Suboccipital Retrosigmoid Surgical Approach for Vestibular Schwannoma (Acoustic Neuroma)

Introduction

Cerebello-pontine angle (CPA) surgery for vestibular schwannomas (VSs) represents a complex surgical setting that requires adequate training, sufficient microsurgical skills and experience, and a meticulous preparation about patient’s anatomy for each case. The surgeon who approaches CPA surgery must be aware that this is a complex and challenging surgery and must be ready to manage and prevent complications.

The retrosigmoid approach is one of the most common surgical routes to the area of the CPA. It allows good visualization and resection of lesions involving that particular anatomical area of the posterior fossa without requiring excessive cerebellar retraction. Tumors of the CPA account for 5% to 10% of all intracranial neoplasms, with the most frequent being VSs (a.k.a. acoustic neuromas), followed by meningiomas and epidermoid tumors. A retrosigmoid approach provides a trajectory that is parallel to the petrous bone. It allows removal of tumors of different sizes and offers the possibility of hearing preservation. It is particularly indicated in tumors with significant mass in the cisternal space and in patients with serviceable hearing, in whom hearing preservation is one of the goals. Conversely, tumors with prominent intrameatal extension and deafness could be considered for other approaches, such as translabyrinthine, presigmoid retrolabyrinthine, or middle fossa. The advantage of the retrosigmoid route is that the approach offers the surgeon a wide view of the cisternal component of the tumor and thus good access to the root entry zone of the acoustic nerve. This approach facilitates identification of the structures of the CPA. It is often possible to dissect the tumor from neurovascular structures while preserving them. Even large tumors extending beyond the CPA can be managed with this approach.

The CPA is a complex triangular space bounded by the brainstem medially, the cerebellum superiorly and posteriorly, and the temporal bone laterally. The anatomic exposure of the posterior fossa provided by the retrosigmoid approach ensures access to a working space limited superiorly by the tentorium cerebelli and inferiorly by the jugular foramen and foramen magnum; it exposes the lateral cerebellar hemisphere and the lateral surface of the pons and upper medulla. Cranial nerves V through XI are visible at their root entry zones and over their cisternal courses with this approach. Different working corridors allow the surgeon to dominate a very broad cisternal space through a relatively narrow approach. Detailed knowledge of the topography of this region and its cisternal anatomy is essential, as a precise orientation allows for safe and unrestrained exposure during surgery, even when neurovascular structures are considerably distorted.

Meticulous microsurgical technique for dissection of the tumor from the adjacent facial and cochlear nerves, electrophysiological adjuncts such as intraoperative cranial nerve monitoring, and careful closure to prevent cerebrospinal fluid (CSF) leakage are critical technical aspects of this surgical approach.

Significant anatomic variations may affect the CPA exposure provided by the retrosigmoid approach, and thus the approach should be tailored according to patient characteristics and pathology volume. Problems of restricted exposure may be overcome by anterior or inferior extension of the retrosigmoid approach, according to patient anatomical characteristics and pathology extension.

The ultimate goal in VS surgery is complete removal of the tumor in a single stage with complete preservation of all cranial nerve function. However, in many cases this is still a therapeutic challenge. Hence, especially when dealing with large tumors or with tumors strictly adherent to neurovascular structures, a combined partial resection with radiation therapy should be considered as an alternative therapeutic option in order to preserve neurological function.

Preoperative Radiological Assessment

A typical preoperative MRI examination consists of a standard study of the entire brain, associated with an internal auditory canal (IAC)–specific protocol. This generally includes thin-section axial precontrast T1-weighted imaging, axial and coronal thin-section postcontrast fat-suppressed T1-weighted imaging, T2-weighted fast spin echo, T2-weighted FLAIR, diffusion-weighted imaging, and cisternography sequences, such as FIESTA sequences (fast imaging employing steady-state acquisition), at the level of the cerebellopontine angle. MRI can routinely be performed at 1.5 T despite greater signal-to-noise ratio and improved resolution with 3.0-T systems. ^,

In our opinion, the key points to assess on preoperative imaging are:

• Cisternal extension of tumor in the lateral cerebello-perimedullary cistern (toward the upper or lower part of the cistern), and degree of brainstem compression.

