In any skull base operation where the operative corridor can be small, deep, and poorly lit, any major vascular injury is a challenge to manage and successfully control. This is particularly true for procedures in the posterior fossa where all the aforementioned conditions can exist and the surgeon is surrounded by highly delicate and sensitive structures, such as the brainstem and cranial nerves. Injury to arterial or venous structures presents its own set of challenges. Arterial injuries can require much more complex techniques, such as microsuturing or bypass to repair while venous bleeding can often be stopped using tamponade with thrombogenic materials. The postoperative complications of arterial injury are often immediate and can cause devastating neurologic sequelae while, in contrast, the consequences of venous injury can be more delayed and unpredictable. The present chapter will review the relevant normal and variant anatomy of the posterior fossa to help surgeons avoid major intraoperative injuries and discuss techniques for controlling bleeding as well as manage some of the downstream consequences.
Key wordsPosterior fossa surgery – vascular injury – dural venous sinus thrombosis – vascular anatomy – anatomical variants
14 Posterior Fossa During Open Skull Base Surgery
14.1 Key Learning Points
Understanding the key vascular anatomy and common anatomical variants relevant to open skull base surgery in the posterior fossa is key to avoiding and dealing with vascular injury.
Identification and direct visualization of the source of bleeding are paramount to appropriately controlling and repairing injuries when they occur. Brain relaxation and wide bony exposure can be key to achieving this in the posterior fossa.
Preparation can be key to controlling arterial bleeding. Having the necessary equipment and hemostatic agents in the room and being ready before catastrophic injuries occur can help control these events and give surgeons the time and tools to appropriately repair them.
Posterior communicating artery and vertebral artery dominance are important collateral relationships to understand in each patient undergoing posterior fossa surgery.
Basilar artery injury may be catastrophic, but can be controlled with focal packing or even focal sacrifice depending on collateral circulation and a nonperforator-bearing segment.
Venous bleeding is often best controlled with thrombogenic materials, tamponade, and time. Attempting to stop venous bleeding with cautery can worsen injuries and lead to increased blood loss.
Injury to a bridging vein at the point where it attaches to the dura or bone can be challenging to repair. If cautery is not sufficient, often a gelatin sponge soaked in thrombin with some fibrin glue or other focal packing can be most useful.
When the mastoid emissary vein is encountered during mastoid craniectomy, careful drilling around the vein can help expose it completely in order to cauterize or ligate it, prior to causing inadvertent injury to the transverse-sigmoid junction.
Intraoperative vascular injuries are among the most feared complications encountered in both open and endoscopic skull base neurosurgery, placing the patient at risk of severe morbidity and potential mortality as well as testing the years of training and mental fortitude of the operating surgeon. Vascular injuries can occur instantaneously to arterial or venous structures and require surgeons to diagnose the source of bleeding and the severity of the injury to choose techniques from an armamentarium of hemostatic strategies. This must be accomplished while simultaneously working through a sea of red that hinders the ability to both precisely visualize the problem and repair it expeditiously.
When performing procedures on pathologies within the posterior fossa, vascular injury is more likely to occur prior to the intradural portion of the procedure. With retrosigmoid, far lateral or transcondylar approaches, vertebral artery (VA) injury can occur during the soft tissue dissection prior to removing any bone. Once bony removal begins, in particular with the retrosigmoid approach, the transverse and sigmoid sinuses are at risk. Preoperative planning, awareness of anatomical variants and dominant supply/drainage, and preparedness can prevent these accidental injuries from having disastrous consequences. During the intradural portion of the operation, the particularly small corridors of the posterior fossa combined with the delicate nature of the surrounding neural structures can make these events even more treacherous.
The techniques for achieving hemostasis and repairing arterial or venous bleeding can differ greatly. Small arterial and moderate venous injuries can often be alleviated with thrombogenic agents and tamponade or bipolar electrocautery while larger injuries may require temporary occlusion and direct repair with suture. Additionally, the types of pathologies being addressed by these approaches can greatly change the risk and likelihood of vascular injury and the surgeon’s ability to repair damage, such as a chordoma encasing the basilar artery (BA) or a meningioma invading the dural venous sinus system. This chapter will review factors affecting the risk of major intraoperative vascular injuries, how to avoid such injuries, and techniques to address these events when they occur as well as examine subsequent effects and postoperative management.
