Endoscopic Transcanal Corridors to the Lateral Skull Base: Initial Experiences

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1 The Laryngoscope VC 2015 The American Laryngological, Rhinological and Otological Society, Inc. TRIOLOGICAL SOCIETY CANDIDATE THESIS Endoscopic Transcanal Corridors to the Lateral Skull Base: Initial Experiences Daniele Marchioni, MD; Matteo Alicandri-Ciufelli, MD, FEBORL(HNS); Alessia Rubini, MD; Livio Presutti, MD Objectives/Hypothesis: Surgical approaches to the lateral skull base, internal auditory canal (IAC), and petrous bone are widely known and have been extensively recorded. Despite the benign nature and limited dimensions of lesions located in this anatomical region, extirpative surgical approaches are often required to reach and remove the disease. The aim of present report was to describe our initial experiences with minimally invasive endoscopic approaches to the lateral skull base. Study Design: Retrospective review of patients charts and video recordings from surgery. Methods: Twelve patients were included in the study. Three main corridors to the lateral skull base were identified: the transcanal suprageniculate corridor, the transcanal transpromontorial corridor, and the transcanal infracochlear corridor. Landmarks, tips, and pitfalls of the approaches have been reviewed and highlighted. Results: These corridors provide a direct approach to pathology involving the fundus, IAC, cochlea, petrous apex and geniculate ganglion region, without any external incision. The pathology was successfully removed in most cases with no important postoperative complications and reasonable facial nerve outcomes. Conclusions: The transcanal endoscopic approaches to the lateral skull base proved to be successful for pathology removal involving the fundus, IAC, cochlea, petrous apex, and geniculate ganglion region. Future widespread application of this kind of approach in lateral skull base surgery will depend on the development of technology, and surgical and anatomical refinements. Key Words: Inner ear, internal auditory canal, transcanal approach, endoscopic ear surgery, acoustic schwannoma, lateral skull base, temporal bone cholesteatoma, transcanal corridors. Level of Evidence: 4. Laryngoscope, 125:S1 S13, 2015 INTRODUCTION The lateral skull base constitutes an anatomic boundary between the fields of neurosurgery and otolaryngology. Surgery in this region has always been a challenge for both disciplines owing to the presence of important anatomical structures such as the internal carotid artery, the otic capsule, and the facial nerve. Several approaches have been developed to reach pathology located in the lateral skull base and in the fundus of the internal auditory canal (IAC) and petrous apex. Despite the benign nature and limited dimensions of the lesions located in this anatomical region, extensive surgical approaches are often required to reach and remove the disease. The transpetrous routes (translabyrinthine, From the Otolaryngology Department (M.A. C., A.R., L.P.), University Hospital of Modena, Modena, Italy; the Neurosurgery Department (M.A. C.), New Civil Hospital Sant Agostino-Estense, Baggiovara, Italy; Otolaryngology Department (D.M.), University Hospital of Verona, Verona, Italy. Editor s Note: This Manuscript was accepted for publication January 20, The authors have no funding, financial relationships, or conflicts of interest to disclose. Send correspondence to Alessia Rubini, MD, Otolaryngology Department, University Hospital of Modena, Via del Pozzo 71, Modena, Italy. rubinialessia@gmail.com DOI: /lary transotic, transcochlear approaches), the retrosigmoid route and the middle cranial fossa approach are the routes most widely used to reach lateral skull base lesions. All of these approaches are open and based on the use of a microscope, and often involve significant amounts of brain retraction and neurovascular manipulation. A gradual introduction of endoscopic techniques for the treatment of middle ear pathology has taken place since the 1990s. 1 Endoscopy was used primarily for visualization of hidden areas such as the posterior epitympanum during classic microscopic tympanoplasties. 2 Gradually, it was also used in surgery to replace the microscope as the main tool used during middle ear operations. 3 6 At present, the main application of endoscopic surgery is in the surgical treatment of middle ear cholesteatoma, but with the natural evolution of the technique, there have been advances in lateral skull base surgery. From our experience over recent years, the authors have progressively noticed that the inner ear and all of the temporal bone could also be accessed in an endoscopic-assisted fashion or even with an exclusive endoscopic approach. So, based on our earlier experiences with transcanal endoscopic approaches to the middle ear, we started to consider the transcanal approach as a surgical corridor to reach diseases with limited extent located in the lateral skull base. 7 9 These minimally S1

