13.4.1 Operative Technique in a Tumor with Subarachnoid Encasement

Proximal control of the exposed artery is obtained, followed by exposure of the tumor while minimizing brain retraction and then distal exposure of the artery beyond the encasement. The artery is then traced through the tumor from both sides, with frequent debulking. Suction, bipolar cautery, fine dissectors, and microscissors are used for dissection of the artery. In case of an artery that has multiple perforators, the surface without perforators is dissected first, followed by the portion containing perforating branches.

13.4.2 Decision to Bypass in Cases of Vascular Occlusion or Sacrifice

The need for bypass in cases in which sacrifice of the ICA or VA is planned is somewhat controversial and depends on many patient-specific factors. Information obtained from preoperative balloon occlusion tests, coupled with monitoring of cerebral blood flow by single photon emission computed tomography (SPECT), transcranial dopplers (TCD), or angiography, can provide insights into whether a vessel can be sacrificed without the need for bypass. The success of this selective bypass approach depends on the accuracy of preoperative testing.

Based on a review of a series of patients operated on by the senior author (LNS) who were not revascularized and who suffered strokes, as well as the reports of other surgeons who had similar experiences,14 ^,​ 15 ^,​ 16 ^,​ s. Literatur we currently practice a universal bypass approach if the ICA must be occluded for tumor cases. For posterior circulation, bypass is not necessary for a markedly nondominant VA. However, if the VA is equal or dominant, we do perform a reconstruction or bypass. The BA should always be reconstructed if injured, although patients can tolerate distal basilar occlusion if there is significant PCom flow. In the event of unexpected intraoperative injury to major arteries, it is best to reconstruct the vessel using either a local, regional, or extraintracranial bypass technique, because the adequacy of collateral circulation cannot be determined.

Indications for bypass in the modern era of skull base surgery can be summarized as follows18:

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  Benign tumor encasing a major artery and the tumor cannot be dissected free for complete resection without damaging the artery. Bypass is typically pursued with recurrent and previously radiated benign tumors. As an alternative, a small amount of tumor can be left behind and re-irradiated if allowable.

  
  
• 
  

  Malignant tumor involving a major artery: complete resection is the goal set preoperatively based on the principles of oncologic treatment, whereby negative margins are imperative for optimal tumor control.

  
  
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  A major artery already occluded by the tumor and the patient is having ischemic symptoms, or there is preoperative evidence of significantly reduced cerebrovascular reserve.

  
  
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  Unplanned intraoperative injury to a major artery that cannot be directly repaired or sutured.

13.4.3 Choice of Bypass Grafts

Bypasses may be divided into two groups: replacement bypasses (e.g., radial artery graft [RAG]/saphenous vein graft [SVG] to replace the ICA/VA) and augmentation bypasses (e.g., STA–MCA bypass performed in a patient who has brain ischemia secondary to ICA occlusion, or occipital artery [OA] to posteroinferior cerebellar artery [PICA] for a patient who has VA occlusion). Most of the bypasses performed for skull base tumors are replacement bypasses using RAG, SVG, or rarely, the superficial temporal artery (STA). Local repair of an injured artery may also be performed in some cases.

The radial artery provides flow rates between 50 and 150 mL/minute acutely, and flow can increase significantly over the ensuing days, as measured by duplex ultrasound.19 We perform preoperative mapping of the arterial tree of the arm as well as an Allen’s test to ensure adequate collateral perfusion. Although the radial artery is easier to harvest than the saphenous vein is, postoperative vasospasm in RAG is an important concern but has been largely alleviated by use of the pressure distention technique, which significantly lowers rates of postoperative vasospasm in arterial grafts.9 ^,​ 20 ^,​ 21

SVGs are an alternative to RAG for high-flow replacement bypasses. We use the saphenous vein when the radial artery is smaller than 1.5 mm or is unavailable for any reason (e.g., prior harvest, insufficiency of palmar arch). In children younger than 12, the radial artery is typically too small, mandating the use of SVG if needed. Flow rates in SVGs are typically higher than in RAGs, in the range of 100 to 250 mL/minute.19 These high flow rates can cause flow mismatches and/or hyperperfusion syndromes when anastomosed to the middle cerebral artery (MCA) or posterior cerebral artery (PCA). Moreover, vein grafts are more technically challenging to anastomose and can be prone to kinking. Accordingly, a RAG that has been pressure-distended to decrease postoperative vasospasm is our graft conduit of choice.