14.3 Arterial Injury and Complications
14.3.1 Arterial Anatomy of the Posterior Fossa
The vertebrobasilar system, which supplies most of the blood to the structures within the posterior fossa, originates from the subclavian arteries bilaterally as the VAs and travels through the transverse foraminae of the cervical spine. After traversing the foramen at C1, the V3 segment of the VA courses posteromedially around the atlanto-occipital joint through the sulcus arteriosus of C1, deep to the muscles bounding the suboccipital triangle. Prior to piercing the dura at the lateral edge of the foramen magnum, the artery gives rise to the posterior meningeal and posterior spinal branches. Once intradural (V4), the VA courses anteromedially in front of or between the hypoglossal rootlets and joins the contralateral VA approximately at the level of the pontomedullary junction to become the BA. 1 Arising from the V4 segment is the paired anterior spinal artery and posterior inferior cerebellar artery (PICA), which courses posteriorly around the medulla and cerebellar tonsils between the lower cranial nerves (CN IX–XII).
As the BA runs across the anterior surface of the pons into the interpeduncular cistern, it gives off the anterior inferior cerebellar artery (AICA), numerous perforating arteries, and the superior cerebellar artery (SCA) within the infratentorial space (Fig. 14.1). The AICA often arises from the lower half of the BA and courses within the subarachnoid space with relations to the pons, middle cerebellar peduncle, and petrosal surface of the cerebellum. 1 Also, it courses around CN VI–VIII and gives off a branch, the labyrinthine artery, which supplies the nerves within the internal auditory canal. The SCA arises near the basilar apex within the interpeduncular space and courses inferior to CN III and CN IV and posterolaterally around the pontomesencephalic junction near the tentorial edge.
14.3.2 Anatomical Variants
There are a number of anatomical variants in arterial anatomy of which skull base surgeons operating in the posterior fossa must be aware to avoid troublesome intraoperative situations. Careful review of preoperative imaging can often identify some of these variants and allow for greater care to be taken when this anatomy is encountered during the operation.
In the majority of cases, the VAs are of different calibers. The incidence of a hypoplastic VA, being defined as having smaller caliber than the contralateral VA but still terminating at the basilar junction, is approximately 20 to 40% and is most common on the left side. 2 , 3 , 4 , 5 As a result, preoperative vascular imaging to understand dominance is critical. Additionally, appropriate preoperative imaging should be obtained to study the course of the VA within the soft tissue as often it can have an ectatic course between C1 and C2 that can increase the risk of injury. The presence of an arcuate foramen, a bony bridge on the posterior arch of C1 that surrounds the VA, can be misleading when performing soft tissue dissection and should be noted on preoperative imaging. Presence of this variant can occasionally cause compression of the underlying VA, which may be notable prior to certain operations. 6 Another common variant that can be encountered during soft tissue dissection is the extracranial-extradural origin of PICA, which can occur in approximately 5 to 20% of cases (Fig. 14.2). 7 Injury to this structure has the potential to cause lateral medullary and cerebellar infarcts resulting in significant swelling that can not only make the intracranial portion of the operation very challenging but also have severe neurological consequences.
14.3.3 Management Strategies for Arterial Injury
In skull base surgery, being prepared for a possible arterial injury can make a large difference in how well it is handled. As always, meticulously studying preoperative imaging for relationships between the targeted lesion and important arterial structures as well as the presence of collateral circulation, such as VA dominance or the presence of posterior communicating arteries, can help surgeons know when these structures will be encountered and provide clues regarding how to proceed at various stages of the operation. Available instrumentation including emergency suctions, temporary aneurysm clips, and fine suture and bypass instrumentation can help reduce time to action in case of an unexpected event. Other instruments that may prove useful during emergent situations, such as endoscopes or mirrors, could help in packing uncontrolled bleeding and give one time to plan an appropriate repair.
