Endoscope assisted retrosigmoid intradural suprameatal approach for vestibular schwannoma [Neurosurgery Wiki]

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=====Endoscope assisted retrosigmoid intradural suprameatal approach for vestibular schwannoma=====


The visualization of [[cranial nerve]]s and vessels behind the tumor in the [[cerebellopontine angle]] and recognition of [[vestibular schwannoma]] located in the lateral [[internal auditory canal]] (IAC) are possible with [[endoscopy]].

These findings are useful for preservation of cranial nerve function and vessels and complete tumor removal. The opened air cells of the posterior wall of the IAC can be recognized with the angled endoscope, and this may be useful for prevention of [[cerebrospinal fluid leakage]]
((Schaller B. [Cerebellopontine angle surgery. Part 2: Specific remarks]. HNO.
2003 May;51(5):375-85. Epub 2003 Mar 28. Review. German. PubMed PMID: 12835853.
)).


====Indications====

The usefulness and problems of endoscope-assisted microsurgery for removal of [[vestibular schwannoma]]s have been reported 
((Gerganov VM, Giordano M, Herold C, Samii A, Samii M. An electrophysiological
study on the safety of the endoscope-assisted microsurgical removal of vestibular
schwannomas. Eur J Surg Oncol. 2010 Apr;36(4):422-7. doi:
10.1016/j.ejso.2009.11.003. Epub 2009 Nov 26. PubMed PMID: 19942394.
))
((Gerganov VM, Romansky KV, Bussarsky VA, Noutchev LT, Iliev IN.
Endoscope-assisted microsurgery of large vestibular schwannomas. Minim Invasive
Neurosurg. 2005 Feb;48(1):39-43. PubMed PMID: 15747215.))
((Göksu N, Bayazit Y, Kemaloğlu Y. Endoscopy of the posterior fossa and
dissection of acoustic neuroma. J Neurosurg. 1999 Nov;91(5):776-80. PubMed PMID: 
10541234.
))
((Hori T, Okada Y, Maruyama T, Chernov M, Attia W. Endoscope-controlled removal 
of intrameatal vestibular schwannomas. Minim Invasive Neurosurg. 2006
Feb;49(1):25-9. PubMed PMID: 16547878.))
((King W A, Wackym P A. Endoscope-assisted surgery for acoustic neuromas (vestibular schwannomas): early experience using the rigid Hopkins telescope. Neurosurgery. 1999;44(5):1095–1100, discussion 1100–1102.))
((McKennan K X. Endoscopy of the internal auditory canal during hearing conservation acoustic tumor surgery. Am J Otol. 1993;14(3):259–262.))
((Miyazaki H, Deveze A, Magnan J. Neuro-otologic surgery through minimally invasive retrosigmoid approach: endoscope assisted microvascular decompression, vestibular neurotomy, and tumor removal. Laryngoscope. 2005;115(9):1612–1617.))
((Tatagiba M, Matthies C, Samii M. Microendoscopy of the internal auditory canal in vestibular schwannoma surgery. Neurosurgery. 1996;38(4):737–740.))
((Valtonen H J, Poe D S, Heilman C B, Tarlov E C. Endoscopically assisted prevention of cerebrospinal fluid leak in suboccipital acoustic neuroma surgery. Am J Otol. 1997;18(3):381–385.))
((Wackym P A, King W A, Poe D S. et al. Adjunctive use of endoscopy during acoustic neuroma surgery. Laryngoscope. 1999;109(8):1193–1201))
((Vaz-Guimaraes F, Gardner PA, Fernandez-Miranda JC. Fully endoscopic
retrosigmoid approach for posterior petrous meningioma and trigeminal
microvascular decompression. Acta Neurochir (Wien). 2015 Apr;157(4):611-5. doi:
10.1007/s00701-014-2332-1. Epub 2015 Jan 18. PubMed PMID: 25596641.
)).

