with progressive hearing loss over many years with no benefit from hearing aid over last year; on examination, no dysmorphic feature was noted; ear examination showed normal external auditory canal with intact tympanic membrane bilaterally; pure-tone audiometry showed severe to profound mixed hearing loss (HL). Decision was made for cochlear implant. CT scan was requested as a part of preoperative planning which showed anomalous FN course where the tympanic segment, second genu, and mastoid segment were extending more posterior and lateral than its predicted course [Figure 1] along with low dura bilaterally. Intraoperatively, the FN monitor was switched on since the start. Drilling was continued following low dura as it was the only available landmark. Subsequently, the FN was identified and facial recess was approached and opened before identifying the mastoid antrum. As a result of the described course, the FN was found lateral to the short process of incus and the lateral semicircular canal [Figure 2].
|Figure 1: Computed tomography scan of the left temporal bone: (a) showing the first genu (black arrow) and lateral semicircular canal (red arrow), (b) showing the tympanic segment extending more posterior and lateral than its predicted course, (c) showing the second genu and mastoid segment being dehiscence to the posterior fossa|
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|Figure 2: Intraoperative picture, showing the facial nerve (blue arrow) where it was identified before the appearance of the lateral semicircular canal and the short process of the incus. Red arrow: Pointing out the low dura|
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| Discussion|| |
The FN within the temporal bone has been divided into four segments: meatal, labyrinthine, tympanic, and mastoid with two genus. The meatal segment of the FN courses in the anterosuperior portion of the internal auditory canal along with vestibular and cochlear nerves. At the fundus which is the end of internal auditory canal, the meatal segment ends and the labyrinthine segment starts. The FN travel in anterolateral direction above the cochlea to end nears the middle ear cavity in the geniculate ganglion giving off the greater superficial petrosal nerve. Then the FN makes an acute angle making the first genu where the tympanic segment starts to travel in a posterolateral direction. At this segment, the FN will be located between the labyrinths medially, and the tympanic cavity laterally passing posterior and superior to cochleariform process, then it passes superior to the oval window, and inferior to the lateral semicircular canal. At its most posterior end, behind the pyramidal process, the tympanic segment angulates again inferiorly forming the second genu to become the mastoid segment.,, A normal CT scan with the predictable FN course is illustrated in [Figure 3]. In our case, the tympanic segment of FN was extending more posterior and lateral [Figure 1]b; similarly, the second genu and mastoid segment were displaced as well. In addition to that, the nerve was dehiscence radiologically into the posterior fossa [Figure 1]c.
|Figure 3: Computed tomography scan of the left temporal bone: showing the normal course of the facial nerve (a) showing the first genu and lateral semicircular canal, (b) showing the tympanic, (c) showing the mastoid segment|
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In a review of the current literature, several intratemporal FN course variations have been described with majority involving the tympanic segment. In a study done by Hao et al. (2018), among 256 cases of isolated congenital malformation of the middle ear, the FN was found to be anomalous in around 32% of cases, such as displacement of tympanic segment inferior to oval window or even covering it. In addition to that, a duplication of the FN near the second genu has been found in an isolated case. Cho et al. (2015) reported a lateralized tympanic segment reaching the tympanic membrane. In a large retrospective study done by Song et al.(2012), they report several FN course variations including the anterior and inferior displacement of the mastoid segment, unlike our case where the FN was displaced posterior and lateral. Reported anomalies in the FN course according to segments are shown in [Table 1].
|Table 1: Reported anomalies in the facial nerve course according to segments|
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| Conclusion|| |
The knowledge of the possible variations in FN course is crucial for safe and successful ear surgeries, and therefore, the risk of FN injury will be reduced. This case report highlights the importance of preoperative CT scanning of the temporal bone and the use of FN monitor with stress on the FN course.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]