PREOPERATIVE EVALUATION

Angiographic evaluation and preoperative endovascular therapy can be invaluable in selected patients with spinal tumors. To properly evaluate and treat patients with spinal tumors, the endovascular therapist must have a thorough understanding of normal vascular anatomy, as well as the goals of the surgeon. In this chapter, we will describe the normal vascular supply of the spine and the goals of preoperative angiography and endovascular therapy, including embolization, balloon test occlusion, and parent vessel occlusion.

NORMAL ANATOMY

The normal arterial supply to the spinal cord consists of a single anterior spinal artery (ASA) and paired posterior spinal arteries (PSAs).¹ The ASA is typically contiguous throughout its course and runs along the ventral surface of the cord. At its most cranial extent, the ASA is formed from paired arteries arising from the distal vertebral arteries. Additional contribution to ASA supply is provided by radiculomedullary arteries at various levels, the most prominent of which include the artery of the cervical enlargement, typically around C5–6, and the artery of Adamkiewicz. The PSAs are paired longitudinal arteries along the dorsal surface of the cord, supplied from multiple radiculopial arteries, with frequent communicating arteries between the two PSAs. There is a pial circumferential network on the surface of the cord connecting the ASA and PSA systems, but these small vessels typically are too small to provide sufficient collateral flow in the setting of occlusion. The ASA typically supplies the anterior two-thirds of the spinal cord, whereas the PSA territory is the remaining posterior third (Fig. 14-1).

Fig. 14-1 Diagrammatic representation of the superficial arterial supply of the spinal cord. 1, Main radicular artery; 2, radiculomedullary branch; 3, radiculopial branch; 4, ASA; 5, PSA; 6 and 7, circumferential arterial plexus; 8, sulcocommissural artery.

At each level in the spine, paired radicular arteries give supply to the vertebra, nerve root, and cord. These radicular arteries enter the thecal sac at the neural foramen and have dural branches as well as radiculomedullary arteries (supplying the ASA) and radiculopial branches (supplying the PSA). In the mid-cervical level, typically around C5–6, a prominent radicular branch supplies the ASA, known as the artery of the cervical enlargement.² The ASA is larger here and in the lower thoracic regions because of increased metabolic needs, given the relatively higher amount of gray matter in the cord at these levels. In the cervical spine, supply also can arise from the ascending cervical and deep cervical arteries, which typically arise from the thyrocervical and costocervical trunks, respectively. Even though these branches typically supply the posterior musculature, and often the posterior elements, they also can supply the ASA and PSA.

In the thoracic cord, radicular arteries of the upper thoracic spine arise from the supreme intercostal arteries, which arise directly from the costocervical trunk, a branch of the subclavian artery, and supply the T1–3 levels. From T4 downward, paired intercostal arteries arise from the thoracic aorta and a rich longitudinal anastomotic network is situated between them. Each intercostal branch gives rise to multiple, small, perforating arteries supplying the vertebral body and then gives off a dorsospinal branch, which further branches to dorsal muscular branches, and the radicular (spinal) artery for that level.³ The major supply to the vertebral body is from the nutrient artery, which is formed from paired spinal arteries along the dorsal surface of the vertebral body and then enters the body in its mid-portion (Fig. 14-2). At each vertebral body level, there are also multiple small, peripheral periosteal branches along the surface of the vertebral body, which supply the peripheral one-third of the lateral and anterior aspects of the vertebral bodies. In addition, there are smaller, metaphyseal arterial branches supplying the metaphyseal regions of the bodies.

Fig. 14-2 Arterial supply of the vertebral body and intervertebral anastomoses. Lateral (A), posterior oblique (B), and axial (C) images demonstrate the arterial anatomy of the vertebral column.

The primary route for venous drainage of the vertebral body is through the basivertebral veins, which coalesce at the dorsal mid-vertebral body to connect to the internal vertebral venous network.⁴ This valveless system has extensive longitudinal connections between the veins of adjacent levels and also connects with the cranial dural venous sinuses at the level of the foramen magnum. There also are connections anterior to the vertebral body along the anterior external vertebral venous network (Fig. 14-3).