In addition to the appropriate tools, preparing the posterior fossa with appropriate brain relaxation can help improve success in the event of a vascular injury. In our practice, we routinely use mannitol during induction, as well as a lumbar drain to improve brain relaxation. More cerebrospinal fluid (CSF) can be drained, and quickly, by opening the cisterna magna early in the dissection or at any point necessary during the operation. All these measures can open the operative corridor to decrease the risk of injury and improve the surgeon’s ability to have control if needed. If there is particular concern preoperatively based on the size or location of the lesion or involvement of vascular structures, a surgeon can pre-emptively prepare for an external ventricular drain at any of the posterior access locations, in particular Keen’s or Frazier’s point. 8 Finally, an adequate size cranial opening can aid in increasing the operative corridor. In an emergent situation, further removal of bone, especially near the foramen magnum, can help in increasing working space to repair injuries, provide proximal control, and aid in brain retraction; as a last resort, cerebellar resection can be performed to open space to achieve control of vascular injury or manage edema related to venous injury.
When arterial bleeding occurs during surgery, it is important to isolate the injury and determine the best way to control the bleeding (Fig. 14.3). Initially, suction should be used to clear the field and specifically locate the injured vessel. This can be aided with the use of tamponade and cottonoids. For massive bleeding, a second suction in the field may be useful. Small injuries to major vessels or from perforating branches can often be controlled using targeted bipolar electrocautery. Larger injuries could require sacrifice of a vessel, which may not be possible depending on the vessel injured and the patient’s collateral vasculature in which case repair of the vessel must be attempted. There are a couple considerations prior to large vessel sacrifice. First, the patient’s collateral circulation must be studied. For instance, if the injury has occurred to a nondominant VA, the patient may tolerate sacrifice if the injury is proximal to PICA, as the vessel will likely adequately fill from the contralateral VA. The presence of sufficient posterior communicating vessels may allow for the sacrifice of a proximal BA injury, which will allow the perforators to continue to fill from the collateral anterior circulation. A second consideration is the effect of large vessel temporary occlusion. If intraoperative neuromonitoring is used, temporary occlusion of the injured vessel can demonstrate changes in signals helping to demonstrate whether or not the vessel can be sacrificed. Additionally, Doppler ultrasonography or indocyanine green (ICG) angiography may reveal if more distal critical vessels are filling after occlusion and hint that large vessel sacrifice can be performed safely.
In any large vessel injury, it is important to obtain proximal and distal control of the injured vessel. Placement of temporary clips can stop bleeding and allow better inspection of the injury to determine how it can be best repaired. When appropriate, primary closure can be attempted with 9–0 or 10–0 suture; however, within the small and deep confines of the posterior fossa, depending on the approach and injury, this can be quite challenging. If vessel sacrifice is necessary but contraindicated based on collateralization, bypass can be attempted via a number of possible techniques, such as removal of the injured segment and re-anastomosis of the vessel, anastomosis with local arterial supply (occipital artery or contralateral PICA), or possibly radial artery or saphenous vein grafts. Injuries to the BA can be particularly challenging because the depth of access to the vessel is long and the working space small. When injury occurs, sometimes bleeding can be controlled with packing of thrombogenic materials, cotton, and/or muscle. If the vessel cannot be sacrificed because of a lack of collateral circulation or the location of the injury in relation to perforators, then one can consider bringing the patient to angiography for repair/reconstruction with thrombectomy, if needed, and/or flow diversion. Unfortunately, this situation can be challenging to manage and often has dire consequences for the patient.
Avoidance of Arterial Injury
As always, preparation is paramount to avoiding intraoperative problems. Recognition of anatomical variants can be valuable to preventing intraoperative bleeding and subsequent devastating ischemia. Asymmetry in the VAs can be important for surgical planning by recognizing a dominant side. This knowledge can allow extra caution during the muscular dissection when approaching the posterior fossa as the artery can be encountered extracranially as it courses superolaterally between the transverse foramina of C1 and C2 and as it runs in the sulcus arteriosus. When possible, such as when approaching midline ventral lesions at the foramen magnum, consideration of the laterality of the approach should be made to avoid injury to the dominant VA. In addition, preoperative recognition of an extradural PICA origin can help prevent inadvertent sacrifice of a presumed muscular branch of the VA, which can help reduce the risk of potentially severe ischemia.