13.4.4 Anesthesia, Monitoring, and Preparation

When a bypass may be needed, preoperative duplex imaging of the radial arteries and saphenous veins and an Allen’s test are performed to facilitate graft selection based on the parameters already outlined (Fig. 13.1). We also monitor pulse oximetry intraoperatively to ensure adequate perfusion of the hand after temporary occlusion of the radial artery prior to harvest. The patient is given 325 mg of aspirin preoperatively. Intraoperative monitoring of electroencephalogram, somatosensory evoked potentials, and motor evoked potentials is employed. Total intravenous anesthesia is used to allow monitoring of motor evoked potentials. If a bypass is performed, the anesthesiologist is asked to raise the patient’s mean arterial pressure by about 20% during temporary occlusion to facilitate collateral perfusion and then places the patient in burst suppression with propofol for brain protection. Approximately 3,000 to 5,000 units of heparin are also administered just prior to vascular occlusion. We do not reverse heparinization at the end of the procedure. Meticulous hemostasis is required prior to heparin dosing to prevent bleeding that can obscure the operative field.

13.4.5 High-Flow (Replacement) Bypass Technique

High-flow bypass techniques are important when major arteries (ICA, dominant VA) must be sacrificed during tumor resection. Replacement of 100 to 250 mL/minute of blood flow can be accomplished using this technique. For a tumor involving the ICA, an ECA or cervical ICA to MCA–M2 segment bypass is preferred (Fig. 13.2). If the MCA vessels are particularly small, the supraclinoid ICA may be used as a recipient vessel. If the proximal ECA or ICA is unavailable for bypass because of tumor involvement, then the V2–V3 segment of the VA can be used as a proximal anastomosis site (Fig. 13.3).

The cervical ICA is exposed in the neck. The tumor is exposed after a craniotomy and an orbital or orbitozygomatic osteotomy. Our practice is to inspect the tumor and determine whether bypass will be needed—a judgment that sometimes requires removal of some of the tumor along the vessel to determine whether the plane is favorable. When there is no clear plane or when vascular invasion is evident, we proceed with bypass prior to aggressive resection of the tumor.

Accordingly, the radial artery (the entire artery from the brachial artery bifurcation to the anterior wrist) or the saphenous vein (in the upper leg and lower thigh) is removed, flushed with heparinized saline, and distended under pressure to relieve vasospasm. The distal anastomosis is performed first, to the MCA (M1 bifurcation or M2 segment) or to the supraclinoid ICA. This is followed by the proximal anastomosis to the ECA (if collateral circulation is poor) or to the ICA (if some collaterals are present). If flow through the grafts is satisfactory as assessed by Doppler/intraoperative angiography, then the ICA is trapped between the cervical and supraclinoid segment, proximal to the bypass. The operation is stopped at this stage and a postoperative angiogram obtained to ensure patency of the bypass.

For VA replacement, an extreme lateral retrocondylar or partial transcondylar approach is used. Proximal anastomosis to the VA is done in the V3 segment extradurally, most commonly as the VA traverses the sulcus arteriosus of C1 and prior to the dural entry point. If the distal anastomosis is distal to the PICA, then the PICA may be reimplanted or a PICA-to-PICA anastomosis performed. Alternatively, the PICA may be occluded if there is good collateral flow from the distal vessel. For BA injury, VA or ECA to PCA–P2 segment bypass is performed using RAG or SVG. A temporal craniotomy with a zygomatic osteotomy or a petrosal approach is used to expose the PCA.

13.4.6 Low-/Moderate-Flow (Augmentation) Bypass Techniques

When the patient has some compensatory flow from collateral circulation, a flow-augmenting bypass such as an STA–MCA may be used to add 25 to 100 mL/minute of perfusion. In the posterior circulation, this can be accomplished using an OA–PICA bypass. The more common technique of STA–MCA bypass is discussed here.

The STA is exposed by a direct cut-down technique. The course of the vessel is traced by Doppler ultrasound or frameless neuronavigation and marked on the scalp. Working under the operating microscope, dissection is started distally and the vessel traced proximally. A small cuff of connective tissue is left around the artery. The vessel is left in situ until the bypass procedure. A T-incision is created from the skin incision made to expose the artery so as to facilitate muscle dissection and a small pterional craniotomy. A middle cerebral branch in the distal sylvian fissure (M3 branch), the largest temporal or parietal cortical branch relatively free of perforators, is used for anastomosis. Ideally the recipient vessel should be at least 1.5 mm in diameter, but it can be as narrow as 1.0 mm. The recipient vessel is dissected free of its arachnoidal covering, and a small rubber dam is placed under the artery. The STA is divided, and an oblique arteriotomy and slight fish-mouthing of the STA is done (Fig. 13.4a). Anastomosis to the MCA is done using diametrically opposing sutures at the ends and either running or interrupted 9–0 or 10–0 nylon sutures (Fig. 13.4b,c,d,e). Prior to the tying of the last suture, the lumen is flushed with heparinized saline and the suture tightened. Flow through the STA is checked using a Doppler probe.

