Microsurgical Anatomy Of Three Approaches Of Vestibular Schwannoma: A Comparative Study

Objectives:To study 3 approaches of vestibular schwannoma surgery-Retrosigmoid, middle fossa and Translabrynthine approach in cadaveric heads by describing the microanatomy of exposed structure with snapshots,label and description, to broaden the understanding of the structures of posterior fossa, cerebellopontine angle, and middle fossa from 3 different views; focusing basically on retrosigmoid approach. This study also describes the advantage and disadvantage of each approach and together it gives a detail anatomy of intracranial, skull base and extracranial structures exposed using three approaches. The related neurovascular structures and their relationship with the adjacent structures were also studied, the knowledge of which is essential for the safe resection of vestibular schwannoma preserving normal structures anatomically and functionally.Observation and measurement of different bony landmarks that provides a valuable information and orientation of different structure in skull base surgery were also performed in dry skull. The study was performed as a part of training to residents, for better learning of the basic microsurgical skills, drilling methods and as a course for resident hand on Cadavers. The methods used for cadaveric preparation, preservation and the factors that are responsible for effective dissection were also pointed out in this study.Study design and settingsThis study was performed at the Skull base micro-neurosurgery Lab of the Department of Neurosurgery of Xiangya Hospital of Central South University(CSU), and was approved by the ethics committee for the Assessment of Research Projects from the Clinical Board of the University Hospital of the Central South University.5 alcohol fixed cadaveric heads (10 sides) were infused with colored silicon and was dissected stepwise to study the microanatomy of the structures exposed through three approaches that are applied for the resection of vestibular schwannoma: retrosigmoid, middle fossa, and translabyrinthine approach. The dissection was performed under magnification with the use of operating microscope Although, retro-sigmoid approach has been exclusively practiced in our center, three most commonly applied approaches middle fossa, translabyrinthine and retrosigmoid approach were studied from surgical perspective, in reference to the location and the exposure of neurovascular structures from different approaches. Together, the course of facial nerve from brainstem to CP angle and IAM and its relationship with surrounding structures were also studied to improve the understanding of microanatomy of facial nerve during VS surgery. Although various studies have already been carried out regarding this topic in different centers and rigid conclusion have already been made by different authors, this study was carried out to explain whether the application of other 2 approaches (Middle fossa and Translabrynthine) is mandatory in our center for effective resection of tumor and to preserve facial nerve and its functions. The advantage and disadvantage of all three approaches that was experienced during dissection has also been explained. In this study, it has been tried to explain all the steps and the microanatomy of the exposed relative structures by means of photos taken during dissection with description for the better understanding of the related structures from different approaches.The photographs were taken with an SLR digital Canon of 6.5 megapixels, Sony Cyber shot (DSC-T90). The images were then transferred to the computer and were processed with neo-imaging software for better demonstration.More importantly, this study was carried out for developing the microsurgical skills, improve drilling methods of temporal bone and to learn the microanatomy of CPA basically VII and VIII nerve complex with related osseous, soft tissue and vascular structures. This study helped to understand the complex anatomy of temporal bone, one of the complex structures of human calvarias. The anatomical detail of the cerebellar arteries basically AICA, which also serve as a landmark for identification of VIIth nerve during vestibular schwannoma surgery was also studied.Bony landmarks of skull base, basically foramens are always in a constant relationship with the corresponding structures. In this study we observed and measured different bony landmarks of middle and posterior fossa, and have calculated the mean distance, range and the standard deviation to explain their significance in skull base surgery in 10 dry skulls provided by anatomy lab of Wuhan Renmin Hospital of Wuhan University. To compare the mean distance between left and right side, we performed the independent T-test at 0.05 level of significance. The measurement includes distance from foramen spinosum to foramen ovale,arcuate eminence, facial hiatus, suprameatal spine (spine of Henle) and orifice of carotid canal; distance from facial hiatus to anterior edge of porus acousticus internus; distance from foramen ovale to foramen rotundum; distance from arcuate eminence to suprameatal spine(spine of Henle);distance from anterior edge of porus acousticus internus to Meckel's cave, and nerve segment of jugular foramen(Pars nervosa); height and length of porus acousticus internus; distance from sigmoid and transverse sulcus junction to posterior edge of porus acousticus internus;lcngth of internal auditory canal in anterior, posterior and