The benefit of endoscopic assistance was shown for those patients whose tumors extended beyond the mid-portion of the IAC but did not reach the [[fundus]]. The significance of this observation was safe removal of the intracanalicular remnant tumor under recognizing clearly the boundary between tumor and cranial nerves
((Kumon Y, Kohno S, Ohue S, Watanabe H, Inoue A, Iwata S, Ohnishi T. Usefulness 
of endoscope-assisted microsurgery for removal of vestibular schwannomas. J
Neurol Surg B Skull Base. 2012 Feb;73(1):42-7. doi: 10.1055/s-0032-1304555.
PubMed PMID: 23372994; PubMed Central PMCID: PMC3424024.
)).


====Problems====
The problems are mechanical or heat injury to the cranial nerve and surrounding normal tissue during manipulation of the endoscope and heat generated by the endoscope’s light source
((Hori T, Okada Y, Maruyama T, Chernov M, Attia W. Endoscope-controlled removal 
of intrameatal vestibular schwannomas. Minim Invasive Neurosurg. 2006
Feb;49(1):25-9. PubMed PMID: 16547878.))
((King W A, Wackym P A. Endoscope-assisted surgery for acoustic neuromas (vestibular schwannomas): early experience using the rigid Hopkins telescope. Neurosurgery. 1999;44(5):1095–1100, discussion 1100–1102.))
((Miyazaki H, Deveze A, Magnan J. Neuro-otologic surgery through minimally invasive retrosigmoid approach: endoscope assisted microvascular decompression, vestibular neurotomy, and tumor removal. Laryngoscope. 2005;115(9):1612–1617.)).


Gerganov et al., evaluated the risk of thermal or mechanical iatrogenic nerve injury related to endoscope application during microsurgical removal of vestibular schwannomas (VS) in a prospective group of 30 patients (Group A). Main analysed parameters were electrophysiological monitoring data (auditory evoked potentials and EMG) during and after endoscopic observation. The structural and functional preservation of facial and cochlear nerves, radicality of tumour removal, and CSF leak rate were evaluated and compared to historical group of 50 patients (Group B), operated consecutively with classical microsurgical technique.

No electrophysiological changes directly related to endoscope were registered. The rate of loss of waves I, II, and V did not depend on application of endoscope and was similar in both groups. The functional and general outcome was also similar. Endoscopic inspection provided early and detailed view of anatomical relations within cerebellopontine angle and internal auditory canal and confirmed completeness of tumour removal. Total tumour removal was achieved in all patients from Group A and in 49/50 from Group B. Useful hearing after the surgery had 17/30 patients in Group A and 26/50 in Group B.

The application of endoscope during microsurgical removal of VS is a safe procedure that does not lead to heat-related or mechanical neural or vascular injuries. The actual significance of this additional endoscopic information, however, is related to the particular operative technique and experience of the surgeon
((Gerganov VM, Giordano M, Herold C, Samii A, Samii M. An electrophysiological
study on the safety of the endoscope-assisted microsurgical removal of vestibular
schwannomas. Eur J Surg Oncol. 2010 Apr;36(4):422-7. doi:
10.1016/j.ejso.2009.11.003. Epub 2009 Nov 26. PubMed PMID: 19942394.
)).


====Technique====

Upon dural opening, the endoscope is inserted into the operative field along the petrotentorial junction. Cerebrospinal fluid drainage provides a wider space for introduction of the endoscope and surgical instruments by opening the [[cerebellomedullary cistern]] for cerebellar relaxation.

Traditional microsurgical techniques are used during the entire procedure.


