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Translabyrinthine and Transtemporal Approaches to Posterior Cranial Fossa Lesions
Acknowledgment
Portions of this chapter are reproduced with permission from Brodkey J, Vrionis FD: Surgical approaches through the temporal bone. In Robertson JT, Coakham H, Robertson JH (eds): Cranial Base Surgery: Management, Complications and Outcome . Edinburgh, Churchill Livingstone, 2000.
The authors are indebted to Bo Jespersen, MD, who provided all illustrations for the translabyrinthine approach.
Traditional approaches to the posterior cranial fossa do not permit direct access to complex lesions of deep-seated lesions of the lateral skull base, cerebellopontine angle (CPA), or clivus. To circumvent brain retraction and allow for complete resection, approaches have been developed that position the dissection both laterally and anteriorly to the brainstem and cerebellum. All of these skull base approaches are combinations and variations of transtemporal bone routes. The temporal bone has a complex anatomy, and unlike craniotomies performed elsewhere, entry to the posterior fossa through the temporal bone poses special challenges for the surgeon to preserve the internal carotid artery (ICA), sigmoid sinus (SS), cranial nerves VII and VIII, and specialized structures for hearing and balance. Despite the widely varied nomenclature, often only subtle differences exist among these approaches. It is imperative that the location, type, and extent of the lesion dictate the type of the approach. Tailored approaches to the lesion instead of standard ones are recommended for minimal disruption of normal structures. In this respect, transtemporal approaches represent an anatomic continuum of temporal bone dissection, with frequent overlaps and minor discrepancies or differences among the various approaches. The complicated nomenclature has arisen because skull base tumors tend to extend into different anatomic compartments, often necessitating combined approaches. Because of the overlap of neurosurgery and otology in this area, collaboration of neurosurgeons and otologists is mandatory.
In this chapter we will have a special focus on the translabyrinthine approach. The other transtemporal approaches are divided into three sections: (1) anterior transpetrosal approaches, describing anterior approaches within the temporal bone through the middle cranial fossa; (2) posterior transpetrosal approaches, for those that are situated more posteriorly through the mastoid process; and (3) combined approaches. We define the external auditory canal (EAC) as the dividing structure between the anterior and the posterior approaches. We divide each approach into sections, giving a brief historical perspective, indications, surgical approach, complications, and disadvantages.
Translabyrinthine Approach
Advantages of the Translabyrinthine Approach
The most obvious advantage of the translabyrinthine route is that it offers a direct approach to the CPA with minimal cerebellar retraction. The tumor is lifted away from the brain stem, avoiding pressure on the brain stem and cerebellum.
Traditionally, the translabyrinthine approach is utilized to resect small tumors. However, larger tumors may also be approached by the translabyrinthine route. In large and giant tumors, it is a significant advantage to be able to go directly to the center of the tumor; after debulking the center of the tumor, the neoplasm collapses and is displaced toward the opening by the surrounding brain structure.
The procedure offers excellent exposure of the lateral end of the internal auditory meatus and allows identification of the facial nerve as it enters the fallopian canal. This identification ensures complete tumor removal from that area and the best chance to preserve the facial nerve.
Disadvantages of the Translabyrinthine Approach
The translabyrinthine procedure destroys the labyrinth and, as a consequence, hearing. This approach is not used if preservation of hearing is attempted. In large tumors the hearing is often poor or may be sacrificed because the chances of preserving hearing are poor.
If the patient has had active otitis media in the past, the approach involves crossing a potentially infected field, and alternative exposure should be considered. In the case of a mastoid cavity, a total obliteration with blind sac closure of the EAC should be performed and healed before the translabyrinthine approach can be done. Finally, the procedure is generally more time consuming than the suboccipital approach, which must be considered if a limited duration of the operation is desirable.
Surgical Anatomy
The bone opening for the translabyrinthine approach is done in the mastoid part of the temporal bone ( Fig. 36.1 ). The mastoid is filled with air cells, and the air cells are connected to the middle ear through the tympanic antrum. In the translabyrinthine approach, the bone is removed between the SS and the external ear canal. The SS is located in the sigmoid sulcus of the temporal bone. From the posterior aspect of the SS, emissary veins run through the mastoid foramen to subgaleal veins.
