Arterial Anatomy of the Spine and Spinal Cord

Introduction

Spinal digital subtraction angiography (SpDSA) is the gold standard imaging modality for evaluating vascular anomalies of the spine and spinal cord. Endovascular treatment of spinal vascular lesions was pioneered in the late 1960s 1–3 shortly after the introduction of selective spinal angiography itself.⁴ Performing diagnostic and therapeutic SpDSA requires a solid understanding of the relevant vascular anatomy. This chapter offers an introduction to the arterial anatomy of the spine and spinal cord and discusses the basic principles of SpDSA.

Each primitive dorsal aorta provides three groups of branches aligned in ventral, lateral, and dorsal rows. The dorsal rami start emerging early at the 6-somite stage, shortly after the still-unfused aortas have branched off the first ventral (or vitelline) arteries and slightly before the appearance of the first lateral branches.⁵ The dorsal branches stem from the primitive aortas in between somites, adopting an intersegmental rather than segmental distribution. These intersegmental arteries (ISAs) initially consist of capillary loops established between a dorsal aorta and the ipsilateral posterior cardinal vein. Later, the arterial and venous sides of the loops develop into the ISAs per se and their corresponding veins (Fig. 88-1, A-B).⁵ Rami sprouting from the arterial limb of the loops connect to similar vessels coming from contiguous ISAs to form longitudinal capillary plexi along the ventrolateral surface of the neural tube (Fig. 88-1, C). These plexi soon extend dorsally to cover the lateral surfaces of the neural tube as well.⁵ Transverse channels developing within this network form the primitive anterior and posterior radicular arteries, which are the first and, at this time, only branches of the ISAs. Capillaries extending further medially from each ventrolateral plexus establish a second set of longitudinal chains along the lateral edges of the floor plate of the neural tube. These chains correspond to the primitive anterior spinal arteries (ASAs), which will later coalesce more or less completely into a single ventromedian channel, the basilar artery cranially and the ASA caudally. In view of this developmental history, the dorsal branches and their capillary plexi for the spinal cord represent the fundamental anatomy of the ISA. At the adult stage, they correspond to the ISA stem and the spinal component of the dorsospinal artery (DA). Later-appearing branches include a dorsal branch (dorsal component of the DA) and a lateral branch (intercostal or lumbar arteries of the adult nomenclature) (see Fig. 88-1, C). The final configuration of the ISA depends upon the branching pattern of these secondary arteries; in particular, a DA exists only when the origin of the lateral branch is proximal to the takeoff of the spinal branch (Fig. 88-2).

FIGURE 88-1 Intersegmental artery (ISA) development in human embryo (adapted from Evans ⁵). Upper left inset identifies reconstruction levels. Simple capillary loop is present in 2.5-mm embryo *(A. Stage 11)*. A capillary network then appears along anterior and lateral aspects of neural tube *(B. Stage 12)*. Midline fusion of primitive aortae has started. In 11/14-mm embryo *(C. Stage 17)*, a ventral radicular branch has developed and forms anastomotic chain along anterior aspect of neural tube, with vessels coming from adjacent levels. First sulcal arteries are identified. Secondary dorsal and lateral branches that will develop into spinal and intercostal/lumbar arteries of adult ISA are now visible. (©2012, Lydia Gregg, CMI.)

FIGURE 88-2 Branching pattern of intersegmental arteries (ISAs) shown by flat-panel catheter angiotomography (FPCA). Left side shows classic branching pattern, including aortic stem *(arrow)*, lateral (or intercostal) branch *(arrowhead)*, and dorsospinal artery (DA) *(double arrow)*, with its spinal *(small arrowhead)* and dorsal *(small arrow)* components. On right, spinal and dorsal branches originate separately from intersegmental stem; there is therefore no DA. Fundamental constituents of ISA are aortic stem and its spinal branch. Lateral and dorsal musculocutaneous branches develop later. (©2012, Philippe Gailloud, MD.)