2 Fig. 1. The transcanal endoscopic suprageniculate approach. On the left, the schematic drawing shows the surgical route from the external auditory canal to the suprageniculate fossa (blue arrow). On the right, the computed tomography scan in the coronal view shows the working area and the bony removal (yellow area) between the middle cranial fossa and the facial nerve into the petrous apex over the cochlea, which may be performed with this approach. [Color figure can be viewed in the online issue, which is available at invasive approaches access the lateral skull base using the natural openings of the external auditory canal (EAC), and allow very good visualization of some anatomical areas lying in the petrous bone, petrous apex, and IAC. Diseases could be removed from these areas without brain or meningeal manipulations and with short and safe postoperative courses. The aim of this article was to describe our initial experiences with transcanal endoscopic corridors to the lateral skull base, describing assumptions, indications, and procedures, and reporting our preliminary results. MATERIALS AND METHODS Between March 2012 and May 2014, 74 patients affected by inner ear and lateral skull base diseases underwent surgery at the otolaryngology department of the University Hospital of Modena. The following patients were included in the present study: those affected by lateral skull base diseases with limited extension into the suprageniculate fossa, cochlea, vestibule, fundus of the IAC and petrous apex, requiring surgery, and those who underwent transcanal endoscopic procedures. Patients with diseases of the lateral skull base who underwent a transmastoid microscopic procedure (with or without endoscopic assistance) were not included in the study. The Fig. 2. Left ear. The transcanal endoscopic suprageniculate approach: on the left, schematic drawing showing a general view of the transcanal anatomy of the medial wall of the tympanic cavity related to the working area of the transcanal suprageniculate approach (orange area). On the right, the details of the endoscopic transcanal anatomical landmarks in this approach; the orange area indicates the bony area between the middle cranial fossa superiorly, the facial nerve and geniculate ganglion inferiorly and the labyrinthine block posteriorly which may be removed to reach the petrous apex endoscopically. aes 5 anterior epitympanic space; ca 5 carotid artery; cp 5 cochleariform process; et 5 Eustachian tube; fn 5 facial nerve; fn* 5 labyrinthine afacial nerve; gg 5 geniculate ganglion; gpt 5 greater petrosal nerve; lsc 5 lateral semicircular canal; mcf 5 middle cranial fossa; pe 5 pyramidal eminence; pes 5 posterior epitympanic space; pr 5 promontory; rw 5 round window; s 5 stapes; ttc 5 tensor tendon canal. [Color figure can be viewed in the online issue, which is available at S2

3 Fig. 3. CT scan in coronal view showing a lesion limited to the suprageniculate area between the middle cranial fossa and the geniculate ganglion, extending over the cochlea involving the petrous apex. In this case, a transcanal suprageniculate route could be attempted. surgery was performed by two experienced surgeons (D.M., L.P.) with similar surgical and endoscopic experience. All operations were video recorded and stored in the clinic s digital database. Between May 2014 and June 2014, operations and patients charts were reviewed, and all data were summarized and gathered in a database for further consideration and analyses. All surgical procedures was performed under endoscopic view. Endoscopes were 3 or 4 mm in diameter, 18 cm in length, and 0 or 45 angled. The surgeon held the endoscope with the left hand and the surgical instrument with the right hand; the endoscope was introduced into the EAC, reaching the tympanic cavity after tympanomeatal flap elevation. A three-chip camera and high-definition monitor (Karl Storz, Tuttlingen, Germany) were used for all of the procedures, and videos were recorded on a computer hard disc. In all procedures, intraoperative facial nerve monitoring (Nerve Integrity Monitoring) was used. Each approach was categorized and analyzed including the ear affected, the area of lateral skull base exposed, and the extent of endoscopic resection based on immediate postoperative computed tomography (CT) scans, compared with the preoperative CT scan. The following were also considered for each patient: intraoperative complications, postoperative complications and facial nerve outcome, when preservation of hearing function was attempted, comparison of the postoperative audiometric exam with the preoperative exam, and the time of recovery and discharge of the patients. Based on the extent and location of the disease in the lateral skull base, three different approaches/corridors were used and are discussed next. Transcanal Suprageniculate Corridor The transcanal suprageniculate corridor is illustrated in Figure 1. When approached through the EAC, the disease was reached located between the geniculate ganglion and the second Fig. 4. Clinical case, left ear. (A) Eardrum before surgery. (B) the tympanomeatal flap was elevated, and the facial nerve and geniculate ganglion were exposed. The middle cranial fossa dura was detected superiorly and the labyrinthine block posteriorly. Drilling between these surrounding anatomical structures was started. (C) After drilling, the pathological tissue was carefully removed from the petrous apex. (D) Final cavity. The petrous apex between the middle cranial fossa, the facial nerve, and the labyrinthine block is exposed. ch 5 cholesteatoma; fn 5 facial nerve; gg 5 geniculate ganglion; lsc 5 lateral semicircular canal; mcf 5 middle cranial fossa; p. apex 5 petrous apex. [Color figure can be viewed in the online issue, which is available at S3

