This review describes the indications and techniques for the retrosigmoid approach for vestibular schwannoma, as performed by the skull base surgery group at the University of Miami. The authors present background of the retrosigmoid approach, surgical steps, and essential “technical pearls” to address complication avoidance, resulting from their expertise with this surgery.
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The retrosigmoid or lateral suboccipital approach is the most commonly used approach for vestibular schwannoma; indicated for tumors of all sizes, particularly when hearing preservation is the goal.
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The excellent exposure of the brainstem and cranial nerves via the retrosigmoid approach permits complete excision of small-to-medium-sized tumors when hearing preservation is the goal and safe resection of larger tumors that compress the brainstem and adjacent neurovascular structures.
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Expertise in retrosigmoid surgery is very much experience dependent, as one incrementally learns new technical pearls in complication avoidance.
As in any other discipline, many of today’s accomplishments in the treatment of acoustic neuromas owe their existence to the brilliance (or good fortune) of the many pioneers who came before and paved an ever more refined path. The year 1777 seems to be the accepted time stamp that marks the recognition by Sandifort during an autopsy of the entity of acoustic neuroma. Fast-forward to 1890, the year that von Bergmann earned the distinct honor of attempting to become the first to resect surgically an acoustic neuroma. The patient sadly succumbed before von Bergmann’s localizing the tumor.
Early surgical approaches
In spite of reports by many authors, most today would recognize Krause as the architect of the suboccipital approach. It is quite amazing to realize that an early version of the “translabyrinthine” approach was suggested by Panse in 1904 ; the tireless debate between proponents of both approaches has been raging ever since. Cushing criticized the translabyrinthine approach for its narrow confines, proximity of vascular structures, risk of CSF leak, and unsuitability for large tumors. Remarkably, a hundred years later, many modern neurosurgeons have similar reservations.
The advent of ventriculography, pneumoencephalography and angiography improved the diagnostic accuracy of acoustic neuromas and Cairns is credited with the first successful total resection of an acoustic neuroma with facial nerve preservation in 1931. While Cushing had advocated a bilateral suboccipital craniotomy in all cases to prevent cerebellar herniation, and intentional subtotal resection, Dandy much favored the small unilateral retrosigmoid flap and gross total resection, which he first reported in 1917. McKenzie, Alexander and Olivecrona modified his technique, and popularized the shift from prone to sitting position, a trend that remained significant for several decades, to be supplanted by the three-quarter prone and supine positions in more recent years.
Surgical microscope
The distinction of introducing the surgical microscope to the resection of acoustic neuromas, undoubtedly, the most influential technical advance in this 120 year history belongs to an otological surgeon, Bill House, although he used it to re-introduce successfully the transtemporal approaches (translabyrinthine and middle fossa).
Surgical microscope
The distinction of introducing the surgical microscope to the resection of acoustic neuromas, undoubtedly, the most influential technical advance in this 120 year history belongs to an otological surgeon, Bill House, although he used it to re-introduce successfully the transtemporal approaches (translabyrinthine and middle fossa).
Early surgical outcomes
Safety of surgery improved steadily through the century. Surgical mortality started in the 80% range in the early days, and dropped to below 10% in House’s hands in the 1960s, and is today below 1%. Similarly, anatomic facial nerve preservation rates started at 0% and today are virtually 100%.
Indications
Treatment options for acoustic neuromas include observation, radiosurgery, and microsurgery. The factors involved in decision-making are discussed in this issue in the article by Sheth et al. Once a decision has been made to operate, the surgeon must choose from among three main surgical approaches:
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Middle fossa
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Translabyrinthine
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Retrosigmoid.
The choice of surgical approach is dependent on several factors including:
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Patient’s age
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Paitent’s hearing status
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Tumor size
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Surgeon preference.
The middle fossa approach is used for small tumors (≈ 1 cm) that are primarily intracanalicular and when hearing preservation is the goal.
The translabyrinthine approach is generally used for tumors of any size when hearing preservation is not an issue.
The retrosigmoid or lateral suboccipital approach is the most commonly used approach and is also indicated for tumors of all sizes, particularly when hearing preservation is the goal. The retrosigmoid approach offers an excellent exposure of the brainstem and cranial nerves IV through XII. This permits complete excision of small-to-medium-sized tumors when hearing preservation is the goal and safe resection of larger tumors that compress the brainstem and adjacent neurovascular structures.
Preoperative assessment
Preoperative evaluation includes:
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Standard preoperative labs
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Electrocardiogram
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Chest radiograph
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Assessment of the patient’s general health.
It is important to obtain a preoperative audiogram with pure tones and speech discrimination to ascertain baseline hearing function.
Magnetic resonance imaging with and without gadolinium administration including IAC views provides vital information regarding the soft tissue anatomy.
The tumor size, including its rostro-caudal and intracanalicular extensions, is determined.
T2-weighted images provide details regarding tumor consistency (hyperintense lesions are suggestive of softer and more suckable tumors), associated edema, and the plane between the tumor and surrounding structures.