During the postoperative period, measures can be taken to avoid worsening of these complications. Strict blood pressure management, with avoidance of hypotension (systolic blood pressure [SBP] ≥ 120 mm Hg) to avoid watershed infarcts and permissive hypertension to preserve penumbra (SBP 140–180 mm Hg based on patient’s baseline blood pressure), can prevent worsening of the initial ischemic effects of these injuries, helping to minimize neurological morbidity. Depending on the necessary techniques used to repair injuries (i.e., if primary repair of a vessel or bypass is necessary), early antiplatelet with Aspirin 81 mg daily may be necessary to prevent thrombus formation and embolization.
14.4 Venous Injury and Complications
14.4.1 Venous Anatomy of the Posterior Fossa
Venous structures are encountered throughout all stages of operations performed to approach the posterior fossa. While performing the soft tissue dissection, significant venous bleeding can be encountered from the venous plexus surrounding the VA. This extensive plexus is usually encountered deep to the muscles of the suboccipital triangle, in particular during the muscular dissection for a far lateral approach and even with the inferior portion of a postauricular incision for a retrosigmoid or transmastoid approach.
The most treacherous venous anatomy encountered during open skull base operations, the dural venous sinus system, is often met at the beginning of the procedure prior to opening the dura. The dural venous sinuses are contained within the endosteal and meningeal dural layers with an inner lining of endothelium. 9 , 10 Unlike normal veins, they lack musculature and valves. The transverse sinus drains from the torcula into the sigmoid sinus and inevitably into the jugular vein (Fig. 14.4). The transverse sinus begins near the inion and runs in a bony groove along the inner surface of the occipital bone entering the sigmoid sinus just medial to the petrous temporal bone. The sigmoid sinus then undergoes a tortuous course where it joins the jugular bulb within the mastoid portion of the temporal bone. Knowledge of surface and bony landmarks for identification of these structures prior to beginning drilling can help avoid injury. The inion can be used to approximate the location of the torcula, while the transverse-sigmoid junction can often be approximated as the midpoint of a line drawn from the root of the zygoma to the inion. Once soft tissue dissection has been completed, the transverse-sigmoid junction can be approximated by a point slightly inferior and medial to the asterion. 11 In addition, there is often a mastoid emissary vein that runs through the occipital bone in the retrosigmoid region that is encountered when exposing the transverse-sigmoid junction, which can bleed significantly.
The intradural venous system within the posterior fossa is divided into four groups based on their drainage and includes the superficial, deep, brainstem, and bridging veins (Fig. 14.5). 12 The superficial veins drain and course along the three surfaces of the cerebellar hemispheres while the deep veins can be found within the fissures between the cerebellum, brainstem, and cerebellar peduncles. The veins draining the brainstem are named based on their drainage and course along the three segments of the brainstem. Finally, the bridging veins that collect drainage from the rest of the posterior fossa are named after their inevitable drainage site and include the galenic group, the petrosal group, and the tentorial group.
Sequelae from injury or sacrifice of the veins of the posterior fossa is uncommon, largely because of extensive anastomosis; however, knowledge of the anatomy can prevent inadvertent bleeding or can aid in finding likely sources of venous bleeding to obtain hemostasis. 12 The bridging veins are among the more relevant veins encountered during open skull base surgery in the posterior fossa. In particular, the superior petrosal veins that course in the rostral aspect of the cerebellopontine angle can be encountered when targeting lesions of the cerebellopontine angle, petrous dura, or trigeminal nerve and can even be inadvertently damaged when operating on other lesions along the petrosal surface of the cerebellum, brainstem, and cranial nerves. In addition, when performing supracerebellar infratentorial approaches, one will encounter the precerebellar vein on the way to the quadrageminal cistern, which can often be sacrificed without consequence, as well as deep venous structures within the cistern, including the vein of Galen and internal cerebral veins as well as the paired basal veins of Rosenthal within the ambient cisterns, laterally, which must be preserved. Although located in the supratentorial space, it is worth noting that the vein of Labbe drains into the dural venous sinuses approximately near the transverse-sigmoid junction. Care must be taken to preserve this vein, particularly when performing transpetrosal approaches for pathologies that may involve both compartments. Injury to this vein can result in venous infarction of the posterior temporal lobe with severe neurologic consequences.
