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.