Removing the air cells creates a space that is bounded posteriorly by the wall of the sigmoid sulcus, superiorly by the tegmen tympani, and anteriorly by the prominence of the lateral semicircular canal. The genu of the facial canal is just inferior to the lateral semicircular canal, and it continues inferiorly to emerge below the skull base at the stylomastoid foramen ( Fig. 36.2 ). The lateral semicircular canal is an important landmark for the location of the entire labyrinth. After removing all three semicircular canals, the vestibule is revealed, which harbors the soft-tissue part of the labyrinth, utricle, and saccule. Through the aperture of the vestibular aqueduct runs the endolymphatic duct that connects the utricle to the endolymphatic sac. The internal auditory canal (IAC) contains four separate nerves. Located laterally are the superior and inferior vestibular nerves separated at the fundus by a bony crest called the transverse crest. Anterior to the superior vestibular nerve, the facial nerve enters the fallopian canal. Laterally, the facial nerve is separated from the superior vestibular nerve by a small vertical bony septum called the vertical crest or Bill bar ( Fig. 36.3 ).
Most vestibular schwannomas arise from one of the vestibular nerves in the IAC. The facial nerve is often displaced in the IAC, and its location may vary. The nerve can always be identified laterally in the IAC.
After maximal translabyrinthine bone removal and opening of the dura, the CPA with its nerves and vessels is seen. Superiorly, the exit of cranial nerve V is seen on the pontine surface near the cerebellum. The exits of cranial nerves VI, VII, and VIII are located on a vertical line on the medulla oblongata near the crossing to the pons. The exit of the cranial nerve VIII is just anterior and superior to the flocculus.
The blood vessels in the CPA display greater variability than do the nerves. The posterior–inferior cerebellar artery emerges from the intracranial segment of the vertebral artery. Loop formations of this artery are often seen to extend cranially to the level of cranial nerves VIII and IX, and in these cases, it may be seen using the translabyrinthine approach. The anterior–inferior cerebellar artery extends from the basilar artery, and in most cases, it forms a loop that protrudes against or into the IAC. The labyrinthine and subarcuate arteries extend from the loop of the anterior–inferior cerebellar artery.
Preparation for Surgery
The patient is placed in a supine position on the operating table. The patient’s head is turned toward the opposite side and maintained in position with a Sugita or Mayfield head frame.
Continuous electrophysiologic monitoring of facial nerve function is performed during the operation. To accomplish this, electrodes are placed in the frontal and oral orbicular muscles. With the patient in a supine position two surgeons can be sitting on each side of the patient’s head. This setup enables both surgeons to be in a comfortable sitting position with a direct view in the microscope and the use of a four-hand technique. The lead surgeon sits on the same side as the tumor. In left-sided tumors, the drill is placed between the two surgeons, and in right-sided tumors, the drill is placed between the scrub nurse and the surgeon on the right side ( Fig. 36.4 ).
The surgical view of the CPA is limited posteriorly by the SS and anteriorly by the horizontal part of the facial nerve.
Surgical Procedure
The use of lumbar drain to relieve hydrocephalus or prevent surgically induced hydrocephalus, is not necessary in translabyrinthine approach with adequate drainage cerebrospinal fluid (CSF) from cistern magna. The skin incision is made using the cutting cautery and starts at the upper edge of the helix, superior to the linea temporalis; it continues 4 to 5 cm posteriorly, turns inferiorly, and ends near the tip of the mastoid process ( Fig. 36.5 ).
Mastoidectomy
An extended mastoidectomy is performed with removal of bone over the SS and the middle cranial fossa using a large cutting drill (see Fig. 36.5 ). In cases with an anteriorly placed SS or large tumors, it is necessary to remove the bone behind the SS. , The anterior margin for cortical bone removal is just behind the external ear canal. The opening is gradually widened backward to the SS and upward to the dura in the middle cranial fossa. Removal of bone over the SS must be done carefully with a diamond drill to avoid tears in the sinus.
There are several methods to skeletonize the SS, including the eggshell method, creation of Bill’s island of bone, and total bone removal. The aim of all three methods is to allow compression of the sinus without direct injury to it.
