Development — this abbreviated, important section precedes discussion of adult anatomy. A more complete discussion is found in the dedicated section of neurovascular embryology.
The basic arrangement of the spinal system consists of a metameric grid of trasversely oriented segmental vessels, connected by various longitudinal channels. This simple bit of knowledge goes a long way in understanding spinal anatomy. Millions of years of speciation have taken place upon a basic building block of the organism — the metameric segment. Just like the fly and the worm, the human body consists of metameric segments, with ecto-, meso-, and endodermal elements. Each vertebral body, its ribs, muscle, nerves, and dermatome, correspond to one level or segment. It is perhaps easiest to appreciate this concept at the thoracic level, where each rib, vertebral body, and other elements constitute the prototyical segment. In the early human embryo, the neural tube is first supplied by simple diffusion. When its limits are reached (200 micrometers perhaps), a primitive vascular system consisting of paired dorsal and ventral aortae (longitudinal vessels) and transversely oriented segmental arteries come into play to vascularize the developing tissue of the embryo.
As the tissue of spinal cord continues to enlarge, new longitudinal connections form between the transverse segmental arteries, most likely to facilitate distribution of blood within the vascular system. This pattern is seen throughout the body, but is somewhat easier to recognize in the vertebrospinal arterial system, where it gives rise to adult anterior spinal artery and numerous extradural longitudinal segmental connections which will be discussed below.
Gradual establishment of dominant longitudinal vessels leads to regression of most transverse segmental arteries, except at some levels where such vessels persist in supplying the longitudinal artery.
This process, in terms of the spinal cord, gives rise to the familiar adult appearance of the anterior spinal artery and its remaining radiculomedullary feeders, while most segmental arteries previously connected to it in early fetal life are limited to supply of the nerve root and adjacent tissues in the adult.
The same pattern of development takes place in the extra-axial, paravertebral space, where longitudinal connections between segmental arteries form a multitude of adult vessels, such as the vertebral, pre-vertebral, pre-transverse, deep cervical, lateral spinal, and other arteries, as will be illustrated below.
Adult Vertebrospinal Arterial Anatomy
The basic arterial vertebrospinal vascular unit consists of two segmental vessels, left and right, arising from the dorsal surface of the aorta. The vessel curves posterolaterally in front of the vertebral body, and sends small branches into its marrow. In front of the transverse process, the segmental artery bifurcates into a dorsal branch and an intrercostal branch. The intercostal segment supplies the rib and adjacent muscle and other tissues. The dorsal branch feeds the posterior elements and, via the neural foramen, sends branches to supply the local epidural and dural elements, as well as a radicular artery to nourish the nerve root. At some levels, the radicular artery is enlarged because, instead of supplying local neural elements, it maintained its embryonic access to the anterior spinal artery. At this level, the artery is called “radiculomedullary” because it also supplies a large segment of the spinal cord. Various other arrangements are seen, for example when radicular artery supplies portions of the dorsal spinal cord, a discontinuous network which is often misrepresented in venerable anatomical texts as a continuous system of two posterior spinal arteries. This is the basic arrangement of spinal supply.
The system varies in the cervical, upper thoracic, and sacral segments (i.e. exceptions are greater than the rule) but the basic principle of segmental dural and radicular vessels supplying neural tube elements is a very useful guide. Variation comes chiefly in form of segmental vessel origin — whereas descending aorta serves this puprose for most thoracic and lumbar segments, the vertebral artery, subclavian branches (costocervical trunk for example), supreme intercostal artery, and median sacral artery (effectively a diminuitive continuation of the aorta below the iliac bifurcation) play this role at the appropriate segments. These vessels of origin are part of the gridline of longitudinal channels which form to connect embryonic segmental vessels. For example, the vertebral artery represents a confluence of discontinuous embryonic channels termed the “longitudinal neural system” into a single trunk. This, in part, explains multiple variations and duplications encountered in the vertebral territory.
Figure 1: Somatotopic organization of the vertebrospinal arterial vasculature, highlighting segmental vascular organization of the vertebrospinal axis and homologous longitudinal anastomoses along its entire length.