Fig. 14-3 Venous anatomy of the vertebral body. Axial (A) and lateral split section views (B) demonstrate the venous anatomy of a lumbar vertebral body. 1, Anterior internal vertebral venous plexus; 2, posterior internal venous plexus; 3, basivertebral venous plexus; 4, posterior external venous plexus; 5, anterior external venous plexus; 6, intervertebral vein; 7, radicular vein; 8, ascending lumbar vein.

ANGIOGRAPHIC EVALUATION

The complete angiographic evaluation of patients with spinal tumors depends on the levels involved. Before the angiogram, a thorough review of all computed tomography (CT) and magnetic resonance (MR) scans should be performed, including assessment of the levels involved and relationship to adjacent vascular structures. Angiography ideally should be performed in a biplane digital subtraction suite using the highest possible matrix size allowed by the equipment. Patient positioning also is critical and should take into account the levels to be imaged. Patients with known thoracic lesions should be positioned with their arms above their heads to facilitate visualization in the lateral plane. Communication between the endovascular therapist and surgeon is paramount, and the angiographer must know the surgical plan in advance of performing the angiogram. Visualization of small radiculomedullary arteries is critical; in patients in whom cooperation is a problem, general anesthesia should be considered.

In patients with cervical spinal tumors, preoperative angiography should include selective catheterizations of both vertebral arteries and both external carotid arteries, and consideration also should be given to catheterizing the supply of the ascending cervical and deep cervical arteries because these can supply the posterior elements of the spinal column as well as collateralize to the vertebral arteries. Injection of the supreme intercostal arteries also should be performed for lesions at the cervicothoracic junction.

In evaluating thoracic or lumbar spinal tumors, one of the primary objectives of diagnostic angiography is to determine the level of origin of the artery of Adamkiewicz, also known as the artery radicularis magna, which provides the primary supply to the anterior spinal artery for the lower thoracic cord. This artery typically arises from between T9 and L2 and is more common on the left than the right.⁵ Angiography will typically reveal a characteristic hairpin turn in the course of the artery of Adamkiewicz (Fig. 14-4). It is very helpful for the surgeon to know whether the artery of Adamkiewicz arises from a radicular artery that is in the planned operative field. Selective injection of the intercostal arteries at least two levels above and below the affected levels should be performed because there is a rich collateral network among the intercostal arteries. The supreme intercostal artery also should be studied in all patients with upper thoracic lesions.

Fig. 14-4 This frontal projection from a left T9 intercostal injection well demonstrates the normal artery of Adamkiewicz. The contrast opacifies the proximal intercostal artery at this level (arrowheads), which gives rise to the artery of Adamkiewicz (arrow). Notice the characteristic hairpin loop (double arrows) that helps identify the artery.

Patients with lesions involving the L5 level or the sacrum may need a pelvic angiogram with catheterization of both internal iliac arteries, preferably the posterior division, to visualize the lateral sacral and iliolumbar arteries. The medial sacral artery typically arises directly from the distal aorta and can be difficult to primarily catheterize, especially in patients with atherosclerotic disease.

Preoperative embolization of tumors can significantly assist the surgeon in obtaining hemostasis. Among the particularly vascular lesions are metastatic renal cell carcinoma, thyroid carcinoma, and aggressive hemangiomas of the bone. Embolization also has been performed for aneurysmal bone cysts of the spinal column.⁶ Embolization for intradural lesions carries higher risk and should be used judiciously based on tumor characteristics and patient symptoms. Embolization of intramedullary lesions carries the highest risk and should only be considered for known hypervascular lesions, such as hemangioblastomas.^7–9

CLINICAL STUDIES

Several series have examined the results of preoperative embolization for spinal tumors. Guzman et al¹⁰ studied 24 patients with hypervascular metastatic lesions, predominantly renal cell carcinoma (15 patients) and thyroid cancer (four patients). Mean intraoperative blood loss in the 22 patients in whom embolization was successfully performed was 1900 cc, compared with 5000 cc in the two patients in whom embolization was only partial. Chatziioannou et al¹¹ examined 26 patients (28 surgical procedures) with renal cell metastases to the spinal column and found a significant difference in blood loss in patients in whom embolization was complete (10 cases, mean blood loss 535 cc) and those in whom it was incomplete (18 cases, mean blood loss 1247 cc). Berkefeld¹² examined the intraoperative blood loss in 50 patients undergoing corpectomy, who underwent either proximal coil embolization (26 patients), particulate and coil embolization (24 patients), or just particulate embolization (nine patients), and compared these with a control group of 10 patients with no embolization. The most common histology was renal cell carcinoma (31 of 69 patients), and 65 of these 69 were either thoracic or lumbar in location. Mean blood loss for the groups were 4350 cc for no embolization, 2650 for coils alone, 1850 for particles and coils, and 1800 for particles alone. They concluded that proximal occlusion alone without distal tumor penetration was not sufficient to limit intraoperative blood loss. Vetter et al¹³ reviewed their experience with 38 patients with cervical tumors treated with preoperative embolization, including particulate embolization, coil embolization, or vessel sacrifice. The procedures were technically successful in all patients, and mean intraoperative blood loss was 2400 cc. Several other studies also have demonstrated a similar benefit to preoperative embolization.^14–19