With blunt dissection, the adjacent dura in the middle and posterior cranial fossa is loosened, and the remaining bone may be removed by either bone punches or drilling.
Labyrinthectomy
As soon as the mastoid cortical bone has been removed and the SS and the middle fossa dura have been outlined, the operating microscope is used. The facial nerve is an important landmark, and its position must be established early in the surgical dissection. After skeletonizing the middle fossa dura, the antrum is opened, and the compact bone of the labyrinth is visualized.
It is essential to open the antrum and to identify the lateral semicircular canal ( Fig. 36.6 ). This canal is a main landmark, and once the positions of this canal and of the antrum are known, the three-dimensional anatomy of the facial nerve is known. After identification of the facial nerve, the labyrinthectomy is performed. The bone in the sinodural angle is removed, followed by opening along the superior petrosal sinus (SPS) until the labyrinthine bone is encountered. The lateral semicircular canal is drilled away until the ampulla is reached anteriorly. Then the posterior and superior semicircular canals (SSCs) are identified and removed to their entrance in the vestibule. After opening the vestibule, the facial nerve is skeletonized from the genu inferiorly to near the stylomastoid foramen, leaving a thin, eggshell bone on the nerve.
After removal of all semicircular canals, the labyrinthectomy is completed, and the vestibule is opened. The endolymphatic duct must be excised from the endolymphatic sac on the posterior fossa dura. The vestibule is removed, and the cribriform area in the saccule marks the most lateral extent of the internal auditory meatus. In the center of the labyrinth, the subarcuate artery is located, and it is usually bleeding when it is opened by the drill.
Internal Auditory Canal Dissection
The dura of the IAC is identified posteriorly, where it continues as the dura of the posterior cranial fossa. The dura at the opening of the canal can be loosened from the bone with slightly bent sharp dissectors. A more than 180-degree arc of bone around the canal is gently removed with a diamond drill. All bone between the internal meatus and the jugular bulb is removed. With a low-riding jugular bulb, all bone removal necessary for tumor removal can be performed without compromising it. A high-riding jugular bulb can be seen as a bluish spot in the bone after removing the ampulla of the posterior semicircular canal (PSC). The surgeon should always be aware of the blue color of the jugular bulb when drilling medial to the facial nerve and inferior to the PSC. All bone covering the jugular bulb must be removed, so that the jugular bulb can be compressed and allow further removal of bone from the inferior part of the porus. Bone is removed until the cochlear aqueduct is identified. It is an important landmark because it identifies the location of cranial nerves IX, X, and XI in the neural compartment of the jugular foramen. Bone dissection should be confined to the area superior to the cochlear aqueduct to avoid injury to these nerves. After removal of bone on the inferior part of the IAC, the dissection is carried out on the superior and anterior parts.
The facial nerve often underlies the dura along the anterior–superior aspect of the IAC, and extreme care must be taken not to allow the bur to slip into the canal, as it is especially vulnerable at this point. The lateral end of the IAC is divided by the transverse crest in an inferior and a superior compartment ( Fig. 36.7 ). The bone around the inferior compartment and superior compartment can be drilled to allow identification of a bar of bone (Bill bar), which separates the superior vestibular nerve from the facial nerve. The bone is removed at the porus and the medial part of the IAC first; the more difficult lateral part is left until last, when most of the bone removal has been completed.
All bone between the middle fossa dura and the IAC must be removed. With the visualization of the most lateral end of the IAC, the bone work is completed. Up to this point, all dissection has been extradural and the morbidity of the approach consequently low.
Dural Opening
The dural incision is started superiorly in the sinodural angle near the SS continuing down to the porus (see Fig. 36.7 ). Care is taken to avoid vessels on the surface of the tumor. Around the porus, the dura often forms a distinct constriction ring that usually adheres to the surface of the tumor, and there are small vessels going from the dura to the tumor that can be coagulated safely. The surgeon must develop a plane between the tumor and cerebellum, because doing so separates the major vessels of the CPA from the tumor.