superior edges. We also studied the relationship of asterion with transverse sinus to locate the safety zone for placement of burr hole.Cadaveric dissection provides the 3D anatomy of the structures which has been of great important for the junior doctors and residents. The methods for preservation and preparation of cadaver before dissection are very important for the effective study. This issue has also been mentioned in brief in this study.Results:1. Snapshots of the dissected structures through 3 different approaches were found to be of moderate quality. In some of the cadavers, the neurovascular tree was poorly colored. In some cases, they even ruptured during injection of dye leading to spillage into cisterns basically in cerebellopontine cisterns and anterior cranial cisterns encasing the important neurovascular structures with solidified silicon solution. Arteries tree was found to be well colored than the venous tree in most of our specimens. The brain parenchyma in most of the cases was hard in consistency due to long course of preservation leading to difficulty in retraction making obstacles in studying the underlying deep structures. Preservation of cadavers in alcohol gave a better effect than in the formalin in a way that it didn't provide the irritating pungent odor during dissection by maintaining the rigidity of normal anatomy.2. Retrosigmoid approach provides a wide exposure of the structures of CPA that includes the cranial nerves from Vth-XIth, vascular structures AICA and its branches, SCA, PICA, petrous vein; and neural structure of brainstem the lateral pons, and medullar along with their junction with cranial nerves, flocculus, and choroid plexus with good visibility. The posterior meatal wall called, acoustic porus internus was exposed with better visibility and RSA provided the enough space for the opening/drilling of meatal wall. The enhanced length achieved by drilling the floor of internal auditory canal provided the access and excellent exposure of the intrameatal segment ofⅦandⅧcranial nerve from their origin at pontomedullary sulcus to the most lateral end of IAC. The facial, vestibular and cochlear nerve shares the constant relationship at the lateral end of the fundus and in the brainstem, so they are easy to identify. Inside the IAC, facial nerve lies superior and anterior; SVN superior and posterior; cochlear nerve inferior and anterior and IVN lie inferior and posterior. In CPA, facial nerve lies ventral to the vestibular nerve and cochlear nerve lies inbetween facial and vestibular nerve. At the lateral end of pontomedullary sulcus facial nerve is located posterior, rostral and lateral to vestibular nerve. Two different variation of AICA was observed:in one kind AICA looped outside or near the IAM (6 cases) and in another AICA looped inside the IAM (4 cases). AICA originated from basilar artery in all our cases. In few cadavers, it was possible to see the thin nervus intermedius.The result of measurement of different bony landmarks that are exposed in retrosigmoid approach are as follows:distance from anterior edge of porus acousticus internus to Meckel's cave was 8.64±1.54mm (range 5.72-11.88mm) and nerve segment of jugular foramen (pars nervosa) was 5.82±1.28mm(range 4.3-9.24mm) height and length of porus acousticus internus was 5.34±0.71 (range 4.1-6.99mm) and 7.94±1.21mm(range 5.66-10.04mm)respectively; distance from sigmoid and transverse sulcus junction to posterior edge of porus acousticus internus was 38.97±2.89mm(range33.87-44.66mm);length of internal auditory canal in anterior, posterior and superior edges are 12.86±1.67(range10.22-15.58mm),8.79±0.91mm(range7.0-10.52mm),10.84±1.54(range9.03-14.04mm) respectively. The relationship of asterion with transverse sulcus was 50% lies on the middle of transverse sulcus, superior 30% and inferior 20%. For all these measurements there was no significance difference observed between the mean distance of left and right side.As there was no any bony landmarks that defines the labyrinth structure in retrosigmoid approach in 2 samples we damaged the posterior-semicircular canal and common crus while drilling the posterior wall of IAM.As the drilling advances to the most lateral part of IAM the stages and the angle of visualization was found to be narrower. Cutting a part of cerebellum and expanding the craniotomy medially gave little better exposure of the lateral part.3. Middle fossa approach provided the better exposure of the most lateral part of the IAC. IAC is exposed extradurally.The facial nerve is approached from the front thus, it lie under direct visualization during dissection. Retraction of temporal lobe exposes the different important structures and landmarks such as, foramen spinosum and middle meningeal artery, foramen ovale, three branches of trigeminal nerve, GSPN, arcuate eminence, facial hiatus, superior petrosal ridge and tegmen. Of the 6 segments of facial nerve; cisternal, meatal, labyrinthine and tympanic segments are exposed via MF approach. The corridor of facial nerve is narrower at the labyrinthine segment. Facial nerve was located anteriorily and superiorly and SVN lies posteriorly separated by Bill's bar. Cochlear nerve was located just below the facial nerve and IVN just below the SVN, separated by horizontal crest. Orientation of VII and VIIIth nerve was evident after the exposure of Bill's bar and horizontal crest. The looped branch of AICA and its labyrinth branch were also evident in some cases. Geniculate ganglion which