14.4.2 Anatomical Variants
Similar to arterial anatomy, there exist normal anatomic venous variants that skull base surgeons must be able to recognize on preoperative studies. One of the most prevalent is asymmetry or dominance of the dural venous sinus system. A number of observational studies have demonstrated asymmetry of the transverse sinuses in approximately 10.0 to 66.9% of patients, with hypo- or aplasia of the left transverse sinus being more common, with an incidence ranging from 25.4 to 59%. 13 , 14 , 15 This disparity is thought to be explained by the fact that the right-sided transverse sinus drains the cerebral hemispheres through the superior sagittal sinus while the left transverse sinus drains the straight sinus and the deep cerebral venous system.
Another variant of the dural venous sinuses is the presence of the occipital sinus, which runs vertically or obliquely underneath the occipital bone. When present, this is usually the smallest of the dural venous sinuses that connect the marginal sinus and vertebral venous plexus to the torcula, inevitably draining into the jugular venous system. Most commonly, the occipital sinus is singular but it can be duplicated bilaterally and rarely triplicated. 16 The presence of the occipital sinus is considered a persistence of a venous channel expected to regress with age as early as in the 6th and 7th months of gestation. 17 One study using magnetic resonance venography (MRV) to examine venous sinus anatomy as it relates to age demonstrated incidence of an occipital sinus in 24% of the population less than 1 year of age trending downward to an incidence of 2.8% in age groups of 16 to 20 years. 18 Other studies have demonstrated similar findings of age-related decreased incidence. 19 , 20
The most clinically relevant of the bridging veins, the superior petrosal or Dandy’s vein, can vary based on its entry point to the superior petrosal sinus relative to the internal acoustic meatus (i.e., medial, intermediate, and lateral) as well as based on the number of tributaries. 12 In cadaveric studies, the medial position was found to be most common (64.7%), followed by the lateral type (26.5%) and intermediate type (8.8%). With regard to the number of tributaries, most commonly the superior petrosal vein was found to have two (50%), followed by one (40%) and three (10%). 21
14.4.3 Management Strategies for Venous Injury
For a surgeon not comfortable managing venous bleeding, skull base neurosurgery is not an ideal subspecialty. Venous bleeding, although of lower pressure than arterial bleeding, can be torrential and rapidly obscure the surgical field, which can present challenges in regaining control. Beyond significant blood loss, venous bleeding can also result in life-threatening complications including air embolism, venous sinus thrombosis and occlusion, embolization, and formation of secondary dural arteriovenous malformations.
The mainstay in controlling venous bleeding is often the use of hemostatic agents, tamponade, and time (Fig. 14.3). Often collagen or gelatin sponges soaked in fibrinogen or thrombin, flowable gelatin matrices, or oxidized cellulose can be successful in controlling small injuries, even in the dural venous sinuses; however, one downside to these products is that they can induce cascades causing clotting and potentially turn a small injury into an occlusive thrombus. 22 Bolstering these products with fibrin glue can be effective in not only achieving hemostasis but to more reliably keep these materials in place.
Many of the techniques for repair of dural venous sinuses have been developed from work done with traumatic injuries and resection of meningiomas invading the superior sagittal sinus. Techniques for direct repair of dural venous sinuses can be more involved and complex, including directly suturing the injury, use of autologous venous or arterial graft with the use of shunts, dural or fascial or muscle grafts, pericranial patching, or allograft or synthetic patch grafts. 23 , 24 , 25 , 26 , 27 , 28 , 29 There are a number of disadvantages to some of these techniques. Direct repair can be problematic when the dura is very fragile and placement of even a single suture can result in further tears, creating a larger problem. Although vessel grafts can restore appropriate sinus flow, the major disadvantage is the requirement for temporary occlusion of the already injured sinus and the time-consuming nature of the repair to both harvest a graft and create an anastomosis. In addition, graft patency rates are generally quite low. Other types of patch grafts can be useful in controlling bleeding; however, the site of harvest must be a consideration. In particular, with operations in the posterior fossa, using a dural graft could increase the risk of CSF leak by making primary dural repair more difficult and leaving larger gaps. In our practice, we often repair large sinus injuries using dural grafts bolstered with thrombin-soaked, hemostatic gelatin sponges. Hemostatic sponges are placed directly over the injury and the dura is then flapped over the sponge and tacked to the edge of the craniotomy through small tack-up holes. This technique balances the need to be efficient and often effectively repairs the injury without requiring sinus occlusion or sacrifice.