Tumor Removal
After opening the dura, the posterior part of the tumor is exposed. Rarely, the facial nerve may lie on the posterior surface of the tumor, and this surface must be carefully inspected for nerve bundles. In large tumors, it is essential to begin tumor removal with intracapsular debulking to reduce tumor size and subsequently develop the extracapsular dissection planes. In small tumors, the surgeon can readily identify the inferior and superior extracapsular dissection planes of the tumor. An ultrasonic aspirator or CO 2 laser is useful for debulking and removing the intracapsular tumor.
Identification of the Facial Nerve in the Fundus of the Internal Auditory Canal
During bone removal, the medial part of the fallopian canal is opened, which uncovers the labyrinthine segment of the facial nerve. The Bill bar separates the facial nerve anteriorly from the superior vestibular nerve. A fine hook is inserted lateral to the Bill bar and gently placed beneath the superior vestibular nerve and the Bill bar. The superior vestibular nerve can then be pulled out from its canal. Likewise, the two nerves inferior to the transverse crest (the inferior vestibular nerve and the cochlear nerve) are pulled out from their canals, along with the tumor ( Fig. 36.8 ). Positive identification of the facial nerve at the lateral end of the IAC is one of the principal advantages of the translabyrinthine approach.
Freeing the Tumor from the Facial Nerve in the Internal Auditory Canal
The arachnoid sheath completely surrounds the tumor, nerves, and vessels in the meatus, and the arachnoid strands that attach the facial nerve to the tumor may be divided with small hooks or fine microscissors. Usually, it is relatively easy to develop the dissection plane between the facial nerve and the tumor in the IAC, but difficulties typically arise at the porus. Around the entire circumference of the porus, dural adhesions to the tumor make dissection of the facial nerve from the tumor difficult. The exact position of the facial nerve in the porus must be established before the adhesions between the dura and the tumor are removed. Inferiorly, freeing of the tumor from the porus is simpler because damage to the cochlear nerve is insignificant. Superiorly, the facial nerve may be at risk. At times, it is difficult to isolate the facial nerve at the porus. In these cases, it is wise to carry out a partial tumor removal, identify the facial nerve medially, and follow the nerve laterally until the porus is reached. During this work, the surgeon must be careful not to push the tumor forward or medially, because stretching the facial nerve, especially at the porus level, can damage the nerve. Early mobilization of the tumor from the IAC has the advantage that the landmarks are well defined and are not obscured with blood, as tends to happen later in the surgical dissection.
Tumor Removal
The principle method for removal of large tumors is intracapsular gutting to reduce tumor bulk, followed by mobilization and removal of the adjacent capsule segment. The point of attack must be changed progressively, and as the limits of mobilization are reached at one point, the surgeon moves to another. Once the interior part of the tumor has been extensively removed, the capsule is displaced into the tumor space. Opening of the arachnoid layers and dissecting within these layers facilitates the isolation of the tumor. Semisharp dissectors and sometimes small cottonoids are used in the proper dissection plane to separate the tumor from the surrounding structures. The dissection is made from four directions: inferior, superior, medial, and lateral.
Inferior Dissection
Dissection at the inferior aspect of the tumor usually leads into the large cerebellomedullary cistern, which allows CSF to escape. This step improves the operative condition, and if any difficulties are encountered because of lack of room in the posterior fossa, draining the cerebellomedullary cistern as soon as possible is valuable.
On the inferior aspect of the tumor, it is possible to localize cranial nerves IX and X, which are best identified near the jugular foramen medial to the jugular bulb. These nerves must be freed from the tumor and isolated. Sometimes they are not well seen because they tend to lie around the corner of the opening. During manipulation of cranial nerves IX and X, changes in the pulse rate may occur. Stopping the manipulation restores the pulse rate.
The posterior–inferior cerebellar artery is at the inferior aspect of the tumor and must be carefully separated from the tumor capsule and preserved. The labyrinthine artery supplies branches to the facial nerve and should be preserved. Dissection inferior to the tumor continues until the brain stem is reached and is completed with removal of that portion of the capsule.
Superior Dissection
The facial nerve is normally located anteriorly to the tumor, but it is not unusual to find the facial nerve over the top of the tumor. If the precise location of the facial nerve is unknown, when starting the dissection at the superior aspect of the tumor, it is imperative to inspect the tumor surface carefully and to use the nerve stimulator to identify the nerve and avoid injury.