====Case series====
===2013===
Consecutive 89 patients with untreated unilateral sporadic [[vestibular schwannoma]] undergoing tumor resection via a [[retrosigmoid approach]] during 2008-2010 were prospectively analysed. Endoscopy-assisted microsurgical (EA-MS) removal was performed in 39 cases (Grade 1: 2, Grade 2: 5, Grade 3: 9, Grade 4: 22, Grade 5: 1) and microsurgical (MS) removal was performed in 50 cases (Grade 1: 1, Grade 2: 3, Grade 3: 9, Grade 4: 34, Grade 5: 3). Minimally invasive approach with craniotomy ≤ 2.5 cm was employed for small tumors (Grade 1 and 2) in the EA-MS group. Endoscopic technique was used for monitoring of neurovascular anatomy in CPA, during dissection of the meatal portion of tumors, assessment of radicality and for identification of potential pathways for CSF leak formation. All cases in MS group were deemed as radically removed. In the EA-MS group, residual tumor tissue in the fundus of [[internal auditory canal]] not observable with microscope was identified with endoscope in four cases. Such cases were radicalized. Tumor recurrence was not observed during the follow-up in EA-MS group. There is a suspicious intrameatal tumor recurrence on the repeated MRI scan in one patient in the MS group. Neither mortality nor infection was observed. The most common complication was [[pseudomeningocele]] (EA-MS 20 cases; MS 23). It was managed with aspiration with or without tissue-gluing in all cases without the need for any surgical revision. Adjunctive use of endoscope in the EA-MS group identified potential pathways for CSF leak formation, which was not observable with the microscope in five patients. Improved cochlear nerve (EA-MS: 22, MS: 14; p = 0.012), brainstem auditory evoked potentials (EA-MS: 3 of 8, MS: 0 of 4) and hearing (EA-MS: 14 of 36, MS: 4 of 45; p = 0.001) preservation were observed in EA-MS group. Despite the trend for better useful hearing (Gardner-Robertson class 1 and 2) preservation (EA-MS: 8 of 26, MS: 1 of 16) there were no significant differences in the postoperative hearing handicap inventory in both groups. There were no differences in the postoperative tinnitus in both groups. Better facial nerve preservation (EA-MS: 39, MS: 44; p = 0.027) and excellent-very good (House-Brackmann 1 or 2) facial nerve function (EA-MS: 31, MS: 29; p = 0.035) were observed in EA-MS group. Postoperative compensation of vestibular lesion, symptoms typical for VS, patients assessed by dizziness handicap inventory, facial disability index were comparable in both studied groups. Adjunctive use of endoscope during the VS surgery due to its magnification and illumination enable superior view in the operative field. It is valuable for assessment of radicality of resection in the region of internal auditory meatus. Improved information about critical structures and tumor itself helps the surgeon to preserve facial nerve and in selected cases also hearing. These techniques can help to decrease incidence of postoperative complications
((Chovanec M, Zvěřina E, Profant O, Skřivan J, Cakrt O, Lisý J, Betka J. Impact 
of video-endoscopy on the results of retrosigmoid-transmeatal microsurgery of
vestibular schwannoma: prospective study. Eur Arch Otorhinolaryngol. 2013
Mar;270(4):1277-84. doi: 10.1007/s00405-012-2112-6. Epub 2012 Aug 4. PubMed PMID:
23010789.
)).

===2012===
The usefulness of endoscope-assisted microsurgical removal of vestibular schwannomas in the internal auditory canal (IAC) was evaluated. Microsurgical removal using the endoscope was done in 28 procedures and microsurgical removal without an endoscope was done in 43 procedures. A retrosigmoid approach was used. The tumor location in the IAC was classified as grade 1 (located up to the mid-portion), 2, 3, or 4 (located up to the fundus with bony destruction) according to the tumor extent, and residual tumor in the IAC was evaluated as grade A (remnant tumor was not observed), B, C, or D (remnant tumor was observed over the mid-portion) according to the extent of remnant tumor. The residual tumor in the IAC was less in the endoscope-assisted group than in the microsurgery group. There was a significant difference only in grade 2, that is, tumor located beyond the mid-portion of the IAC. There was no significant difference in the results of preservation of useful hearing, facial nerve function, and tumor recurrence between the two groups. The benefit of endoscope-assistance microsurgery was shown for those patients whose tumors extended beyond the mid-portion of the IAC but did not reach the fundus
((Kumon Y, Kohno S, Ohue S, Watanabe H, Inoue A, Iwata S, Ohnishi T. Usefulness 
of endoscope-assisted microsurgery for removal of vestibular schwannomas. J
Neurol Surg B Skull Base. 2012 Feb;73(1):42-7. doi: 10.1055/s-0032-1304555.
PubMed PMID: 23372994; PubMed Central PMCID: PMC3424024.
)).