4 Fig. 5. The transcanal endoscopic transpromontorial approach. On the left, a schematic drawing shows the surgical route from the external auditory canal to the fundus of the internal auditory canal (IAC) through the promontory (blue arrow). On the right, the computed tomography scan in the coronal view shows the working area and the bony removal (yellow area), which may be performed with this approach through the promontory, removing the cochlea and the vestibule, and reaching the fundus of the IAC. [Color figure can be viewed in the online issue, which is available at portion of the facial nerve inferiorly, the middle cranial fossa (MCF) lying superiorly, and the labyrinthine bloc posteriorly. Working above the cochlea and labyrinth, sensorineural hearing function was preserved (Fig. 2), although an ossiculoplasty was required after disease removal. The correct indication for this approach required disease with limited extension into an anatomical triangle composed of the MCF, facial nerve, and labyrinthine bloc as shown in Figures 2 and 3. This area can be reached endoscopically by a transcanal route avoiding an MCF approach. The surgical steps required the creation of a wide tympanomeatal flap to expose the tympanic cavity; the flap was then transposed inferiorly and detached from the handle of the malleus to obtain good exposure of the medial wall of the tympanic cavity. When the ossicular chain was present and intact, removal of the incus and head of the malleus was required to expose the entire second portion of the facial nerve, from the second genu to the geniculate ganglion and the greater petrosal nerve area. The cog and the cochleariform process were identified endoscopically and used as an anatomical landmark for the geniculate ganglion area owing to the close anatomical relationship between the geniculate ganglion and these structures. 4 The dura of the MCF was exposed by removing bone from the tegmen of the anterior epitympanum, and represented the upper surgical limit. The lateral semicircular canal was also detected endoscopically, posteriorly and superiorly, with respect to the second genu of the facial nerve, representing the posterior limit of the surgical dissection (see Fig. 4 B). The cog was gently removed Fig. 6. The transcanal endoscopic transpromontorial approach of the left ear. On the left, a schematic drawing shows a general view of the transcanal anatomy of the medial wall of the tympanic cavity related to the working area of the transcanal transpromontorial approach (orange area). On the right, details of the endoscopic transcanal anatomical landmarks of this approach. The orange area indicates the bony area, which may be removed to reach the internal auditory canal (IAC) passing through the cochlea and the vestibule. The IAC projection with the anatomical position of the nerves is also shown. ca 5 carotid artery; fn 5 facial nerve; fn* 5 labyrinthine afacial nerve; gg 5 geniculate ganglion; gpn 5 great petrousal nerve; lsc 5 lateral semicircular canal; mcf 5 middle cranial fossa; pr 5 promontory; psc 5 posterior semicircular canal opening; rw 5 round window; sph 5 spherical recess; vc 5 vestibular crest. [Color figure can be viewed in the online issue, which is available at S4

5 Fig. 7. Transcanal endoscopic transpromontorial approach of the left ear. (A) The medial wall of the tympanic cavity after ossicular chain and eardrum removal. (B) The stapes was removed exposing the medial wall of the vestibule. (C) The promontory was drilled, detecting the basal, medial, and apical turns of the cochlea. (D) The cochlea was opened just below the vestibule exposing the tumor in the internal auditory canal (IAC). (E) A careful dissection of the tumor from the facial nerve in the IAC was performed.(f) Final cavity after tumor removal. coh 5 cochlea; cp 5 cochleariform process; et 5 eustachian tube; fn 5 facial nerve; fn** 5 facial nerve in the IAC; lsc 5 lateral semicircular canal; ve 5 vestibule; pr 5 promontory; rw 5 round window; s 5 stapes; tum 5 tumor. [Color figure can be viewed in the online issue, which is available at using a microcurette, increasing the exposure of the geniculate ganglion area, and when required, the greater petrosal nerve was also exposed just anteriorly to the geniculate ganglion. Based on the extent of the disease, a diamond burr or a Piezosurgery device (Mectron, Carasco/Genova, Italy) was used to remove the bone between the MCF superiorly, the labyrinthine bloc posteriorly, and the facial nerve inferiorly, reaching the pathological tissue in this area (Fig. 4C,D). After disease removal, a fragment of temporal muscle was used to obliterate the cavity created, an ossicular chain reconstruction was performed at the same time as the surgery, and the tympanomeatal flap was replaced. Transcanal Transpromontorial Corridor The transcanal transpromontorial corridor is illustrated in Figure 5. When approached through the EAC, the fundus of the IAC was reached by passing through the promontory, and disease located in the vestibule, cochlea, and/or IAC was removed. With this surgical approach, hearing loss was expected owing to the extirpation of the promontory area to form a passage through the cochlea to reach the IAC (Fig. 6). The surgical steps were as follows: a circumferential incision was made on the EAC, 2 cm from the tympanic membrane, and a wide tympanomeatal flap was created by skin degloving, and all of the eardrum with the meatal skin flap of the EAC was removed. A diamond burr was used to perform a wide meatoplasty, and the bony annulus was drilled circumferentially until the hypotympanum, retrotympanum, and protympanum were exposed endoscopically so as to obtain optimal surgical space and control of the whole medial wall of the tympanic cavity. The scutum was removed to uncover the incudomalleolar joint. The incus was removed, and the tensor tendon was cut removing the malleus (Fig. 7A). Using a hook, the stapes was removed to expose the vestibule. The spherical recess, representing the cribrose area where the inferior vestibular nerve was attached and a landmark for the internal auditory canal, 8 was detected endoscopically in the saccular fossa (Figs. 6 and 7B). When the disease involved the vestibule without IAC extension, the oval window opening was enlarged inferiorly on the promontory using a microcurette, allowing direct access to the medial wall of the vestibule, and the disease from the depth of the vestibule was S5