The petrosal vein and cranial nerve V are located in the superior aspect of the tumor. These structures must be identified and carefully separated from the tumor. The trigeminal nerve is a broad white structure running in the inferior–posterior direction. Near the brainstem, the nerve lies in close contact with the tumor capsule but is usually easy to separate from the tumor. Handling of the nerve must be avoided if recovery of sensory loss in the face is to be achieved. The petrosal vein and vein branches are stretched over the tumor and enter the SPS. Although it is often tolerated, the petrosal vein must be preserved to prevent venous infarction. The superior dissection is continued until the pons is reached and the attachment of the trigeminal nerve to the brain stem is visualized.
Medial Dissection
The medial dissection is the most difficult part of the tumor removal because of the risk of damaging the facial nerve or the pons. After the posterior part of the tumor is debulked, a small portion of tumor capsule is left attached to the cerebellum and the pons. The tumor capsule is lifted, and the proper cleavage plane is identified. In large tumors, the posterior pole may protrude far under the cerebellum and deep into the brain stem. This protrusion requires maximal rotation of the operating table toward the surgeon to visualize the dissection plane. The vessels are dissected away, and the branches that extend into the capsule are coagulated and divided. The capsule and tumor remnant are pushed anteriorly, and arachnoid and veins are dissected from the capsule. When the brain stem has been reached from all directions and the tumor has gradually been removed, the remaining small portion of tumor covers a part of the brain stem, facial nerve, and cochlear nerve.
Anterior Dissection and Final Tumor Removal
In small tumors, the main approach is from the anterior aspect. In these cases, the facial nerve can be easily located at the brain stem, and the precise position of the nerve is easy to ascertain early. Separating the tumor from the facial nerve is done from the porus and against the brain stem.
In large tumors, the dissection of the facial nerve is done in an anterior–posterior direction. The small portion of tumor that covers the facial nerve from the brain stem to the porus is the most difficult to remove ( Fig. 36.9 ). The anterior part of the tumor capsule usually contains a number of small vessels from arteries and veins, and dissection can easily provoke bleeding. Veins are often present around the facial nerve’s entry zone from the brain stem. Coagulation of the bleeding vessels is dangerous and must be precise to avoid facial nerve injury.
For several reasons, dissection on the anterior aspect of the tumor is troublesome and time consuming:
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The facial nerve is often stretched and spread out over the largest prominence of the tumor, which sometimes makes the nerve nearly invisible.
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The facial nerve is not protected by an epineurium and is vulnerable to injury.
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At the porus, the facial nerve is often embedded in the tumor capsule, causing problems when loosening the tumor.
Although complete tumor resection is the goal of the surgery, it is also acceptable to leave a small piece of tumor on the facial nerve in order to preserve function. Preoperative counseling and discussion with the patient are paramount, especially given the option of radiosurgery if the remnant enlarges in the future.
Care must be taken not to leave fragments of tumor on the nerve, because the dissection continues inside the tumor and obscures the location of the nerve, which may be lost.
Once the facial nerve has been separated from the tumor, the last bit of tumor is removed from the brain stem. The adhesions between the tumor and the brain stem are not usually dense, but if they are, clearing the brain stem of tumor demands particularly careful dissection. Once the tumor has been removed, the field is inspected and the facial nerve is examined with the stimulator. If a signal can be obtained by stimulating the nerve with a low stimulus at the brain stem, it is likely that the patient will have normal facial nerve function. The wound is then irrigated with saline, and all bleeding points are controlled. Absolute hemostasis is required and may be time consuming.
Facial Nerve Repair
If the nerve has been divided during operation, the surgeon may get an acceptable level of function by anastomosing the nerve ends, provided that the central stump can be found and isolated. If the nerve ends can reach each other, they are anastomosed end to end and fixed with a suture and fibrin glue. If a gap exists, a great auricular nerve or sural nerve graft is used to bridge the gap between the nerve ends.
Closure
An essential part of wound closure involves preventing rhinorrhea. CSF may escape through the middle ear and the eustachian tube. The antrum is packed with a small piece of fat. The dural opening and the mastoid cavity are filled with large pieces of adipose tissue taken from the abdomen. An artificial glue is injected around the adipose tissue. The muscle fascia/pericranium flap is closed watertight with absorbable sutures. The postauricular skin incision is closed in two layers with absorbable subcutaneous suture and nylon suture in the skin. A solid compression of the wound by gauze supported by a solid elastic bandage is used for 24 hours.
