Summary
Given the varied configurations of the venous system in both the normal and pathologic states, it is crucial to take venous anatomy into consideration when performing complex skull base surgery. In order to have a thorough understanding of the venous drainage for a particular case, dedicated imaging such as computed tomography venogram (CTV), magnetic resonance venogram (MRV), or diagnostic angiography is necessary. The best strategy remains avoidance of injury. Unlike arteries, veins are delicate and more easily susceptible to injury and therefore special attention has to be paid to this avoidance. This can be achieved by recognizing the relevant anatomy preoperatively and choosing the most direct and safest approach accordingly. However, in the case of sinus injury or involvement by the tumor, skull base surgeons must be prepared to repair and reconstruct major venous structures. Finally, surgeons must also be able to recognize and manage iatrogenic venous sinus thromboses in the postoperative setting.
Key words
Venous complications – venous sinus thrombosis – venous thrombosis – venous sinus injury – venous sinus repair20 Venous Considerations in Skull Base Surgery
20.1 Key Learning Points
Skull base approaches often traverse important venous structures such as temporal lobe draining veins, the petrosal vein, the torcula, the sigmoid sinuses, and the jugular bulbs.
Although the venous system can be assessed using magnetic resonance venogram (MRV) or computed tomography venogram (CTV), the only dynamic modality for understanding the venous outflow is an actual catheter-based angiogram. The optimal surgical approach can be planned while mitigating the risk of venous injury.
Intraoperative techniques that can be used to maximize exposure of the tumor while preserving veins and sinuses include:
Arachnoid dissection to release the veins from the surface of the brain and the dural base.
When accessing the clivus, it may be advantageous to work on both sides of the sigmoid sinus.
Strategic placement of subtemporal retractors (e.g., placing a retractor underneath the posterior leaflet of a divided tentorium to elevate it with the temporal lobe can be used to prevent occlusion or avulsion of temporal lobe veins in a presigmoid transpetrosal approach).
Venous sinuses or veins can be accidentally lacerated. Immediate repair and reconstruction have to be considered on an individual basis given that by the time brain edema or hemorrhage sets in, it is too late.
If division of a dominant sigmoid or transverse sinus is contemplated, measurement of the intrasinus pressure on both sides of a temporary clip at the proposed site of ligation is helpful to predict the safety of such ligation.
Patch grafts and venous interposition grafts are options to be considered in repairing sinus injuries.
Postoperative sinus thrombosis after skull base surgery is rarely symptomatic and does not require intervention. However, occasionally patient may develop elevated intracranial pressure and may need a shunt or therapeutic anticoagulation.
20.2 Introduction
Venous sinuses and cerebral draining veins may be involved by tumors or may be encountered during the approach to most skull base tumors. Inadvertent injury to these structures can result in serious problems such as bleeding, air embolism, venous infarct, and intraparenchymal hemorrhage leading to intracranial hypertension requiring intervention. All of these can produce neurological deficits that range from asymptomatic to severe (Fig. 20.1 and Fig. 20.2). In addition, excessive manipulation of these venous structures can result in thrombosis. Brain retraction coupled with sacrifice of some cortical veins can further complicate the situation, such as under the temporal lobe and in the sylvian fissure. Avoidance of venous injury is the best strategy. However, the surgeon needs to be aware of the options available for recognizing and dealing with these problems, should they arise.
20.3 Venous Anatomy
Although the venous drainage at the skull base follows a general pattern, normal variations can exist.
20.3.1 Temporal Lobe Draining Veins
Cortical veins from the basal temporal lobe drain into the lateral tentorial sinus which then joins the transverse sinus. Cortical veins from the lateral temporal lobe can drain directly into the transverse sinus or can also drain into tentorial venous lakes for a short course prior to draining into the transverse sinus (Fig. 20.3). The anastomotic vein of Labbe typically drains to the transverse sinus directly but can occasionally drain into the lateral tentorial sinus. 1 , 2 , 3 Many skull base approaches require manipulation of or transection of the tentorium, which can lead to significant bleeding. Excessive coagulation of the tentorium can lead to compromise of the venous drainage of the temporal lobe leading to temporal lobe venous infarct as mentioned above.
20.3.2 The Petrosal Vein
The petrosal vein is composed of multiple tributaries that come together and drain into the superior petrosal sinus. The petrosal vein drains the lateral aspect of the cerebellum and the brainstem into the superior petrosal sinus which joins the transverse sinus to form the sigmoid sinus. 4 , 5 , 6 , 7 Although complication secondary to sacrifice of the superior petrosal vein is thought to be rare, there are reports of an incidence of up to 30%. The reported complications can range from mild cerebellar edema, peduncular hallucinosis, hearing loss, and cranial nerve palsies to more serious complications such as cerebellar venous infarct (with possible hemorrhagic conversion), midbrain or pontine infarct, acute hydrocephalus, or death. 2 , 8 , 9 , 10 , 11 , 12 , 13 , 14 Given that some of these complications can be very serious and even fatal, sacrifice of this vein should be avoided as much as possible. 15
20.3.3 Torcula
The sinuses converge at the torcula in various configurations, which is best characterized on conventional angiogram or CTV. Anatomic studies show that the most common configuration is that the superior sagittal sinus (SSS) drains predominantly to the right transverse sinus (TS). The second most common configuration is that the SSS duplicates into right and left channels but still drains to a single transverse sinus. Another configuration seen is that the SSS drains to a true confluence of sinuses and then splits into bilateral transverse sinuses. The least commonly seen configuration is when the SSS drains predominantly to the left transverse sinus. 3 , 6 , 16 , 17 , 18 , 19 The drainage pattern of the SSS at the torcula becomes important in the preoperative planning of skull base approaches.