13.4.7 Staged Operations

Our typical practice is to stage operations in which a bypass is needed. Craniotomy, exposure osteotomies, and bypass are done during the first surgery, followed by tumor resection 3 to 7 days later. This delay allows some degree of hemodynamic equilibrium and graft maturity after bypass is performed and allows the tumor resection to be done after heparinization has worn off. In addition, tumor resection and skull base repair can be lengthy. The tumor resection and the skull base repair are lengthy and to couple them after the exposure, osteotomy and bypass would make a longer procedure.

13.4.8 Postoperative Management

Postoperative monitoring of graft patency is usually done by Doppler evaluation. An angiogram is performed in the immediate postoperative period or within 24 hours of surgery. If this is not possible, a CT angiogram is obtained to ensure graft patency. Patients are maintained on 325 mg of aspirin. Patients who have SVG are maintained on subcutaneous heparin, 5,000 U every 8 hours for 3 days, in addition to aspirin. Duplex ultrasound studies are performed to follow the volume flow through the graft. The systolic blood pressure should be maintained below 140 mm Hg for 2 to 3 days.

Graft occlusion is most common at the time of surgery or within 24 hours of surgery. If occlusion is noted intraoperatively, it is corrected as needed. Typically the anastomotic sites at the proximal and distal end are examined for thrombus and are reexplored to resolve any kinking or flow-related stasis in the vessel. Further heparinization can be employed if clot continues to form despite good flow. If the graft occludes in the postoperative period, the patient is typically taken back to the operating room for reexploration. Our general practice is not to employ endovascular thrombolysis in these cases, because some mechanical reason for thrombus formation typically needs to be dealt with to ensure long-term patency. Vasospasm occurs occasionally with RAG (despite use of the intraoperative pressure distention technique) and can be successfully treated by endovascular angioplasty or administration of intra-arterial vasodilators such as nicardipine. Following discharge (7–10 days), patients are kept on aspirin for life in case of vein graft and for at least 1 year in cases of RAG. We typically obtain duplex flow measurements and CT angiogram or MR angiogram at 3 months, 1 year, and then subsequently every 1 to 2 years, depending on the situation.

13.4.9 Results

From 1988 to 2006, 130 patients underwent bypasses for tumors (79 for skull base meningiomas, 7 for chondrosarcomas, 7 for chordomas, and 5 for adenoid cystic carcinomas, in addition to other tumors such osteogenic sarcoma, schwannoma, hemangiomas, and hemangiopericytomas; Table 13.1). The immediate patency rate for bypasses was 95.4%, and gross total resection was achieved in 82 (63%) patients, with 29 RAGs and 101 SVGs used. Two patients had delayed graft occlusions (after 2 years), which were revised. Sixteen patients had disease progression or recurrence and died. One patient, who was wheelchair-bound and had multiple lower CN palsies, died 8 months after surgery. One patient had a major stroke despite functioning of the graft and died after 7 days.

As already noted, modern practice patterns have led to a decrease in the number of tumors treated using aggressive resection and bypass. The senior author examined his modern series of 20 bypasses in 18 patients operated on for skull base tumors between 2003 and 2012.18 Mean age was 41 years, with 14 anterior circulation bypasses and 4 posterior circulation bypasses. Long-term patency was 100%, although one patient did require a revision of the graft after stenosis was found on surveillance imaging. Seventy-seven percent of patients had had prior treatment, whether surgery or radiation therapy. In this series of largely recurrent or radiated tumors, gross total resection (GTR) was possible in 72% of patients. Long-term outcomes were good for 14 of the patients (mRS < 2), with a mean follow-up of 47 months. One patient died during the perioperative period from complications related to aspiration pneumonia. Of the five patients operated on for malignant disease involving a major artery, three died from progression of their disease (osteosarcoma, synovial sarcoma, chordoma). Similarly poor long-term survival results with highly malignant head and neck disease have been reported by other authors.22 Accordingly, bypass and aggressive resection in cases of highly malignant disease should be considered on a case-by-case basis.

Treatment of the majority of benign lesions or recurrent/higher-grade meningiomas resulted in excellent long-term outcomes. These results have been borne out in several smaller case series of skull base lesions from other institutions.23 ^,​ 24 ^,​ 25 Accordingly, bypass for the management of complex skull base tumors involving major vessels can be beneficial with good patient selection and surgical technique that minimizes morbidity.