The location and size of the sigmoid sinus and jugular bulb should also be assessed, particularly to be aware of a high jugular bulb that may hinder drilling of the caudal posterior lip of the IAC.
Computed tomography of the temporal bone provides information regarding aeration of the mastoid process and the location of the posterior semicircular canal in relation to the IAC.
Anesthesia for vestibular schwannoma
The anesthetic technique is similar to that of other brain tumors:
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After anesthesia is induced an arterial line is obtained and an indwelling Foley catheter inserted.
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All pressure points are padded.
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Sequential compression devices are placed to prevent deep venous thrombosis.
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Preoperative antibiotics are administered one-half hour before skin incision and 10 mg of dexamethasone is given at the beginning of the operation. Antibiotics are re-dosed as necessary based on the length of the procedure.
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At the time of skin incision, a 20% solution of mannitol in a dosage of 0.5 to 1 mg/kg body weight is administered intravenously over a period of 15 to 20 minutes.
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The patient is hyperventilated to obtain an end-tidal CO 2 of 28 mmHg.
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Although a lumbar drain can be inserted, we have found that, in most cases, adequate brain relaxation can be obtained with corticosteroids, hyperventilation, and cisterna magna drainage.
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It is important not to give paralytics to allow monitoring of facial nerve function. However, once the tumor has been removed, inhalation anesthetics may be administered to allow a rapid and smooth awakening from anesthesia.
Monitoring during surgery
Neurophysiologic monitoring during acoustic neuroma surgery allows continuous assessment of brain stem, facial nerve, and auditory nerve function throughout the procedure. Facial nerve function is monitored by inserting EMG electrodes into the orbicularis oculi and oris muscles and placing a motion sensor on the patient’s face. Brainstem auditory evoked potentials and electrocochleography using a transtympanic electrode allows monitoring of auditory function in cases where hearing preservation is attempted. We have been very satisfied with near-field direct cochlear nerve recordings as a reliable measure of cochlear nerve functioning during dissection. More details about intraoperative monitoring are discussed in another Chapter.
Surgical technique
Position
The patient can be positioned sitting, supine, or in a lateral decubitus.
In the sitting position:
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The patient is placed upright on several cushions and hips and knees are bent approximately 90°.
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The head is turned 30° toward the side of the lesion, flexed slightly in an antero-posterior plane, and fixed in a 3-pin Mayfield head clamp.
The sitting position facilitates drainage of blood and cerebrospinal fluid out of the wound and thus provides a bloodless and clear operative field. However, it carries a higher risk of air embolism and spinal cord ischemia and, in our opinion is more cumbersome and exhausting for the surgeon’s arms as compared with the other positions. For these reasons, we prefer either the supine or lateral decubitus position based on the patient’s body habitus.
We used the supine position in our earlier experience, but more recently have adopted the lateral decubitus position exclusively.
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In the supine position, the shoulder that is ipsilateral to the tumor is elevated with a gel roll to allow turning of the head without excessive tension on the neck.
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In the lateral position, the patient is placed with the side of the lesion upward while the dependant arm hangs off the bed and is cradled in a padded sling between the table and edge of the Mayfield head holder.
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With the lateral position, it is important to retract the patient’s superior shoulder toward the patient’s feet using adhesive tape, taking special care not to create a brachial plexus stretch injury.
Regardless of the position used, the head is fixed in a 3-pin Mayfield clamp and positioned with 3 movements:
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Contralateral rotation so that the temple is parallel to the floor.
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Contralateral bending so that the vertex is slightly tilted toward the floor.
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Flexion in the anteroposterior plane until the chin is two finger breadths from the clavicle to open the cervical-suboccipital angle. Over-flexion could compress the jugular veins.
The operating table is placed in slight reverse Trendelenberg so as to position the patient’s head above the level of the heart to reduce cerebral venous congestion.
Incision and Exposure
The retroauricular region of the scalp is shaved. Several surface landmarks can be identified and marked to plan the skin incision. We use 3 reference points:
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External auditory canal (EAC)
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Inion
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Tip of the mastoid process.
The course of the sigmoid sinus is represented by the posterior border of the mastoid process. The EAC-inion line is generally parallel to and slightly lower than the lower border of the transverse sinus.
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We mark a point along the EAC-inion line at which we start our drilling to expose the transverse sigmoid junction. This point is placed 4 cm and 4.5 cm from the external auditory canal in females and males, respectively.
Technical pearl
Although the asterion may be used to approximate the transverse-sigmoid junction, it is frequently difficult to accurately identify intraoperatively. We have found that these points are useful safe areas to start the drilling to expose the venous sinuses. They are almost always medio-caudal to the actual TS junction.
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The size of the planned craniotomy is approximately 3.0 to 3.5 cm, immediately posterior to the sigmoid sinus.