As you can see, numerous longitudinal vessels exist throughout the vertebrospinal axis, often with the same vessel going by several different names, for historical reasons. For example, see above for homology between the lateral spinal, pre-transverse, and deep cervical arteries. The segmental arrangement is particularly modified in the cervical region, where longitudinal vessels are dominant — most obviously the vertebral arteries. It is important however to recognize the existence of segmental vessels connecting the three dominant cervical longitudinal arteries (ascending cervical, vertebral, and deep cervical) in terms of their anastomotic potential and its implications for both collateral revascularization and inadvertent embolization during interventional procedures.
The following diagrams provide a basic view of relevant arterial anatomy of the spinal elements, serving as a guide for interpretation of subsequent catheter angiography illustrations.
A – aorta; B – segmental artery; Ba – intersegmental arterial anastomosis; C – prevertebral anastomotic network; D – direct vertebral body feeding arteries; E – dorsal spinal artery; F – intercostal/muscular artery; G – pretransverse anastomotic network; H – dorsal division of the dorsal spinal artery; I – post-transverse anastomotic network; J – muscular branches of the post-transverse anastomotic network; K – ventral division of the dorsal spinal artery; Ka – radicular artery; La – ventral epidural arcade; Lb – dorsal epidural arcade; M – nerve root sleeve dural branch of the ventral division dorsal spinal artery; N – dural branch of the ventral division dorsal spinal artery; O – radiculopial artery; P – radiculomedullary artery; Q – anterior spinal artery; R – mesh-like pial arterial network; S, T – posterior spinal artery; U, V – pial arterial network (a.k.a. vasocorona) anastomoses between anterior and posterior spinal arterial systems, W – sulco-commissural artery, X – rami perforantes of the peripheral (centripetal) system, Y – central (centrifugal) system of sulcal arteries, originating from pial network of the cord; altogether, the pial network and rami perforantes (R+Y) are called the vasocorona or corona vasorum; Z – rami cruciantes (a.k.a. crux vasculosa, a.k.a. rami anastomotici arcuati)
In the following examples, nomenclature using the above letters will be used for correlation.
Aorta and segmental vessels. Many spinal angiograms start with imaging the biggest vessel in the body. Some are surprised to discover that these segmental lumbar and intercostal arteries (red) are actually not that small (between 1 and 2 mm diameter typically) — most can be easily engaged (and occluded) with a 5F catheter. The aortic injection gives a roadmap, may identify a particularly large fistula, and show which levels may have missing segmental arteries, thereby obviating a frustrating search. In this angiogram of a patient with a dural fistula, a congested spinal cord vein (light blue) can be seen in the venous phase (dark blue). Celiac trunk (orange) and renal arteries (yellow) are also labeled.
Typical Lumbar artery (segmental artery) injection. During spinal angiography, the segmental artery is selected with an appropriate 4F or 5F catheter (RDC, SAS). Injection rates are 1-2 cc/sec for as long as you think you need it, typically 2-4 seconds. Frame rates vary from 1-3 per second, and should not exceed 3 unless particularly necessary (to visualize microanatomy of a high flow fistula, for example). When dural or other fistula is suspected, multiple levels may need to be interrogated. One can easily go through 300 ml or more of contrast, so be aware. For metastatic disease, the search may be more focused. It is helpful to view the angiogram in both subtracted and native views to appreciate both fine vascular detail and bony landmarks.
The lumbar artery (purple, B) is relatively selectively injected, with trace opacification of hte contralateral left L3 lumbar artery due to proximity of the left and right orifices to each other. Since there is no rib, the artery does not have a prominent “intercostal” component. The arteries of the dorsal branch (red, H, J) supply the lamina and adjacent tissues, with anastomosis to the spinal process arterial arcade (yellow, I). You can see continuation of this arcade inferiorly, NOT to be mistaken for the anterior spinal artery or other spinal artery. The anterior spinal artery is straighter and has a characteristic radiculomedullary hairpin turn (see below). A large paravertebral anastomotic branch (green, G) is present, which opacifes ipsilateral L4 level dorsal branches (blue, H, J). No radiculomedullary artery is seen at this level.
Common lumbar trunk: Especially in the lower spine, single left and right lumbar artery origins are common. Absent levels are also common, usually supplied via paravertebral and prevertebral anasomoses.