An understanding of the ISA based on its developmental anatomy is applicable at any vertebral level. Figures 88-3 and 88-4 show the typical configuration of thoracic and lumbar ISAs. Caudally, the median sacral artery (MSA) is the true continuation of the abdominal aorta beyond the takeoff of the common iliac arteries, derived themselves from the umbilical arteries^6,7 (Fig. 88-5). The MSA remains temporarily connected with the capillary network supplying the hindgut and with the primary metanephric plexus.^8,9 Through these connections, the MSA occasionally provides branches normally arising from the abdominal aorta,⁸ such as a renal artery (Fig. 88-6, A) or a superior rectal artery (Fig. 88-6, B). At the cervical level, longitudinal anastomoses linking the first six ISAs form the vertebral arteries (VA)¹⁰ (Fig. 88-7). According to Bracard, the VA is better described as a hemodynamic solution than a true artery.¹¹ The distal V3 and V4 segments of the VA are formed by the enlarged spinal branch accompanying the first cervical nerve,^12,13 a persistent segment of the proatlantal artery.¹⁴ The original intersegmental configuration, fairly apparent in children, accounts for the typical sources of collateral supply observed at the adult stage (Fig. 88-8).

FIGURE 88-3 Flat-panel catheter angiotomography (FPCA) of left T6 showing thoracic internal segmental artery (ISA) anatomy. Typical anatomy of left thoracic ISA in coronal **(A)**, sagittal **(B)**, and axial **(C)** planes. Two-dimensional nature of angiography results in superposition of many branches that can at times be difficult to distinguish (e.g., prominent dorsal muscular branch *(white arrowhead)* may adopt course reminiscent of spinal contributor). Because of leftward position of thoracic aorta, ISAs are longer on right side than on left, in particular at upper thoracic levels, where left ISAs adopt an initial recurrent course before curving sharply backward to pass behind mediastinal attachment of endothoracic fascia *(white arrow)*. Recurrent osseous branch is stretched over lateral aspect of vertebral body *(gray arrowhead)*. Gray arrow points at dorsal branch of ISA (or dorsal component of dorsospinal artery) as it divides into its terminal musculocutaneous branches. (©2012, Philippe Gailloud, MD.)

FIGURE 88-6 Anomalous branches of median sacral artery (MSA). A shows an inferior polar artery *(arrow)* coming from MSA *(arrowhead)*. In B, superior rectal artery *(black arrow)* originates from MSA *(white arrow)*. Although rare, these variants must be kept in mind when seeking elusive sources of hemorrhage from kidney or bowel. (A courtesy Dr. Ali Sultan, University of Miami, Coral Gables, Florida; B ©2012, Philippe Gailloud, MD.)

Arteries of the Vertebral Body

The vertebral body is vascularized by anterolateral and posteromedian osseous branches (Fig. 88-9). The anterolateral arteries originate from the stem of the ISA; they are divided into ascending, descending, and recurrent branches. Because of the leftward position of the aorta, they are more numerous on the right side than on the left.¹⁵ Ascending and descending branches connect with corresponding vessels from adjacent levels; these anastomoses often act as collateral supply pathways. They also participate in several anatomic variations described later in this chapter. After a short ascending course, the recurrent branch curves medially with a typical 90-degree angle to establish an anastomosis with the contralateral recurrent artery¹⁵ (Fig. 88-10).

FIGURE 88-9 Arteries of vertebral body. Subtracted axial reconstruction of three-dimensional digital subtraction angiography (3D-DSA) acquisition of left L3 documents two main groups of branches vascularizing vertebral body: anterolateral *(white arrowheads)* and posteromedian *(white arrows)* osseous arteries. Posteromedian arteries are branches of retrocorporeal artery that penetrate posterior wall of vertebral body through basivertebral foramen *(asterisk)*. Gray arrowhead points at recurrent osseous branch, splayed over anterior wall of vertebra. (©2012, Philippe Gailloud, MD.)