===2010===
From July 2003 to July 2007, intraoperative auditory monitoring using auditory brainstem response (ABR) and electrocochleography (EcochG) was conducted in 18 of 138 patients undergoing surgery for acoustic neuroma who had preoperative hearing. The retrosigmoid approach was used for 16 complete resections and 2 patients underwent partial resections. Assisted endoscopic surgery was conducted for 10 ears. The main outcome measure was preservation of hearing, assessed using the classification method of the American Institute of Otolaryngology-Head and Neck Surgery.

Hearing was preserved in 11 of the 18 patients (61.1%): 2 of 5 patients whose tumors were larger than 20 mm (40%), and 9 of 13 patients with smaller tumors (69.2%). Among the 10 cases of ear endoscope-assisted surgery, hearing was preserved in 8 (80%). Intraoperative monitoring revealed that the waveform was influenced when the posterior labium of the internal acoustic meatus was ground and drilled, or when traction or electrocoagulation was performed near the opening of the internal acoustic meatus, especially when the internal auditory artery was clamped, the tumor in the internal acoustic meatus was treated, and the arachnoid vessels in the inner-most layer of the tumor surface were clamped or electrocoagulated
((Han DY, Yu LM, Yu LM, Ji F, Young WY, Yang SM. Acoustic neuroma surgery for
preservation of hearing: technique and experience in the Chinese PLA General
Hospital. Acta Otolaryngol. 2010 May;130(5):583-92. doi:
10.3109/00016480903402999. PubMed PMID: 20406132.
)).
----
Gerganov et al., evaluated the risk of thermal or mechanical iatrogenic nerve injury related to endoscope application during microsurgical removal of vestibular schwannomas (VS) in a prospective group of 30 patients (Group A). Main analysed parameters were electrophysiological monitoring data (auditory evoked potentials and EMG) during and after endoscopic observation. The structural and functional preservation of facial and cochlear nerves, radicality of tumour removal, and CSF leak rate were evaluated and compared to historical group of 50 patients (Group B), operated consecutively with classical microsurgical technique.

No electrophysiological changes directly related to endoscope were registered. The rate of loss of waves I, II, and V did not depend on application of endoscope and was similar in both groups. The functional and general outcome was also similar. Endoscopic inspection provided early and detailed view of anatomical relations within cerebellopontine angle and internal auditory canal and confirmed completeness of tumour removal. Total tumour removal was achieved in all patients from Group A and in 49/50 from Group B. Useful hearing after the surgery had 17/30 patients in Group A and 26/50 in Group B.

The application of endoscope during microsurgical removal of VS is a safe procedure that does not lead to heat-related or mechanical neural or vascular injuries. The actual significance of this additional endoscopic information, however, is related to the particular operative technique and experience of the surgeon
((Gerganov VM, Giordano M, Herold C, Samii A, Samii M. An electrophysiological
study on the safety of the endoscope-assisted microsurgical removal of vestibular
schwannomas. Eur J Surg Oncol. 2010 Apr;36(4):422-7. doi:
10.1016/j.ejso.2009.11.003. Epub 2009 Nov 26. PubMed PMID: 19942394.
)).


===2008===
Assisted by hard-tube ear endoscope, 11 patients with acoustic neuroma were operated via labyrinthine approach and retrosigmoid approach.