6 Fig. 8. (A) A left-ear transcanal endoscopic transpromontorial approach was performed until the tumor was exposed in the internal auditory canal (IAC). (B) The tumor was dissected from the IAC after detecting and preserving the facial nerve. (C) Endoscopic magnification of the facial nerve in the IAC, during dissection of the tumor. (D) After tumor removal, a fat pad was used to close the promontorial defect. cp 5 cochleariform process; fn 5 facial nerve; fn** 5 facial nerve in the IAC; lsc 5 lateral semicircular canal; pr 5 promontory; tum 5 tumor. [Color figure can be viewed in the online issue, which is available at carefully removed endoscopically. This latter surgical maneuver was performed carefully to avoid trauma to the spherical recess and consequent cerebrospinal fluid (CSF) leakage. When the disease was located in the cochlea with or without IAC involvement, a more anterior transcochlear approach was performed. The round window membrane was detected, and the tegmen of the round window was removed. Using Piezosurgery, the promontory was completely removed to gain access to the cochlea. During this procedure, the basal, middle, and upper turn of the cochlea were progressively identified (Fig. 7C). The upper portion of the medial turn of the cochlea was identified and used as a landmark for location of the labyrinthine tract of the facial nerve. Dissection of the cochlea was performed progressively until the helicotrema was identified. Dissection of the cochlea was performed progressively, detecting the fundus of the IAC and allowing the cochlear nerve entrance to be seen. When the disease was located in the cochlea, a gentle dissection of the tumor was performed, trying to avoid CSF leakage. When the tumor was located in the IAC, the fundus of the IAC was carefully opened through the cochlea just below the vestibule and the spherical recess exposing the lesion in the IAC (Fig. 7D). During this surgical maneuver, an outflow of CSF occurred. By gently maneuvering the tumor mass, the facial nerve was detected endoscopically, then the tumor mass was dissected from the IAC and from the facial nerve (Fig. 7E,F), paying careful attention not to damage the nerve (Fig. 8A C). Afterward, a final inspection of the surgical cavity and the IAC was made to confirm complete removal of the mass. Closure of the IAC was performed with abdominal fat, packing the promontorial defect and closing the communication between the inner and middle ear (Fig. 8D). Fibrin glue was used to secure the closure of the promontorial defect. Depending on the extent of the disease and on the extent of tissue extirpation, either repositioning of the tympanic membrane and ear canal skin with a tragal graft underlay or a cul-de-sac closure of the skin of the EAC was performed. Transcanal Infracochlear Corridor The transcanal infracochlear corridor is illustrated in Figure 9. When disease was located inferiorly with respect to the IAC in the petrous apex, it was removed by drilling the bone between the cochlea superiorly, the carotid artery anteriorly, and the jugular bulb inferiorly, using this opening as a surgical endoscopic corridor to the inferior portion of the petrous apex. This approach allowed removal of disease located in the petrous apex below the IAC with limited extent, preserving hearing function. The ossicular chain was preserved and the surgical corridor was bounded by the anatomical triangle between the cochlea superiorly, the carotid artery anteriorly, and the jugular bulb postero-inferiorly (Fig. 10). In these cases, a circular incision of the skin of the EAC was made with a round knife, and a wide tympanomeatal flap was elevated superiorly detaching the eardrum from the handle of the malleus, exposing endoscopically the mesotympanic spaces, the protympanum, and the hypotympanum (Fig. 11A). A circumferential correction was made to the bony EAC, reducing the overhang of the bony annulus anteriorly and inferiorly to obtain good access to the S6

7 Fig. 9. The transcanal endoscopic infracochlear approach. On the left, a schematic drawing shows the surgical route from the external auditory canal to the infracochlear region below the cochlea (blue arrow). On the right, the computed tomography scan in coronal view shows the working area and the bony removal (yellow area), which may be performed with this approach between the cochlea superiorly and the jugular bulb inferiorly to reach the petrous apex cells. [Color figure can be viewed in the online issue, which is available at www. laryngoscope.com.] hypotympanum. The inferior limit of the promontory with anatomical structures forming the round window niche (posterior pillar, tegmen, and anterior pillar with the finiculus bone) was detected endoscopically. Exposure of the finiculus bone indicated the jugular bulb at its caudal end. Furthermore, under the finiculus bone, a subcochlear canaliculi was identified when present, showing the route to reach the petrous apex cells (Fig. 10, Fig. 11B). 