The spinal branch of the ISA usually divides into a radicular artery, coursing medially along the corresponding nerve root, and a retrocorporeal artery that provides the posteromedian osseous branches. Over the posterior aspect of the vertebral body, the retrocorporeal artery participates in the formation of a rich anastomotic network by joining analogous branches from adjacent levels. The retrocorporeal artery is frequently targeted during preoperative spinal embolization, so the presence of dangerous anastomoses within this network has to be kept in mind (Fig. 88-11). The cluster of posteromedian osseous branches is often associated with a prominent blush that can mimic a vascular anomaly (Fig. 88-12). Similarly, an enhancing Schmorl’s node can simulate a more aggressive pathology such as a metastasis (Fig. 88-13). The three branches of the ISA passing through the neural foramen are also known as the anterior, intermediate, and posterior spinal canal arteries.¹⁵ The anterior and intermediate branches are the retrocorporeal and radicular arteries, the posterior one is the prelaminar artery.¹⁶ These branches can have a common origin (i.e., the spinal branch in its complete form¹⁶) or originate separately (Fig. 88-14). The left and right prelaminar arteries establish an anastomotic network along the anterior aspect of the posterior vertebral arch that is often difficult to differentiate from the retrocorporeal network on angiographic projections. A modification of the retrocorporeal anatomy is seen at the level of the odontoid process, which is principally vascularized by two branches of the VA, the anterior and posterior ascending arteries^17,18 (see Fig. 88-7). The posterior ascending artery is a cranial extension of the C2 retrocorporeal artery that courses upward within the anterior epidural space. It assumes a typical ogival shape around the odontoid process as it connects with the contralateral posterior ascending artery to form the apical arcade^17,20 (see Fig. 88-29, B). The odontoid process receives additional supply from the ascending pharyngeal artery, notably a branch joining the apical arcade via the hypoglossal canal.¹⁹

FIGURE 88-11 Retrocorporeal network in a 3-year-old boy. Left T12 injection shows diamond-shaped retrocorporeal network over three levels *(T11, T12, L1)*. A prominent posterior spinal contributor originating from left L1 *(arrows)* is opacified through the network. Visibility of retrocorporeal network decreases with age, so dangerous connections must always be looked for during spinal embolization. (©2012, Philippe Gailloud, MD.)

A selective ISA angiogram usually results in visualization of a hemivertebral blush. The extent and conspicuity of that blush varies among individuals. Anastomoses linking osseous branches from adjacent/opposite ISAs explain the common visualization of partial or even complete hemivertebral blushes in adjoining vertebrae, or of a bilateral blush at the studied level. In children, the vertebral blush appearance depends upon the maturation of the growth plate and the progressive dominance of the anterolateral group of osseous branches over the posterior median one (Fig. 88-15) (Gailloud, unpublished data). The evolution of the blush also reflects the transition of the vertebral vascularization from the rich arterial network seen in children to the end-artery configuration of adults, a switch completed at about age 15.²⁰ The conspicuity of the hemivertebral blush decreases with age as bone marrow is progressively replaced with fat.

FIGURE 88-15 Prominent endplate blush *(arrowheads)* seen in this left T6 angiogram in an 8-year-old boy is typical for children and should not be interpreted as an abnormal finding. (©2012, Philippe Gailloud, MD.)

Muscular and Cutaneous Branches

The stem of lumbar ISAs gives several superficial and deep branches to the psoas muscle (see Fig. 88-4, B).¹⁵ The paraspinal musculature is principally vascularized by the dorsal component of the DA via its medial, intermediate, and lateral muscular branches. Posterior perforators of the lateral branch of the ISA also contribute, either directly or by anastomosing with the lateral muscular branch of the DA (see Fig. 88-4, B). Despite extensive interconnections, the muscular branches continue to exhibit a segmental pattern at the adult stage. The terminal skin arteries, on the other hand, have less predictable territories owing to cutaneous shifting and vascular redistribution.⁵ Anastomoses between the osseous, muscular, and cutaneous branches of adjacent or opposite ISAs offer a wide range of potential collateral pathways. Figure 88-16 illustrates some typical connections, in particular the anterior and posterior laterovertebral anastomoses, which are commonly involved in anatomic variations and represent potential dangerous anastomoses during endovascular procedures.