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A C-shaped retroauricular incision is then marked extending approximately 2 cm below the tip of the mastoid to a point approximately 2 cm above the pinna. It is crucial that the apex of the C-shaped incision extend beyond the medial edge of the planned craniotomy because it is difficult to retract the skin posteriorly. It is for this reason that we do not use a linear incision, which generally results in the skin edges lifting up with traction and compromising the medial exposure.
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The scalp is prepped and draped in a standard fashion and the planned incision infiltrated with 0.5% lidocaine with epinephrine 1:200,000.
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The scalp incision is elevated sparing the pericranium above the superior nuchal line and the fascia overlying the suboccipital musculature.
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The suboccipital fascia and muscles are then incised in a T-shaped fashion.
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The horizontal limb is made leaving a small muscular cuff attached to the superior nuchal line.
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The vertical limb is carried down caudally until the occipital bone changes orientation from a horizontal to a more vertical plane (floor of the posterior fossa).
Technical pearl
Caution must be exercised during the lower end of the exposure. Overzealous use of the Bovie in the region between the occipital bone and C1 may result in a vertebral artery injury. In this region it is only necessary to incise the superficial muscle fibers. Adequate exposure can then be achieved with several fishhooks retracting in an upward direction, a maneuver that creates significant depth.
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Once the muscular incisions have been made, the muscles are elevated in a subperiosteal fashion to uncover the suboccipital bone and the base of the mastoid process including the digastric groove. The exposure is maintained by retracting the scalp flap and suboccipital muscles with fish hooks.
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The transverse and sigmoid sinuses are then exposed using a combination of 5 mm round cutting and diamond burrs.
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An initial burr hole is placed just medio-caudal to the presumed location of the transverse-sigmoid junction based on the surface landmarks described above.
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Once the dura is identified, the drilling is continued in a superolateral direction until the transverse-sigmoid junction is identified.
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Drilling is then continued in a caudal direction to expose the edge of the sigmoid sinus. It should be noted that the outer wall of the sigmoid sinus is curved and more adherent to the overlying bone than the transverse sinus.
Technical pearl
Bleeding from the mastoid emissary vein is frequently encountered during drilling. This may be controlled with bone wax until its bony canal has been drilled, at which point it may be coagulated and sectioned. The emissary vein may also be used as a landmark and followed to the sinus.
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Once the sigmoid sinus has been exposed, the dura is carefully separated from the overlying bone in the region of the transverse sinus and cerebellum.
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A craniotomy is then performed extending from the transverse sinus to the caudal level of the sigmoid sinus where it curves anteriorly. Frequently, it is necessary to remove additional bone inferiorly beyond the craniotomy flap to expose the floor of the posterior fossa to gain access to the lateral cerebellomedullary cistern. Any mastoid air cells that have been entered during the drilling are occluded now with bone wax.
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At this stage of the procedure the operative microscope is used.
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The dura is opened in a C-shaped fashion based medially.
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Initially only a small horizontal dural opening at the caudal portion of the exposure is made to avoid outward herniation of the cerebellum. It is helpful here for the surgeon to position himself at the patient’s vertex, looking caudally with the microscope.
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A 0.25 × 3 in telfa is placed over the cerebellum and the latter is depressed to expose the lateral recess of the cisterna magna.
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The arachnoid membrane is then sharply incised under direct vision and cerebrospinal fluid is allowed to drain for several minutes.
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Once the cerebellum is adequately relaxed, the remaining dura is opened approximately 5 mm parallel to the sigmoid and transverse sinuses, and the dural flap placed under tension medially to avoid it shrinking during the surgery. The dural edge along the sigmoid and transverse sinuses is also retracted with 4–0 neurolon sutures.
Tumor Exposure and Removal
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The midportion of the cerebellum is covered with telfa patties and gently mobilized to expose the cerebellopontine angle. A self-retaining retractor is never used, no matter how small or large the tumor is.
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The next step is to open the arachnoid over the inferior tumor pole and the lower cranial nerves and establish the appropriate plane of dissection.
Technical pearls
Acoustic neuromas are invested in two arachnoidal layers. The first of these layers represents the peripheral arachnoidal layer of the posterior fossa whereas the second layer comprises the arachnoid membrane that envelops the tumor itself and was invaginated by it. Ideally the dissection should be performed between these two layers so that cranial nerves and major arteries can be shielded from the dissection. However, in the event that there is a poor arachnoidal plane, it may be necessary to carry out the dissection between the tumor and the deep arachnoidal layer.
Unless the tumor is very large and occupies all available space, the initial view reveals the lateral portion of the tumor and the anterior inferior cerebellar artery with its subarcuate branch. The lower cranial nerves and posterior inferior cerebellar artery are identified caudally and the trigeminal and petrosal vein rostrally.
Occasionally, with small-to-medium-sized tumors the eighth nerve complex may be identified entering the inferomedial side of the tumor with its fibers splayed over the posterior surface of the capsule. In such cases the vestibular nerve remnant proximal to the tumor is divided, enabling dissection of the tumor from the cochlear and facial nerve.
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