Avoidance of Venous Injury
Most important to avoiding intraoperative catastrophe is preparation. Thoroughly studying the preoperative imaging for variant anatomy and prominent venous structures, having all necessary materials and tools needed to manage excessive bleeding readily available, and having a checklist of techniques to regain control when an injury occurs can turn a near-fatal event into a routine episode. There are some common points during a procedure where venous bleeding can occur, and knowing these points and how to pre-emptively control them can be helpful. When sinus injury occurs, it is important to take precautions to prevent air embolism, which may include copious irrigation and lowering the head of bed. When there is particular concern for sinus injury, precordial Doppler is recommended to help identify air emboli and appropriately treat them expeditiously.
As previously discussed, soft tissue dissection of the suboccipital musculature at the level of the foramen magnum can result in bleeding from the vertebral venous plexus. Careful, subperiosteal dissection of this tissue can help minimize this bleeding and often the veins of the plexus can be identified without injury, if particular caution is taken. However, when bleeding is encountered, often the best way to control it is through the use of hemostatic materials (we prefer flowable gelatin matrices and gelatin sponges soaked in thrombin), tamponade, and time. Attempting to use electrocautery on this fragile venous system can sometimes result in larger injuries, especially if the walls of the veins become stuck to instruments.
When performing a midline, suboccipital approach, the presence of an occipital sinus can be disastrous. The presence of this structure should be noted on preoperative imaging. When present, the dural opening must be performed carefully and, when approaching the midline, use of bipolar cauterization and/or clips can help coagulate or ligate the sinus prior to incising this structure.
Often sinus injury can occur at the transverse-sigmoid junction when the mastoid emissary vein is encountered during the mastoid craniectomy portion of a retrosigmoid approach. Furthermore, this vein can even travel through the bone and can cause significant venous bleeding when clearing the soft tissue. When encountered at this point, it is important to clear the soft tissue fully from around the exit site of the vein at which point bone wax can be effective in stopping bleeding. While drilling, it is sometimes possible to clear the bone around the vein in a controlled fashion so that the vein can be ligated or cauterized prior to being torn from the sinus.
Additionally, the sigmoid sinus can be directly injured during mastoid drilling. This can occur via direct injury from a drill or rongeur or potentially from heat created by the drill. The risk of direct injury can increase if the sinus is particularly protuberant into the mastoid bone. When drilling, care should be taken to adequately thin the bone and dissect in from the sinus in a controlled fashion.
Once intradural, the highest risk of injury can be to the bridging veins of the posterior fossa, in particular, the superior petrosal vein through a retrosigmoid approach. When CSF has been significantly drained and the cerebellum begins to sag, tension can be placed on this vein. In combination with drying out, injury can occur inadvertently, even when working some distance away. Often this vein can be sacrificed in a controlled fashion, without consequence, which can prevent unexpected injury and is reported in many large case series as being safe; however, there exist reports of complications associated with its sacrifice ranging from auditory hallucinations to venous infarction and cerebellar hemorrhage. 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 In our practice, we routinely preserve this vein and rarely sacrifice it intentionally. For the majority of pathologies within the posterior fossa, this goal can be achieved because most pathology is inferior to the vein. Situations in which we would consider sacrificing this vessel would be when the vein is in the way of accomplishing the goal of surgery; for example, resection of a trigeminal schwannoma from a posterior approach, or occasionally with very large tumors where increasing the working space by sacrificing the vein can prevent inadvertent injury and bleeding and aid in tumor resection and preservation of neurologic function.
When sacrificing the vein it is best to coagulate with bipolar electrocautery and then cut the vessel half way to ensure it has been fully cauterized. If necessary, further cautery can be used prior to finishing dividing the vessel. This technique can help minimize bleeding if the vessel has not been fully cauterized. If a bridging vein is injured at the point where it attaches to the dura or bone, this can be challenging to repair. In some instances cautery can be sufficient; however, often gelatin sponge soaked in thrombin with some fibrin glue can be most useful. The fibrin glue can help hold the sponge in place if gravity is working against keeping the material in place.