10 The jugular bulb was thus identified below the finiculus bone in the floor of the hypotympanum, and identification of the vertical tract of the carotid artery was performed by drilling the protympanic cells just below the eustachian tube orifice. Drilling of the medial aspect of the tympanic cavity into the round window fossa, the hypotympanic and protympanic cells between the jugular bulb, carotid artery, and basal turn of the cochlea was performed with a diamond burr (Fig. 11C). The removal of the above-mentioned bony anatomical triangle allowed the disease in the petrous apex to be reached (Fig. 11D), which lay below the IAC and medially compared to the vertical portion of the internal carotid artery. At the end of the surgical procedure, the tympanomeatal flap was replaced. RESULTS The final study group was composed of 12 patients (Table I). Pathology Six of the 12 patients were affected by cholesteatoma of the tympanic cavity with involvement of the inner ear (vestibule, geniculate ganglion and middle cranial fossa, IAC). Two of the 12 patients were affected by an acoustic schwannoma of the fundus of the IAC, with Fig. 10. The transcanal endoscopic infracochlear approach of the left ear. On the left, a schematic drawing shows a general view of the transcanal anatomy of the medial wall of the tympanic cavity related to the working area of the transcanal infracochlear approach (orange area). On the right, the details of the endoscopic transcanal anatomical landmarks in this approach. The orange area indicates the bony area between the cochlea superiorly, the carotid artery anteriorly, and the jugular bulb inferiorly, In the middle of this anatomical area, the finiculus bone with the tunnel (the subcochlear canaliculi) (red arrow) may help the surgeon to reach the petrous apex cells lying under the IAC. ap 5 anterior pillar; jb 5 jugular bulb; fn 5 facial nerve; pp 5 posterior pillar; pr 5 promontory; rw 5 round window; s 5 stapes; sty 5 styloid prominence; su 5 subiculum. [Color figure can be viewed in the online issue, which is available at S7

8 Fig. 11. (A) A transcanal endoscopic infracochlear approach of the right ear was performed. The eardrum was removed exposing the hypotympanum and protympanum. (B) Endoscopic magnification of the hypotympanum; the tunnel was detected. (C) The carotid artery and the jugular bulb were detected, and drilling of the infracochlear area was started. (D) The tumor in the petrous apex was reached. ca 5 carotid artery; in 5 incus; jb 5 jugular bulb; ma 5 malleus; pr 5 promontory; rw 5 round window; scc 5 subcochlear canaliculi; tum 5 tumor. [Color figure can be viewed in the online issue, which is available at hearing loss on the same side, and with growth of the schwannoma detected by magnetic resonance imaging (MRI) during follow-up. One patient was affected by a cochlear schwannoma involving the fundus of the IAC, with hearing loss on the same side and with growth of the schwannoma detected by MRI during follow-up. One patient was affected by a meningioma of the IAC eroding the cochlea with only residual hearing on the same side. One patient was affected by an indefinite lesion in the petrous apex (chondroma) with internal carotid artery involvement requiring biopsy. One patient was affected by a hemangioma of the facial nerve (geniculate ganglion) involving the middle cranial fossa superiorly with intradural extension. Surgical Procedures and Intraoperative Findings Out of 12 endoscopic transcanal lateral skull base approaches, three patients underwent an endoscopic transcanal suprageniculate approach, five patients underwent an endoscopic transcanal transpromontorial approach, and four patients underwent an endoscopic transcanal infracochlear approach. In 10/12 patients, the disease was completely removed from the lateral skull base. In one patient, a minimally invasive biopsy was performed via the transcanal infracochlear route to reach disease located in the inferior aspect of the petrous apex, preserving the ossicular chain and hearing function. As a result of adhesion of the lesion to the inferior aspect of the IAC, CSF leakage occurred that required repair with Lyodura. In one S8 patient, a meningioma of the IAC was found, partially extending into the cerebellopontine angle. The pathology was not radically removed because of adhesions to the facial nerve, and a subtotal removal was performed, preserving the facial nerve function. In 2/6 patients affected by cholesteatoma, a posterior extension of the disease into the mastoid cells was found; therefore, a microscopic mastoidectomy was added after the endoscopic transcanal lateral skull base management to remove disease from the mastoid cavity. In 5/12 patients (four transpromontorial approaches and one infracochlear approach), CSF leakage occurred during surgery because of opening of the fundus of the IAC. In all transpromontorial procedures, a fat pad pushed into the promontorial defect was used to stop the leakage, closing the promontorial defect. In the infracochlear approach, it was repaired by pulling a muscle graft into the defect. These grafts were secured with fibrin glue. In all 12 surgical procedures, the facial nerve was detected endoscopically and preserved in 11/12 patients. In one patient, facial nerve sacrifice was necessary to remove the neoplasm (hemangioma of the facial nerve), and reconstruction of the facial nerve was attempted during the same surgical procedure using an auricular nerve cable graft. No significant intraoperative complications were recorded in the present case series. Postoperative Course A CT scan was performed in all cases several hours after the surgery (Fig. 12). In the case of IAC