20.3.4 Sigmoid Sinuses and Jugular Bulb
The venous structures surrounding the vertebral artery in the suboccipital region form a complex network that is highly interconnected and has been likened to the cavernous sinus, hence the term the “suboccipital cavernous sinus.” This venous plexus connects to the jugular bulb via the anterior, posterior, and lateral condylar emissary veins. 2 , 3 , 6 This relationship of the sigmoid sinus and jugular bulb with condylar emissary vein becomes important in cases of sigmoid or jugular bulb occlusion. In these cases, the condylar emissary acts as the alternate pathway for venous drainage and therefore care must be taken to preserve this vein during skull base approaches.
20.4 Avoidance of Injury
20.4.1 Understanding the Relevant Anatomy
The surgeon must have a thorough understanding of the relationship between the tumor and venous structures. There are several ways of assessing the venous system including MRV, CTV, and digital subtraction angiography (DSA). The only dynamic modality for understanding the venous outflow is an actual catheter-based angiogram or venogram. Interventionalists are becoming increasingly adept and comfortable with exploring the venous anatomy, which typically involves retrograde access from the ipsilateral internal jugular vein. We suspect this will become increasingly common as new techniques to assess venous flow or pressure, in combination with the ability to place venous stents, becomes mainstream.
Example 1: A 40-year-old female presented with facial numbness from a clival meningioma. As part of her workup, she also had an MRV performed (Fig. 20.4a, b). The images suggested narrowing of a portion of the right transverse sinus (blue arrow). A preoperative cerebral arteriogram (Fig. 20.4c, d) confirmed the area of stenosis on MRV (blue arrow). The temporal lobe drainage (via two major veins, green arrows) was coming into the transverse-sigmoid junction that ultimately drained into the jugular bulb. Therefore, during the performance of a presigmoid supra- and infratentorial approach, it was imperative to avoid injury to the sinus which was the predominant pathway for draining the temporal lobe.
Example 2: A 37-year-old female suffered from palsy of the right hypoglossal nerve with atrophy of the right half of the tongue. Further workup with a cerebral arteriogram showed that the right jugular bulb and internal jugular veins were the dominant venous drainage (Fig. 20.5). Direct surgery for removing the tumor could jeopardize the large jugular bulb. Also, given that she had a complete hypoglossal palsy, surgery was not considered and she was treated with stereotactic radiosurgery.
20.4.2 Using Alternate Approaches
Detailed study of the venous anatomy can reveal variations that may require a change in the planned approach. This becomes particularly important in cases involving an occluded sinus when collateral venous channels have to be carefully preserved.
Example case: A 63-year-old male had a dumbbell schwannoma arising from the left tenth cranial nerve (Fig. 20.6a–c). A preoperative angiogram showed the left transverse and sigmoid sinus were the dominant venous outflow but the jugular bulb was occluded by the tumor. The venous drainage was via the condylar emissary vein (Fig. 20.6d). A transtemporal approach was initially planned for a one-stage removal of the intracranial and extracranial portions of the tumor. However, on reviewing the angiogram this approach would carry a high risk of injuring the condylar emissary vein with possibly serious consequences. It was therefore decided to perform the resection in two separate stages. A preauricular infratemporal approach was carried out in the first stage to allow resection of the extracranial and foraminal portion of the tumor. At a second stage, a retrosigmoid craniectomy was performed to remove the remaining posterior fossa portion of the tumor (Fig. 20.6e). In this way, the venous drainage was safely avoided and preserved.
20.4.3 Dissection and Preservation of Veins
Cortical veins draining into a dural sinus travel in an oblique path from the cortex to the sinus. As they come from the brain surface, they have a subarachnoid course. This surrounding arachnoid can be carefully dissected to release the vein and provide room to mobilize the brain and open up a pathway for dissection. Similarly, veins can be freed up from the dura through careful microdissection until the point of entry into the venous sinus. Combining these two maneuvers can allow the surgeon to work around the veins while simultaneously preserving them.
In a presigmoid transpetrosal approach, careful and deliberate placement of a retractor after cutting the tentorium can also assist in preventing stretch injuries to temporal lobe veins. In this approach, the tentorium is divided into an anterior leaflet and a posterior leaflet after visualization and protection of the fourth cranial nerve. A retractor can be placed underneath the posterior leaflet such that the temporal lobe draining veins (that can drain into tentorial venous lakes prior to draining into the transverse sinus) are elevated with the posterior tentorial leaflet and the temporal lobe. The elevation of the veins with the temporal lobe together prevents stretch injury or avulsion of the veins.
20.4.4 Working on Both Sides of the Sinus
The clivus is flanked by the petrous temporal bones. Depending on the depth of the clivus, using the combined presigmoid retrolabyrinthine and subtemporal approach alone may pose a limitation for the surgeon to visualize the ventral surface of the clivus and the base of the tumor, especially the ipsilateral junction of the petrous bone and the clivus. A useful strategy is to use the presigmoid corridor to visualize and debulk the tumor and separate the brainstem and tumor interface (Fig. 20.7a, b). Then the surgeon can open the retrosigmoid dura and access the ventral aspect of the clivus from a different angle (Fig. 20.7c, d). This retrosigmoid dissection is useful to access the lower pole of the tumor that may be extending below the caudal cranial nerves.
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