• Intrameatal component of tumor, to plan necessity and extension of IAC drilling/exploration.

• Degree of mastoid pneumatization.

• Prominence of jugular bulb and transverse sinus dominance.

• Endolymphatic sac and anatomical rapports with IAC.

As will be further discussed point by point in the following sections, the correct preoperative assessment of these anatomical variables is mandatory to decide the type of approach, tailor it according to individual anatomic characteristics, facilitate tumor resection, plan limits of IAC drilling, prevent reconstructive complications, and to preoperatively evaluate risks for neurovascular damages.

Patient Preparation and Surgical Position

A short-duration muscle relaxant is used to facilitate endotracheal intubation. A perioperative prophylactic antibiotic with good CSF penetration (e.g., ceftizoxime, 2 g) is administered intravenously. The use of corticosteroids is usually recommended preoperatively in cases associated with peritumoral edema and for routine post-anesthesia comfort.

We do not routinely use a lumbar CSF drain: for small tumors the CSF drainage readily obtained during the initial phase of intradural exposure provides more than adequate cerebellar relaxation; in cases of exceedingly large tumors, in order to ease cerebellar relaxation, a controlled spinal drainage can be an option. We rarely place ventriculostomy for this purpose; also in cases in which a tumor is causing obstructive hydrocephalus we primarily address tumor removal, which, in most cases, resolves the problem.

Several positions are commonly used for CPA surgery. Positioning can affect surgical exposure and surgery ergonomics; in addition, positioning affects venous outflow, and poor positioning can result in cerebellar swelling. In our experience, the park bench position guarantees the best balance between surgical comfort and risks related to patient position. This position is advantageous for approaching lower cranial lesions and provides the surgeon with access to the anterior brainstem and foramen magnum, as well as the cerebellopontine angle for resection of acoustic neuromas.

As a general rule, patient positioning is typically attended to after induction of general anesthesia and placement of arterial and venous lines. Positioning is the joint effort of the surgeon, anesthesiologist, and nursing staff. Positioning of the neurosurgical patient is challenging and requires adequate anesthetic depth, maintenance of hemodynamic stability, evidence of appropriate oxygenation, and preservation of invasive monitoring.

The park bench position is a modification of the lateral position, and provides the surgeon with better access to the posterior fossa than the lateral position. After intubation and placement of a three-point head fixation device, the head is kept in a relatively neutral position and the body is slightly elevated (reverse Trendelenburg); best working angle is reached with the head (A) gently rotated toward the floor, (B) slightly flexed toward the chest and (C) laterally flexed to favor the exposition of the occipito-mastoid region, with the mastoid tip being at the upper edge of the surgical field. In addition, the aforementioned maneuvers on the head also favor the surgeon’s working-angle comfort by moving the mastoid region away from the patient’s shoulder in order to “open” the surgical field. This is of great importance, especially in relatively muscular patients with short mastoid-shoulder distance. However, excessive head hyperflexion, hyperextension, lateral flexion, or rotation should be avoided in order to avoid cervical injury and to reduce the risk of cerebellar swelling secondary to compromised flow through the jugular venous system. The head should be elevated above the level of the thorax. This maneuver facilitates additional cranial venous drainage.

A gel roll between the bed and the upper ribs and axilla of the dependent side of the body should be placed to avoid pressure injury. The dependent arm and shoulder are placed lower than the body on a support attached to the head of the bed. The nondependent arm can be placed either on a support attached to the bed parallel to the dependent one or supported along the body. In our experience, a vacuum-pack positioning device can be very effective in stabilizing the patient’s torso and maintaining body alignment ( Fig. 35.1 ).