9 TABLE I. Characteristics of the Final Study Group. Patient Age, yr Sex Disease Surgical Route Preoperative Facial Function (RGS) Postoperative Facial Function (RGS) Preoperative Hearing Function (PTA) Postoperative Hearing Function (PTA) Complications Hospitalization, d A.A. 77 M Facial nerve hemangioma Suprageniculate 2 6 SN BC , AC 5 60 SN BC , AC B.G. 44 M Acoustic neuroma Transpromontorial 1 1 Anacusis Anacusis Eardrum perforation C.M. 58 M Chondroma Infracochlear (biopsy) 1 1 NA DX BC , AC M.E. 17 F Cholesteatoma Suprageniculate (combined approach) O.D. 25 F Cholesteatoma (recurrence) K.A. 40 M Cochlear schwannoma 1 1 SN BC5 15, AC Suprageniculate 3 1 SN BC , AC SN BC 5 10, AC SN BC , AC None 7 Eardrum perforation Transpromontorial 1 1 Anacusis Anacusis Delayed facial palsy, granulation tissue in the external auditory canal T.I. 62 F Acoustic neuroma Transpromontorial 1 1 SN BC , AC 5 43 B.V. 27 F Cholesteatoma (recurrence) Anacusis Transitory facial palsy 6 6 None 3 None 3 Transpromontorial 1 1 Anacusis Anacusis None 1 S.O. 29 M Cholesteatoma Infracochlear 1 1 DX BC 5 7, AC 5 25 NA None 1 A.D.M. 13 F Cholesteatoma Infracochlear 1 1 DX BC 5 10, AC 5 50 DX BC 5 5, None 3 AC D.G.G. 45 M IAC meningioma Transpromontorial 1 1 Anacusis Anacusis None 7 N.M.D. 63 F Cholesteatoma Infracoclear (combined approach) 1 1 SN BC , AC SN BC , AC 5 NA 10 5 None 5 PTA was calculated as a pure-tone air-conduction (AC) and bone-conduction (BC) threshold average at 500, 1,000, 2,000, and 4,000 Hz. F 5 female; GG 5 geniculate ganglion; IAC 5 internal auditory canal; M 5 male; MCF 5 middle cranial fossa; NA 5 not available; PTA 5 pure-tone average; RGS 5 Rough Facial Nerve Grading System; SN 5 left; DX 5 right. S9

10 involvement, an MRI scan was also planned after 1 to 2 months. In all cases, the postoperative pain was well controlled using intravenous paracetamol. Antibiotic therapy (1 g of intravenous cefazoline twice a day) was administered for 48 hours after surgery. Out of the three patients who underwent a transcanal suprageniculate corridor approach, all of them had conductive hearing loss preoperatively, and postoperatively they maintained approximately the same hearing function from both bone and air PTA values. Transcanal Suprageniculate Approach In the three patients who underwent an transcanal suprageniculate approach, no postoperative complications were observed. The postoperative CT scan showed a regular outcome for the transcanal suprageniculate approach. All these patients were discharged without complications between the third and seventh postoperative day. Transcanal Transpromontorial Approach Four out of five patients who underwent a transcanal transpromontorial approach were kept in bed for 3 days after surgery owing to CSF leakage during surgery after opening the IAC. Three of five patients in the same group reported postoperative horizontal nystagmus in association with vertiginous and neurovegetative symptoms. The dizziness gradually improved during the hospital stay. All of these patients were discharged without complications: 2/5 patients were discharged on the ninth postoperative day, and 2/5 patients were discharged on the sixth postoperative day. One patient was discharged on the first postoperative day, because the extent of disease was limited to the vestibule without IAC involvement. Transcanal Infracochlear Approach No postoperative complications were observed in any of the four patients who underwent the transcanal infracochlear approach. One patient was discharged on the sixth day after surgery, because CSF leakage occurred during surgery, and the patient was kept in bed for 3 days after surgery. Three of four patients were discharged on the third day after surgery. Hearing Results Out of the five patients who underwent a transcanal transpromontorial corridor approach for removal of their pathology, five patients had postoperative anacusis. Preoperatively, 4/5 patients had profound hearing loss or anacusis. Out of the four patients who underwent a transcanal infracochlear corridor approach, two had normal hearing preoperatively, whereas two had middle ear conductive hearing loss. Of the two patients with preoperative normal hearing, one maintained a normal hearing function, whereas the other patient reported middle ear conductive hearing loss postoperatively. Of the two patients with preoperative hearing loss, one maintained the same bone- and air-conduction pure-tone audiometry (PTA) values postoperatively, whereas in the other patient, PTA air conduction was not available, and bone PTA was preserved. S10 Facial Function Outcome Preoperatively, 10/12 patients presented with grade I of the Rough Facial Nerve Grading System (RGS). 11 One patient affected by hemangioma of the geniculate ganglion presented with RGS grade II, and one patient affected by cholesteatoma involving the geniculate ganglion with extension into the petrous apex presented with RGS grade III. Analyzing facial nerve function after surgery, 11/12 patients maintained their RGS grade I score during the postoperative follow-up. Of these patients, one patient presented a delayed facial palsy (RGS grade III) 2 days after surgery. After 6 days, recovery of facial function was observed (RGS grade I). One patient presented with RGS grade III facial palsy in the immediate postoperative period because of difficult dissection of the tumor from the nerve. In this case, a recovery of facial nerve function (RGS grade I) was also observed 1 month after surgery. For the patient who presented preoperatively with RGS grade III (cholesteatoma involving the petrous apex), facial nerve paresis improved to grade II at discharge. During the follow-up period, the facial nerve recovered completely to RGS grade I after some months. One patient (hemangioma of the facial nerve) presented