Surgical anatomy of the temporal bone undoubtedly is amongst the hardest to study let alone identifying its pathology. It requires a three-dimensional picturization in addition to sound working knowledge of radiological mapping in relation to the many nooks and crannies of temporal bone. Temporal bone is made of squamous, petrous, tympanic and mastoid part along with several bony projections; the styloid, zygomatic and mastoid processes. It forms the lateral skull base bounded by sphenoid bone anteriorly, occipital bone posteriorly while cusping the temporal lobe medially. Important neurovascular structures run through and adjacent to it; the sigmoid sinus later into jugular bulb, petrous part of ICA, the facial and vestibulocochlear nerves.
Managing petrous bone lesions adheres to the cardinal rules of ascertaining the status of hearing and facial nerve function. This foreordains the choice of approach taking into account of the extent of lesion, especially intracranial involvement and internal carotid encasement; the latter likely resulting in an inoperable scenario.
Petrous bone lesions can be divided into intradural and extradural. The former commonly being schwannomas and meningiomas whilst the latter being cholesterol granulomas, cholesteatomas, epidermoid cysts and osteomyelitis [4]. Surgical planning with DW-MRI is indispensable in these cases to establish the diagnosis of cholesteatoma per se [5]. HRCT may only delineate bony erosions and tegmen dehiscence which are inadequate to rule out the possibility of intradural pathologies; specifically, in this case a meningocele in view of prior surgery with known tegmen dehiscence.
Access to petrous bone via endoscopic trans-canal technique has been well described in the landmark paper by Marchioni in 2015. Three routes were illustrated; the supra-geniculate, trans-promontorial and infra-cochlear route [6]. The selection is dependent primarily on the location of lesion in addition to preoperative facial nerve function and hearing status.
Pertaining to this patient, albeit a revision case with non-communicating cholesteatoma, we had an advantage of a well performed canal wall down procedure which paved an excellent per-meatal access. A state of bony erosion of the superior and lateral semi-circular canals further created an opportune window into the petrous region. Hence, the choice of a certain technique also relies on individual anatomical handicaps requiring the surgeon to improvise accordingly.
Furthermore, the bulk of lesion sat at the supra-labyrinth portion sparing the cochlea with an incomplete dysfunctional facial nerve (grade four HBGS). Compounding these factors with the existing labyrinth erosions; naturally the supra-geniculate route was deemed most suitable. This corridor as described by Marchioni et al is formed by a triangle bounded by the middle cranial fossa superiorly, tympanic segment of facial nerve inferiorly and labyrinth bloc posteriorly [6].
Despite preoperative dysfunctional facial nerve and profound hearing loss; a conservative approach is still mandated whenever possible. Endoscope use provides a wide-angle view and assist greater manoeuvrability of instruments as well as presenting a clear magnification for dexterous dissection around the minute inner ear structures principally in this case the middle cranial fossa dura as the roof of petrous bone was mostly dehiscent, the intact cochlear and first part of facial nerve [7, 8]. This is to allow rehabilitation with cochlear implant should the opportunity arise for this patient while removal of cholesteatoma mass may even reverse the compressive effect on the facial nerve of the meatal segment [6].
Preoperative balance examination is also vital as to allow judicious drilling of the eroded semi-circular canals for access. In this case, vestibular hypofunction had been established with a positive Halmagyi test despite no vertiginous episodes which correlated well to the absence of severe vertigo post operatively; further indicating the chronicity of the condition that has been well compensated by contralateral neural-vestibular system.
Common challenges encountered with endoscopic surgery remains; single hand use which demands absolute and adept drilling with adequate irrigation and suctioning while able to manage complications of bleeding and CSF leak. Certain extensive cases still benefit form a combined assistance of a microscope particularly to address the aforementioned pitfalls [8, 9]. Nonetheless, it is a perfectible approach that is capable of minimizing morbidity faced in open trans-mastoid, middle cranial fossa and retrosigmoid approaches such as excessive temporal lobe retraction, dural injury, sigmoid sinus bleed and even surgical site wound infections and breakdown.
Obliteration of petrous cavity post extirpation is not always done to aid surveillance, unless a dural tear causing CSF leak occurs; as experienced in this case which requires adequate repair with composite materials using fat, muscle, bone wax or tissue glue.