lies at the lateral end of IAC appears as a bulbulous enlargement. GSPN passed through a small hiatus into the middle fossa before coursing parallel to the petrous ridge of the temporal bone and entering the foramen laeerum lateral to mandibular segment of facial trigeminal. Trigeminal nerve and all its three division converging from the Gasserian ganglion were also well exposed. The localization of medial end of the IAM was quite difficult to initiate the drilling. Drilling at the lateral wall was quite difficult as the working corridor was very small which put the nerves at risk. Risk to the cochlea was high while drilling IAM at the lateral end.The result of measurement of different bony landmarks that are exposed during middle fossa approach is as follows:distance from foramen spinosum to the foramen ovale is 2.95±0.82mm (Rangel.34-4.82mm, P≥0.833) arcuate eminence is 23.8±2.62mm (range 19.57-27.99mm, P≥0.599), facial hiatus is 11.26±2.54 (8.05-15.81mm), orifice of carotid canal is 15.40±1.23mm(13.5-17.87mm), suprameatal spine(spine of Henle) is 32.26±1.90mm(range 29.04-35.99mm); distance between foramen ovale to foramen rotundum is 11.75±2.68mm (7.55-17.72mm); distance from arcuate eminence to suprameatal spine(spine of Henle) is 23±3.13 mm (15.08-31.7mm); distance from facial hiatus to anterior edge of porus acousticus internus is 13.86±1.83mm(10.01-17.52mm).The t-test revealed to significance difference between the left and right side in all measurements. For all these measurements there was no significance difference observed between the mean distance of left and right side.4. Translabyrinthine approach provides the exposure of the CPA anterior to the sigmoid sinus. Lateral aspect of the pons, ventral surface of lateral cerebellar hemisphere and cranial nervesⅤ,Ⅶ,Ⅶ,Ⅷand vascular structures:AICA, SCA are well exposed through this approach. Visibility and the access to the lower cranial nerves are not satisfactory in this approach. Facial nerve lies deeper in the IAC and vestibular nerve is placed laterally, thus the vestibular nerve is encountered initially during IAC dissection. This approach requires extensive removal of the mastoid bone, labyrinth and cochlear structure thus the identification of bony landmarks are important to perform stepwise drilling. The important landmarks:suprameatal spine, external orifice of ear, mastoid process and tip, groove for diagastric muscle, asterion, stylomastoid foramen, McEwen's triangle, structures of inner ear and ossicels were also well exposed. The horizontal semicircular canal serves as an important landmark for the identification of facial nerve which lies just medial to it. The cisternal, meatal, labyrinth, tympanic and mastoid segment of the facial nerve along with its temporal division chorda tympani nerve was also exposed. Facial nerve is identified earlier and is followed putting the nerve under visualization throughout the surgery, thus reducing its injury. Jugular bulb, middle dura plate and sigmoid sinus form the Trautman's triangle and it overlies the dura lateral to CPA. Being an unfamiliar approach it was difficult to locate certain landmarks and to perform drilling process precisely, thus the time required for performing this approach is longer than other 2 approaches.Conclusion:l.The dissection of cadaveric head as a part of training to residents provides better knowledge in understanding the 3-D microanatomy of the relative structures. The information and the explanation achieved from snapshots helped to understand the microsurgical anatomy of the structures that are exposed during VS surgery via 3 approaches. Together, the whole process of dissection helped to develop microsurgical skills and drilling methods. The head was preserved for long duration which made it difficult to retract and study the underlying structures. Other technical limitation like proper way of preparation, injection of dye under constant pressure and dissection under proper guidance were the other causes for moderate quality snapshots. The more fresh the cadaver head it is, there is a better chance of preparation of vascular tree which will eventually lead to better dissection and good outcome. The age of the cadaver, proper fixation and positioning of the head, availability of standard micro-dissection instruments, accurate formulation of the chemicals while preparation and mixing them stepwise, the knowledge of micro-neuroanatomy of the resident together with the skills of using micro-instrument, good quality camera with expertise in photography are all the factors that will lead to the standard quality of research. A detailed knowledge of the complex relationship of the tumor to cranial nerves, cerebellar arteries, and brain structures to the VS is a prerequisite for optimizing the outcome of surgery.2. Retrosigmoid approach was observed to be best and more familiar approach among three approaches. Retrosigmoid approach together with drilling of the bony roof of IAM provides the sufficient exposure of the CPA, the contents of IAC (VII and VIII nerves, AICA) without destructing the labyrinth and cochlea as compared with other 2 approaches. VII and VIII nerve can be visualized almost in a full length till the fundus through retrosigmoid approach. As vestibular schwannoma can arise anywhere within this length, dissection of the tumor can be carried out from the origin keeping the nerves under direct visual control with increased safety during dissection from the brain stem and lower cranial