a RGS grade VI after surgery. Dissection of the tumor had required sacrifice of the second portion of the facial nerve, and a great auricular nerve graft was used to reconstruct the continuity of the facial nerve. Follow-up The mean postoperative follow-up period was 12 months (range, 1 51 months). One patient required a surgical ambulatory procedure under local anesthesia for the presence of granulation tissue in the external auditory canal. Two patients presented a slight perforation of the eardrum. During the follow-up period, no recurrence of disease was found. DISCUSSION In recent years, there has been an increasing number of studies in the literature 3,6 on the exclusive endoscopic approach to the tympanic cavity. This approach represents a minimally invasive technique through the EAC, and is used when disease is located in the tympanic cavity without mastoid involvement. The development of this approach, together with the development of dedicated instruments, 12 and technological advancements allow endoscopic tympanoplasty to be performed, removing the diseases and reconstructing the middle ear without any external incision or mastoidectomy. 3,13 15 Nevertheless, transcanal endoscopic approaches to middle ear diseases, such as tympanic cavity cholesteatoma,

11 Fig. 12. Postoperative computed tomography scan in the coronal view (A, B) and axial view (C, D) of a transcanal endoscopic transpromontorial approach. Note the surgical corridor extending from the external auditory canal to the internal auditory canal. are not yet widely accepted as preferable to conventional microscope-based canal wall down or canal wall up approaches. Certainly, with the recent application of the endoscopic technique for cholesteatoma removal, outcome studies that directly compare the results with respect to extent of disease, patients quality of life, recurrence, residual disease, morbidity, length of stay, and cost are lacking. And despite the surgical evolution of the endoscopic technique in anterior skull base surgery and in the management of malignant tumors of the paranasal/anterior skull base, few ear surgeons are routinely adopting the endoscope during surgery for ear disease, because there is a lack of adequate training to reach the optimal skill levels with this tool. Advances in the field of skull base surgery aim to maximize anatomical exposure while minimizing patient morbidity. Owing to complex anatomy, the petrous bone in the lateral skull base is wedged between the sphenoid and occipital bones, containing or in close relationship to the facial nerve, the IAC with the cochlea and vestibular labyrinth, and the main vascular structures such as the jugular and internal carotid artery. To reach lesions lying at this anatomical boundary represents a challenge for surgical access, despite the limited dimensions and the benign nature of the disease. To reach lesions lying in the lateral skull, brain retraction, structural manipulation, and facial nerve palsy are often associated with open procedures. This is also the case for removal of benign lesions with limited dimensions. It is obvious that a detailed knowledge of the anatomy of this area and the characteristics of the lesion are essential in choosing and executing the best surgical route. The anatomy of this area has been well described in the literature from a transtemporal, transmastoid microscopic point of view, but studies on the endoscopic anatomy through the EAC are scarce. From our experience with endoscopic approaches to the middle ear, we have gradually come to the opinion that the medial wall of the tympanic cavity could represent a surgical corridor to reach the lateral skull base and inner ear 8,9 (vestibule, fundus of the IAC, suprageniculate area, petrous apex), in the same fashion as the nasal/sphenoid surgical corridor is used for access to sellar disease and the anterior skull base. Several dissections were performed by our group to gain the necessary anatomical knowledge and surgical skills, studying the anatomy from the EAC to the IAC from an endoscopic point of view, and using the EAC as a natural opening to access the lateral skull base. 8 From these dissections and from our initial clinical experiences reported above, three main endoscopic corridors have been identified in the lateral skull base: the transcanal suprageniculate route, the transcanal transpromontorial route, and the transcanal infracochlear route. Some considerations on these surgical routes are outlined next. Regarding the infracochlear route, when performed with a microscope, this is a route that has been well known for decades to gain access to pathologies located in the infracochlear region. It was mainly used in the past for cholesterol granuloma drainage. As mentioned above, by using an endoscope for visualization, it is possible to extend the indications for petrous apex lesions, and for pathology extending inferiorly to the IAC. Cholesteatoma can also be treated with this approach. Three main structures represent the limits and, at the same time, the landmarks for this approach: the carotid artery anteriorly, the internal jugular vein posteriorly, and the cochlea inferiorly. The subcochlear canaliculus, a small tunnel that connects the round window chamber region to the petrous apex, can be followed from a lateral to a medial direction, indicating the correct route. 10 The suprageniculate route is considered when a disease with limited dimensions is lying in the suprageniculate fossa. Anatomically, we can consider the suprageniculate fossa to be a triangular anatomical area S11