Postoperative Management and Complications
Surgery on large tumors in the posterior fossa is not without risks. Large tumors sometimes adhere strongly to the surface of the cerebellum and the brain stem. Dissection of the tumor from the surroundings may elicit cerebellar edema and infarction. Veins can often be coagulated, but sometimes packing is necessary, especially of the bulb of the SS and the petrosal vein. Hematomas or reactive edema of the cerebellum may necessitate immediate response. Preservation of the anterior inferior cerebellar artery (AICA) is paramount to avoid pontine strokes. Close observation of the patient is necessary in the initial postoperative phase by trained nurses in the intensive care unit. Acute hydrocephalus resulting from obstruction of CSF drainage from the ventricles may occur because of edema or hematoma at the level of the fourth ventricle.
Hematoma
Postoperative hematoma after a translabyrinthine approach is a rare but serious complication. This complication is the cause of most of the mortality seen after translabyrinthine removal of vestibular schwannomas. Even when excellent hemostasis appears to have been achieved, however, postoperative hemorrhage may occur, and it seems most likely among elderly patients and patients with large tumors. The hematoma is most likely to occur shortly after the operation but may be seen days after surgery. Loading the patient with an antifibrinolytic agent such as aminocaproic acid or tranexamic acid can frequently prevent this complication.
The clinical course is often insidious. If the patient does not regain consciousness after surgery or if the patient—after a period with normal sensorium—develops decreasing consciousness, a hematoma must be suspected. If the symptoms develop quickly, it may be necessary to open the wound immediately in the intensive care unit. If the symptoms develop more slowly, computed tomography (CT) can confirm the diagnosis. We always perform a control CT scan the day after surgery to avoid clinical deterioration or death because of slowly progressive hematomas.
Acute Hydrocephalus
Acute dilatation of the cerebral ventricles may be observed immediately postoperatively or in the following days. It is frequently caused by a postoperative hematoma. Failure to wake up or decreasing consciousness should arouse suspicion, and a CT scan should be obtained. If the complication is diagnosed soon after surgery, external ventricular drainage is undertaken. If the complication arises slowly and persists after surgery, a permanent CSF shunt may be considered.
Facial Paralysis
Facial paralysis is often difficult to detect immediately postoperatively, when full wakefulness and cooperation are absent and, for unknown reasons, eye closure may be present. If the face is paralyzed, eye care is of great importance. In the first days after surgery, the eye is covered with a protective shield and viscous eyedrops are used to prevent corneal abrasions. Shortly thereafter, a tarsorrhaphy is performed. The patient is warned to protect the eye and to wear protective spectacles. An ophthalmologist should be consulted.
At least 6 months of observation should be allowed before any further treatment is considered. If permanent facial paralysis exists, a faciohypoglossal (VII to XII) anastomosis is carried out after a delay of approximately 10 to 12 months.
Cerebrospinal Fluid Leakage
CSF leakage is a common complication after vestibular schwannoma surgery, and it is seen in about 5% of the cases. The leakage may occur either through the skin at the wound site or through the nose. Rhinorrhea is more common than leakage through the wound. The diagnosis of postoperative CSF rhinorrhea is often obvious within a few days of surgery, but its recognition may be delayed. Some patients report only a sensation of postnasal dripping or a salty taste in the mouth. In these cases, the diagnosis may be subtle. The patient should be tested in a head-down position for watery escape from the nose before discharge.
When rhinorrhea is diagnosed, a lumbar drain is inserted for 3 to 5 days; however, the success with CSF drainage is not as high as in the wound leakage, probably because the defect is located in the bone, which is not as easily overgrown as is a defect in soft tissue. If the lumbar drainage fails to close the defect, reoperation and resealing the communication to the middle ear and eustachian tube with muscle graft and bone wax in the communicating air cells is necessary.
CSF escaping through the postauricular wound may be prevented by meticulous closure of the wound followed by a tight head bandage. If wound leakage occurs, spinal drainage is often sufficient to solve the problem, and reoperation is rarely necessary.
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