EMBOLIZATION TECHNIQUE

The technique for embolization initially involves identifying the normal arterial supply to the region, including selective injections of appropriate vessels (as discussed previously). Once the anatomy has been delineated, super-selective catheterization of the vessels supplying the lesion can be performed.^7,10 After this, embolization is typically performed using particulate agents, such as polyvinyl alcohol (PVA) spheres. Again, care should be taken to ensure that there are no anastomoses seen between the vessel being embolized and supply to the anterior or posterior spinal arteries. Typically, initial particle size will be between 100–300 μm, with progressive increase in size as distal flow slows. The use of proximal occlusion with Gelfoam should be reserved until after the distal tumor bed has been embolized with small particles. Proximal occlusion alone carries the risk of recanalization and recruitment of new blood flow into the tumor bed and has been shown to be not as effective as particulate embolization.^7,12 In patients in whom tumor supply is shared with supply to the ASA, extreme care should be taken in performing the embolization. In a recent series, Prabhu et al²⁰ described their experience with embolizing spinal tumors, and in 29 of their 51 patients, tumor supply was shared with the radiculomedullary artery. In 80% of those patients, they were able to embolize more than 80% of the tumor with careful microcatheter placement beyond the origin of the radiculomedullary artery.

The timing of preoperative embolization also is critical. Patients with cord compression should be embolized immediately before surgery because there can be swelling or hemorrhage of the lesion post-embolization, which can worsen the cord compression. Patients without myelopathy or compression ideally should be embolized within 48 hours of surgery to prevent recanalization or recruitment of collateral flow. Special care also should be taken to observe the femoral access site and distal pulses during the operation because the patient might be prone for several hours. In select cases, the use of a groin closure device might be appropriate to minimize the risk of groin hematoma.

BALLOON TEST OCCLUSION AND PERMANENT VESSEL OCCLUSION

For patients with cervical spinal lesions, it may be beneficial to perform a balloon test occlusion (BTO) of the vertebral artery on the side of maximal tumor involvement to determine the safety of sacrificing this artery either surgically or endovascularly. When performing BTO, the patient typically is kept awake, and neurophysiological monitoring (including physical examination, as well as continuous electroencephalography (EEG), somatosensory evoked potentials (SSEPs), and brainstem auditory evoked potentials) is performed. Typically, the balloon is inflated in the vessel distal to the highest level of tumor, if possible, so as to simulate the level of vessel sacrifice at surgery, and kept inflated for 15–20 minutes.¹³

If the patient passes the BTO, permanent vessel occlusion (PVO) may be appropriate in certain situations. Either coils or balloons can be used for vessel sacrifice. In patients with hypervascular tumors such as renal cell carcinoma involving the cervical spine in whom resection is planned, it may be beneficial to perform a BTO followed by PVO in advance of particulate embolization of the tumor bed. This allows a more aggressive embolization because the risk of reflux of embolic particles into the vertebrobasilar system has been mitigated. For patients with relatively hypovascular tumors, it may be beneficial to endovascularly trap the vessel after BTO by occluding the vessel both proximal and distal to the involved levels.

SUMMARY

Preoperative angiography and embolization are useful adjuncts in selected patients with spinal tumors. Endovascular therapy should be considered in all patients with known hypervascular tumors, such as renal cell carcinoma or thyroid carcinoma. In patients with cervical spinal tumors, preoperative angiography with possible test occlusion, parent vessel occlusion, and tumor embolization can facilitate safe resection of difficult lesions. Close communication between the surgeon and endovascular therapist is critical to ensure safe, effective therapy.