12 between the tympanic portion of the facial nerve, the geniculate ganglion and the greater petrosal nerve inferiorly, the middle cranial fossa superiorly, and the labyrinthine bloc posteriorly. In this preliminary experience with endoscopic visualization, the suprageniculate route allowed us to reach this anatomical triangle directly, and the bony tissue between these anatomical structures was removed to reach the petrous apex cells lying superiorly to the geniculate ganglion, the labyrinthine facial nerve, and the IAC. The possibility to control the pathologic tissues lying in the suprageniculate fossa through the EAC may help the surgeon to perform minimally invasive surgery, avoiding middle cranial fossa craniotomy and brain tissue manipulation. The transpromontorial route is considered when a disease with limited dimensions involves the cochlea, the vestibule, and the fundus of the IAC. Classically, benign pathologies such as cholesteatoma, acoustic schwannomas, or cochlear schwannomas involving the fundus of the IAC or the cochlea have been treated with open approaches with translabyrinthine or transotic surgery to reach the fundus of the IAC and cochlea. These techniques provide wide exposure and manipulation of the dura of the middle cranial fossa, and can also require facial nerve skeletonization. Anatomically, the removal of the promontory area gives good access to the cochlea and vestibule. These anatomical structures are in close relationship with the fundus of the IAC. As shown in earlier articles, 8 the medial wall of the vestibule and the spherical recess represent crucial anatomical landmarks used to detect the fundus of the IAC, because the end of the inferior vestibular nerve is attached in the spherical recess. On the other hand, the cranial portion of the apical and medial turn of the cochlea are crucial landmarks for detection of the labyrinthine facial nerve, which runs just above these structures. Anatomical knowledge of these structures from an endoscopic point of view allowed us to reach the fundus of the IAC, removing diseases lying in this area and avoiding the transpetrous open approaches. In particular, in the future development of this approach, management of acoustic schwannomas of the fundus could be modified, with the alternative to wait and carry out scans in patients with hearing loss and growing tumors. Although these initial experiences are promising in the authors opinion, some surgical risks should be considered. During the transcanal transpromontorial approach, possible risk of injury to the internal auditory artery or of some anterior inferior cerebellar artery, loops must be considered, and a preoperative MRI evaluation of the vessels in the IAC is mandatory. Moreover, management of the facial nerve in the IAC could be difficult, and careful surgical maneuvers are required to avoid damage to the nerve. A possible transitory facial nerve palsy should be considered with this approach, although total recovery was present in our case series. In fact, one patient presented transitory facial nerve palsy on the second postoperative day, and another patient presented facial palsy grade III because of dissection of the tumor, which was tightly adherent to the facial nerve. The present article is based on a limited number of cases and represents the initial experiences using a transcanal endoscopic approach to lateral skull base lesions, describing the transcanal surgical corridors. Despite this, the techniques seem promising, and may represent an advance in the lateral skull base surgery field. These approaches require long-term validation and follow-up that may possibly indicate drawbacks or confirm our results. Certainly, these endoscopic transcanal approaches are only indicated at present in limited cases, especially when a benign lesion is present with limited extension into the inner ear and lateral skull base lying beyond the tympanic cavity. When a lesion is lying in the suprageniculate fossa, the petrous apex, the cochlea, vestibule, and IAC, a CT scan and MRI study of this lesion are mandatory to understand its anatomical relationship with the tympanic cavity, and to decide whether a transcanal endoscopic route should be attempted through the EAC, avoiding open approaches. Last but not least, these approaches require the development of adequate endoscopic skills, because several endoscopic ear dissections may be necessary to understand the transcanal anatomy from the EAC to the IAC. Neuro-otological skills are necessary because the endoscopic approach may be changed or combined with traditional microscopic approaches, if required, during the surgical procedures. CONCLUSION The transcanal endoscopic approach to the lateral skull base has proven to be successful for removal of pathology involving the fundus, IAC, cochlea, petrous apex, and geniculate ganglion region, with lower complication rates and less invasive procedures compared to traditional microscopic approaches. Three main corridors to the lateral skull base have been identified: the transcanal suprageniculate corridor, the transcanal transpromontorial corridor, and the transcanal infracochlear corridor. Future applications of this kind of approach in lateral skull base surgery will depend on the development of technology, and surgical and anatomical refinements. BIBLIOGRAPHY 1. Thomassin JM, Korchia D, Doris JM. Endoscopic guided otosurgery in the prevention of residual cholesteatomas. Laryngoscope 1993;103: Presutti L, Marchioni D, Mattioli F, Villari D, Alicandri-Ciufelli M. Endoscopic management of acquired cholesteatoma: our experience. 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