Total removal was achieved in 9 (81.8%) cases, while subtotal removal was achieved in 2 (18.2%) cases. Facial nerves and cochlear nerves were preserved completely during operation in all cases. After operation 9 (81.8%) cases had no facial paralysis, while the other 2 cases (18.2%) had mild peripheral facial paralysis. The same hearing level as that of preoperation in 2 patients (18.2%), hearing impairment in different degrees in 9 patients (81.8%), among which moderate sensorineural hearing loss in 1 patient, moderate to severe in 2 patients, severe in 3 patients, profound in 3 patients.

The application of ear endoscope in acoustic neuroma surgery can improve the total removal rate of tumors and the salvage rate of vessels and nerves. However, its disadvantages make it an assistant method for microsurgery
((Chen S, Xu Y, Ou Y, Zheng Y, Deng Y, Chen B. [Endoscope-assisted surgical
resection of acoustic neuroma]. Lin Chung Er Bi Yan Hou Tou Jing Wai Ke Za Zhi.
2008 Aug;22(16):729-31. Chinese. PubMed PMID: 18975773.
)).

===2006===
Clinical series included 33 consecutive patients (23 women and 10 men; mean age 50 +/- 15 years). A bayonet-style rigid endoscope with 70 degrees angle of view and 4 mm outer diameter was found to be optimal for observation of the internal auditory canal. Its insertion in the cerebellopontine cistern should be preferably done under control through an operating microscope. Endoscope-controlled manipulations necessitate the use of a special holder system, which provides a stable position of the device and allows bimanual manipulations by the surgeon. A thermographic evaluation did not reveal a significant increase of the local temperature due to use of the endoscope. Use of the endoscope permitted removal of the neoplasm from the most lateral part of the internal auditory canal and identification of the nerve of tumor origin. In total, 28 tumors underwent total removal, and anatomical preservation of the facial nerve was attained in 31 cases. Damage of the facial nerve by the endoscope was met once. In 8 out of 16 patients, who showed serviceable hearing before surgery, this was preserved after tumor removal. In conclusion, endoscope-controlled removal of the intrameatal vestibular schwannomas seems to be a technically feasible, effective and safe procedure. Nevertheless, good equipment and special training are absolutely necessary for attainment of optimal results
((Hori T, Okada Y, Maruyama T, Chernov M, Attia W. Endoscope-controlled removal 
of intrameatal vestibular schwannomas. Minim Invasive Neurosurg. 2006
Feb;49(1):25-9. PubMed PMID: 16547878.)).

===2005===
Eighteen patients were included in a prospective study. Their average tumor diameter was 3.9 cm. The [[retrosigmoid approach]] was used in all cases. The endoscope was applied during all stages of tumor removal.

The facial nerve was visualized endoscopically at early stages of surgery in 9 patients and the abducent nerve in 7 patients. The source of bleeding was identified in 1 case. Tumor remnants in the region of the fundus of the internal auditory canal after apparently total removal were identified in 2 cases. Exposed and unobliterated temporal bone air cells were not observed.

Even in cases of large schwannomas, the location of the [[facial nerve]] can be determined endoscopically early in the procedure. The application of endoscope-assisted microsurgery increases the rate of cranial nerve preservation and of total tumor removal. Although the application of the endoscope did not provide useful information in some cases, it is a safe procedure that did not lead to any complications and/or to considerable prolongation of the operative time. Its application is justified in all cases
((Gerganov VM, Romansky KV, Bussarsky VA, Noutchev LT, Iliev IN.
Endoscope-assisted microsurgery of large vestibular schwannomas. Minim Invasive
Neurosurg. 2005 Feb;48(1):39-43. PubMed PMID: 15747215.)).