nerves. There is a possibility to preserve facial function and hearing even in large VS as the facial and cochlear cranial nerves can be identified clearly in their proximal (close to the brain stem) as well as lateral part (in the IAC) thus increasing the chances for their preservation. There are still some surgeons who do not prefer to open the IAC or open a small length and rather try to pull the tumor from the meatal wall resulting in residual tumor and neurovascular injury. Retrosigmoid approach without drilling IAM does not provide enough full length exposure of theⅦ,Ⅷnerves and AICA and the origin of the tumor from vestibular nerve within the canal cannot be identified. The current mode of treatment of VSs is radiosurgery, observation, surgical removal by one of the approach retrosigmoid, middle fossa and translabrynthine approach or the staged treatment. However, complete surgical resection of tumor via suboccipital retrosigmoid approach with preservation of facial nerve and cochlear nerve in possible ways has been the best methods of treatment of VSs. The same modalities have been exclusively practiced in our center.The surgical landmarks of retrosigmoid approach can be classified as 3 levels:The first level of surgical landmarks includes:mastoid process and tip, external occipital protuberance, superior nuchal line. The second levels of surgical landmarks are: asterion, the groove of the digastrics, Henry space, transverse sinus, sigmoid sinus. The third level:glossopharyngeal nerve, jugular foramen, porus acousticus internus, AICA, foramen of Luschka and its choroid plexus, and trigeminal nerve3. Middle fossa approach provides the better exposure of the lateral part of I AM, and failed to expose the structures of CPA, so this approach is ideal for removal of small laterally placed vestibular schwannomas (intracanalicular tumor). However, if it is combined with different subtemporal approaches and Kawase approach which is also called extended middle fossa approach, the pathology of prepontine area and CPA, clivus, medial cavernous sinus Meckel's cave and trigeminal nerve till the brainstem, the pathology related to the basilar and SCA artery can be dealt with wide exposure. Facial nerve lies in front of the surgeon when the tumor is approached via this route, so the risk of damage to facial nerve is very less, and dissection can be performed putting the nerve under direct visualization. However, if the tumor lies below the facial nerve then it needs more manipulation and retraction leading to the injury of facial nerve. The risk of injury to cochlea and labyrinth is very less, so hearing is not affected in this approach. This approach needs the retraction of temporal lobe, which might lead to post-op seizure and ischemia due to the injury to neurovascular structure of temporal lobe.The most important part of this approach is to have the clear anatomical knowledge of the middle fossa floor, courses of facial nerve and the different landmarks, that guide to carry this approach smoothly. The surgical landmarks can be classified under 3 levels. The first level:zygomatic arch, spine of Henle, external auditory canal. The second level:foramen spinosum, foramen ovale, facial hiatus, GSPN, arcuate eminence, trigeminal ganglion, and superior petrous ridge. And the third level includes facial nerve, geniculate ganglion, vertical crest and transverse crest. 4. Translabyrinthine approach also provides wide exposure of the CP angle to most lateral part of the internal meatus; tumor of any size can be approached via translabyrinthine. Facial nerve is identified earlier in this approach thus performing the surgery under direct visualization reduces its injury. There are certain landmarks that facilitate the safe dissection and identification of facial nerve which in needed to be considered and followed during surgery. However, the exposure of long segment of facial nerve buried inside the mastoid bone requires meticulous drilling, sometimes leading to injury. As the labyrinth structure is completely distorted in this approach, hearing cannot be preserved. So, this approach is not recommended for hearing preservation. Being an unfamiliar approach to neurosurgeon, it requires great understanding of the micro-anatomy of the temporal bone, bony labyrinth and inner ear to perform this approach which can be practiced in the dissection lab. The involvement of ENT surgeons while carrying out this approach leads to better outcome.Levels of surgical landmarks that facilitate the translabyrinthine approach are can be grouped under 3 levels:The first level of surgical landmarks is mastoid process and tip, spine of Henle, McEwen's triangle. The second level includes facial canal, posterior semicircular canal, lateral semicircular canal, sigmoid sinus, transverse sinus, Trautman's triangle. And the third level is IAM, Bill's bar, transverse crest, AICA.5. Bony landmarks of skull base are always in a constant relationship with the corresponding structures; the knowledge of which keeps the surgeon under orientation while performing surgery of skull bases. It also provides the information about the distance of certain structures from fixed landmarks, and guides the surgeon in performing safe dissection/procedure throughout the surgery. It gives the information like the extent of craniotomy and retraction to be made; distance to be explored to reach certain structures, distance between the structures so as not to injure them during drilling and dissections process.