Lateral positioning leads to gravitational changes in the ventilation-perfusion relationship in the lung. The best perfusion occurs in the dependent lung zones.

Other positions can be used for CPA surgery such as standard supine position , prone oblique position , and semi-sitting position.

Anatomical Landmarks and Skin Incision

Before skin incision we mark relevant cranial landmarks relevant for surgical and incision planning, such as: (A) root of the zygoma , (B) the inion , and (C) the mastoid tip , and (D) a line that connects the root of the zygoma to the inion (inion-zygoma line) that corresponds to the projection of the transverse sinus and landmarks the borders between posterior cranial fossa and middle cranial fossa and corresponds to the superior nucal line. A line that connects (E) the posterior profile of the mastoid process to the inion-zygoma line can be used to preoperatively identify the transverse-sigmoid junction in order to plan the surgical incision ( Fig. 35.2 ).

Each superficial landmark is associated with a specific section of the sinus anatomy. For example, the inion-zygoma line and the superior nucal line approximate the transverse sinus, whereas the intersection of the projection of the mastoid process with the inion-zygoma line approximates the transverse-sigmoid junction.

Several skin incision variants have been described, such as the “C-shape” incision, “linear” incision, or “lazy-S” incision. The basic principles behind these variants are the same. They all aim to expose the posterior margin of the mastoid process and the suboccipital area. The incision should be placed in the retroauricular region, approximately 2.5 cm behind the auricle, going from approximately 1 cm above the transverse sinus projection down to the mastoid tip. In our opinion, a linear slightly curved 5 to 6 cm incision extended from the mastoid tip to just above the inion-zygoma line (transverse sinus projection) is sufficient to guarantee adequate exposure of the suboccipital area. The inferior limb of the skin incision should be directed slightly anteriorly to avoid direct or indirect injury to the lesser occipital nerves. ^,

The cutaneous incision must include the superficial tissues up to the pericranium that is elevated and overturned with the skin anteriorly toward the auricle. In the upper part of the incision the pericranium is well represented over the calvarium and can be harvested for reconstructive purposes.

The muscle-fascial plane is dissected separately from the cutaneous limb to obtain an additional plan for reconstruction during the closure phase. The dissection must continue until the posterior profile of the mastoid process and the digastric groove are exposed. Traditionally, the mastoid tip is exposed; however, in our opinion, this is not mandatory, and muscle dissection exposing the upper half of mastoid process is generally sufficient to perform the approach.

Once the mastoid process is exposed, the foramen of the mastoid emissary vein can be identified. The mastoid emissary vein puts in communication the transverse-sigmoid junction with the suboccipital superficial venous circle. The mastoid emissary vein can be used as an additional landmark to identify the transverse-sigmoid junction. The asterion should be also identified behind the mastoid process, and traditionally it is considered the osseous landmark for the transverse-sigmoid junction ( Fig. 35.3 ). However, several cadavers and radiological studies showed the asterion as being unreliable for localizing the transverse-sigmoid junction. Several studies highlighted that the superior nucal line and inion-zygoma line are more accurate anatomical landmarks to localize the distal transverse sinus and that the asterion was unreliable to localize the transverse-sigmoid junction. This corresponds to several studies that have found the transverse-sigmoid junction at the asterion in only 60% to 70% of patients. ^,

In our opinion, most reliable landmark for identification of the transverse-sigmoid junction corresponds to the intersection of a line drawn from the groove of digastric muscle on the posterior edge of the mastoid process with the inion-zygoma line. Correct identification of transverse-sigmoid junction projection on the bone surface is mandatory to correctly plan the surgical approach and place the burr hole.

Navigation devices can be used to confirm its position prior to craniotomy.