===1999===
Between 1989 and 1998, 32 patients underwent removal of acoustic neuroma (AN) via a combined retro-sigmoid-retrolabyrinthine approach. Endoscopes were used at different stages of the operation, and their use was evaluated with regard to elimination of the disadvantages of the posterior fossa approach. All patients in whom AN had been diagnosed underwent surgery in which a standard retrosigmoid-retrolabyrinthine approach was used. Standard sinus endoscopes of 0 degree, 30 degrees, and 70 degrees were introduced into the cerebellopontine angle before debulking the tumor, and the IAC was inspected at the end of the operation. Neurovascular integrity as well as the relationship between the AN and surrounding structures were evaluated. The IAC was inspected for residual tumor, and if any was found, endoscopically guided tumor dissection was performed.

Endoscopes have facilitated an understanding of the anatomy between an AN and neighboring neurovascular structures. For surgery in which the posterior fossa approach is used, endoscopes can make operations safer by eliminating the disadvantages of the approach. In addition to allowing inspection of the fundus, it is possible to perform endoscopically guided tumor dissection within the IAC
((Göksu N, Bayazit Y, Kemaloğlu Y. Endoscopy of the posterior fossa and
dissection of acoustic neuroma. J Neurosurg. 1999 Nov;91(5):776-80. PubMed PMID: 
10541234.
)).
----

The lateral end of the IAM up to the transverse crest could be viewed by the 30-degree rigid angled endoscope. This landmark could, therefore, be utilized in the endoscopic-assisted technique to predict the optimal amount of bone to be removed at a stage before the internal auditory meatal dura is opened when the intact dura affords added protection to the meatal contents during drilling. Well designed dural flaps on the posterior petrous bone could be created by making a longitudinal incision not more than 7 mm from the superior border of petrous bone and a transverse incision at least 17 mm from sigmoid. These flaps minimize injury to the endolymphatic sac and protect the cochlear nerve and vasculature that when damaged, may result in hearing loss
((Low WK. Enhancing hearing preservation in endoscopic-assisted excision of
acoustic neuroma via the retrosigmoid approach. J Laryngol Otol. 1999
Nov;113(11):973-7. PubMed PMID: 10696373.
)).

===1997===
A study compared CSF rhinorrhea rates of 38 consecutive suboccipital acoustic neuroma operations, in which conventional techniques were used to pack the temporal bone defect around the internal auditory canal, with the succeeding 24 consecutive operations, in which endoscopes were used to visualize all exposed air cells directly. After locating all patent air cells endoscopically, they were specifically sealed with bone wax, and then a small fat graft harvested from the wound margin was used to fill the remaining defect.

Postoperative CSF rhinorrhea occurred in 7 of 38 (18.4%) operations in which no endoscopic technique was used and in 0 of 24 operations in which endoscopes were used.

The use of endoscopes to visualize the temporal bone air cells that cannot be directly observed otherwise appears to reduce the incidence of postoperative CSF leak in suboccipital acoustic neuroma surgery
((Valtonen HJ, Poe DS, Heilman CB, Tarlov EC. Endoscopically assisted prevention
of cerebrospinal fluid leak in suboccipital acoustic neuroma surgery. Am J Otol. 
1997 May;18(3):381-5. PubMed PMID: 9149835.
)).

====Training===
Twelve bilateral retrosigmoid dissections were performed in 6 glutaraldehyde embalmed, colored silicone injected, adult cadaveric heads. Using a standard retrosigmoid approach, Baidya et al., first implanted the tumor model at the cerebellopontine angle (CPA) and then we resected the tumor under simultaneous endoscopic and microscopic visualizations. The resection was performed by first creating a corridor by removing the lower portion of the tumor and then by inserting through the same corridor the flexible endoscope mounted on a surgical instrument in order to accomplish early visualization of the VII-VIII complex. This early visualization of the VII-VIII complex made possible expeditious removal of the model with preservation of the VII-VIII complex.

They were able to successfully implant the artificial tumor in all the specimens. The post-tumor implantation CT scan confirmed the optimal CPA location of the model with its intra-porus extension. The exposure of the facial and the adjoining neuro-vascular structures was excellent during all stages of the surgical removal and was accomplished with minimal cerebellar retraction, under intermittent endoscopic-assisted control. Early visualization of the facial and vestibular cochlear nerves complex led to unhindered removal of the tumor model.

The endoscopic-assisted microsurgical removal of a tumor model simulating a medium size vestibular schwannoma was feasible in this tumor model study emulating real surgery. Visualization of the acousticofacial bundle at the early stage of the surgical removal should theoretically decrease the risk of its inadvertent injuries as well as facilitate complete removal of the tumor. Clinical studies to validate this laboratory study are necessary
((Baidya NB, Berhouma M, Ammirati M. Endoscope-assisted retrosigmoid resection
of a medium size vestibular schwannoma tumor model: a cadaveric study. Clin
Neurol Neurosurg. 2014 Apr;119:35-8. doi: 10.1016/j.clineuro.2013.12.023. Epub
2014 Jan 18. PubMed PMID: 24635922.
)).
----
A 20 mm hole made backwards from the midpoint of the [[asterion]] to the mastoid process is suitable for a retrosigmoid keyhole approach with the aid of an endoscope. The endoscope-assisted [[retrosigmoid keyhole approach]] can be considered an effective and safe method for removal of vestibular schwannoma.

With the aid of an endoscope, we simulated surgical procedures in 30 sides of 15 formalin-fixed cadaver heads.

(1) For 24 (80%) sides, the midpoint of the top notch to the mastoid process was in the anterior edge of the sigmoid sinus; for 27 sides (90%) the midpoint of the asterion to the mastoid process was in the posterior edge of the sigmoid sinus. (2) The IAC and CPA were exposed by the retrosigmoid keyhole approach with the aid of the endoscope
((Sun JQ, Sun JW. Endoscope-assisted retrosigmoid keyhole approach for
cerebellopontine angle: cadaveric study. Acta Otolaryngol. 2013
Nov;133(11):1154-7. doi: 10.3109/00016489.2013.817682. Epub 2013 Sep 3. PubMed
PMID: 23998700.
)).
----
A retrosigmoid approach to the IAC was performed on 10 whole fresh cadaveric heads after acquiring high-resolution computed tomographic scans (120 kV; slice thickness, 1 mm; field of vision, 40 cm; matrix, 512 x 512) with permanent bone-implanted reference markers. Drilling of the posterior wall of the IAC was executed with image guidance. Its most lateral area was visualized using endoscopy.

Target registration error for the procedure was 0.28 to 0.82 mm (mean, 0.46 mm; standard deviation, 0.16 mm). The measured length of the IAC along its posterior wall was 9.7 +/- 1.6 mm. The angle of drilling (angle between the direction of drill and the posterior petrous surface) was 43.3 +/- 6.0 degrees, and the length of the posterior wall of the IAC drilled without violating the integrity of the labyrinth was 7.2 +/- 0.9 mm. The surgical maneuvers in the remaining part of the IAC, including the fundus, were performed using an angled endoscope.

Frameless navigation using high-resolution computed tomographic scans and bone-implanted reference markers can provide a "roadmap" to maximize safe surgical exposure of the IAC without violating the labyrinth and leaving a small segment of the lateral IAC unexposed. Further exposure and surgical manipulation of this segment, including the fundus without additional cerebellar retraction and labyrinthine injury, can be achieved using an endoscope. Use of image guidance and an endoscope can help in exposing the entire posterior aspect of the IAC including its fundus without violating the labyrinth through a retrosigmoid approach. This technique could improve hearing preservation in vestibular schwannoma surgery
((Pillai P, Sammet S, Ammirati M. Image-guided, endoscopic-assisted drilling and
exposure of the whole length of the internal auditory canal and its fundus with
preservation of the integrity of the labyrinth using a retrosigmoid approach: a
laboratory investigation. Neurosurgery. 2009 Dec;65(6 Suppl):53-9; discussion 59.
doi: 10.1227/01.NEU.0000343521.88537.16. PubMed PMID: 19935002.
)).