Craniotomy and Dural Stage

It is important to underline that a correct bony approach is a central phase in CPA surgery: the approach must be tailored to the patient’s anatomical characteristics and in relationship with the size and extension of pathology. It must allow a correct working corridor toward the pathology. Preoperative radiological assessment must be carefully evaluated in order to determine size and extension of the craniotomy and anticipate possible strains. An insufficient approach, or a poorly positioned one, can be open to neurovascular damages and intraoperative complications and can create non-ergonomic work corridors with insufficient exposition of the relevant anatomy and blind working angles.

After bony exposure and identification of the abovementioned landmarks, a burr hole (performed with a diamond drill or with a standard perforator) is placed just underneath the transverse-sigmoid junction. As explained previously, each superficial landmark is associated with a specific section of the sinus anatomy but different studies disagree on which landmarks are most reliable. The intersection of the projection of the digastric groove line with the inion-zigoma line and the position of the mastoid emissary vein give a good estimation of the position of the sinuses’ junction. Navigation system information can be used for validation. After burr hole placement this can be extended with a Kerrison rongeur in a supero-lateral direction to safely identify the sinuses’ junction. Craniotomy is then performed with a craniotome; a window of approximately 3 cm is generally sufficient for most procedures. It is bounded anteriorly by the sigmoid sinus and superiorly by the transverse sinus ( Fig. 35.4 ).

Particular care should be taken on the anterior edge of the craniotomy, as the sigmoid sinus lies on a more superficial plane compared to the transverse sinus and its course runs in a bone groove behind the mastoid process; thus it could be easily pinched with the craniotome. A safe margin should be left on the anterior edge of craniotomy, which can then be finished with punches or with a diamond drill to reach the sigmoid sinus dura.

In our experience, we prefer to perform a craniotomy more than a craniectomy, for reconstructive purposes. A drilled burr hole allows minimal bone loss, compared to a perforator.

An important step of the approach is the removal of the inner edge of the craniotomy using fine punches while protecting the dura: this enhances the working angle and allows a direct line of sight down the posterior surface of petrous bone.

Also, the size of the craniotomy should be tailored according to pathology volume and individual anatomical characteristics of the patient : an overly large approach does not necessarily determine a more favorable corridor; it enables inadvertent damage to the cerebellum and neurovascular structures that should be exposed only for the necessary extension.

Careful preoperative assessment of individual anatomical characteristics of the patient should be, therefore, carefully assessed on preoperative imaging. Anatomic variations may affect the CPA exposure provided by the approach. For example, if the sigmoid sinus course is more posterior, the anterior edge of the craniotomy should be accordingly shifted posteriorly, ^, while a low transverse sinus course, especially if the patient has a short neck and a prominent shoulder, could further limit the working channel. These drawbacks may be overcome by anteriorly extending the approach by a careful roenger unroofing of the sigmoid sinus and anterosigmoid decompression to allow anterior mobilization of the sigmoid sinus. Conversely, a highly placed jugular bulb restricts access to the IAC.

Other important considerations in tailoring the craniotomy are side of operation and sinus dominance. A prevalence of right-side transverse sinus dominance has been demonstrated in the general population. Hence, before surgery, preoperative imaging should establish if sinus dominance is present; on the dominant side, the transverse sinus course is more caudal and much closer to the inion-zygoma line, and the burr hole is more likely to be closer to the sinus, potentially damaging it, or may even be supratentorial.

Craniotomy could lead to the opening of the mastoid air cell system, to a variable degree. When the craniotomy is complete, the whole craniotomy perimeter should be inspected and subsequent opened mastoid air cells should be sealed with bone wax. The edges of the craniotomy should be inspected with exceeding care, both at the moment of craniotomy and at the end of the procedure, in order to ensure an effective obliteration during the reconstructive stage. ^,

Variations in shape, position, and size of craniotomy should be tailored according the anatomical features of the lesion. The craniotomy should be shifted superiorly for lesions extending toward the upper cerebello-perimedullary cistern, and inferiorly to reach lesions extending toward the region of the lower cranial nerves.