Paravertebral anastomotic network – typically, this is the dominant longitudinal anastomotic connection between adjacent segmental arteries. It is particularly well visualized in young, normotensive patients. Technical considerations are also important — having the catheter well-wedged into the ostium of the segmental artery, as well as longer, higher volume injections (within reason, of course), are key to opacifying all kinds of collaterals. The paravertebral network is located along the lateral aspect of the vertebral body, adjacent to the sympathetic chain, for example. A well developed paravertebral network (blue, G) is present. The catheter (red) is engaged in a lumbar artery (brown, B) and via this network opacifies the lumbar artery of the level immediately above (purple) and immediately below (pink). Notice the spinous process arcade again (black, I). This network ensures virtual impunity for atherosclerotic or iatrogenic occlusion of a proximal segmental artery. More care should be excercised at radiculomedullary artery levels.
Multiple longtitudinal anastomotic networks — prevertebral, paravertebral, spinous process
In this patient, all three networks are demonstrated — stereoscopy is very helpful to decide which is which. Also notice prevertebral transverse and retrocorporeal networks at same level.
C – prevertebral anastomotic network; G – paravertebral anastomotic network (can opacify adjacent levels with strong injection, or supply adjacent level in case of intercostal artery hypoplasia/aquired stenosis); I – spinous process branch and associated anastomotic network connecting spinous processes; Blue — precorporeal anastomotic network (not shown in diagram); blue — retrocorporeal anastomotic network (pink color vessels in diagram, and see section below); light blue — left L1 segmental artery; brown – left T12 segmental artery; dark green — right T12 segmental artery; pink – radiculopial artery.
Another demonstration of multiple longitudinal anastomoses:
Lumbar segmental artery injection, demonstrating a well-developed post-transverse anastomotic network (I) visualized through the ventral division (H) of the segmental artery (B), with its muscular branches (J), as well as the pre-transverse anastomosis (G), both contributing to collateral visualization of the adjacent cranial segmental artery (B). F – muscular artery, homolog of the intercostal artery.
Retrocorporeal arterial network
Another view of cervical radiculomedullary artery (of Lazorthes) arising from inferior vertebral (C6 segment). This kind of dominant supply is seen less frequently for the cervical spinal cord than it is for the thoracolumbar enlargement in case of the artery of Adamkiewicz.
A, B – Frontal and C — lateral stereo pair projection digital subtraction and native angiographic views of right vertebral artery injection, visualizing a dominant cervical radiculomedullary artery (P, artery of Lazores) and the anterior spinal artery (Q), anastomosing with its basilar homolog (long white arrowhead). Very faint posterior spinal artery (T) is best seen in stereo, as well as the lateral spinal artery (short white arrow).
Although balanced supply to the cervical cord is more common, and most of the time it comes from the cervical vert, occasionally the typically small distal intracranial vertebral artery supply is dominant, as in this case. It is important to pay attention to this when flow diversion methods are used in the distal vertebral artery.
Lateral view of the same, in stereo
Anterior spinal artery duplication
Embryologic considerations dictate that both anterior and posterior spinal arteries are, so to speak, secondary vessels arising as a conduit between metameric arteries. This is also true in case of the vertebral artery, coming together as a fusion of the longitudinal neural system. This notion helps explain unusual variants such as anterior spinal artery duplication. In this patient with bilateral superior cervical dissections, it is likely that hemodynamic need resulted in hypertrophy of a rudimentary duplicated anterior spinal artery — at least it can be said that in the majority of patients were similar constraints invoke support of the anterior spinal collateral, there is still only one anterior spinal, as opposed to this case, where there are two. Frontal view of right vert injection shows high servical dissection (orange), the anterior spinal system (red, pink arrows) contributes to supply of the right PICA (black) via the C4 segment radiculomedullary artery (brown). Notice midline location on the frontal view and superimposition of the anterior spinal channels on the lateral view, confirming their anterior spinal nature. The odontoid arcade (white) also does the same, via its usual C3 segment radiculodural artery (purple)
Stereo of the same
Injection of the left vertebral artery reveals a healed, less severe superior vertebral dissection. In a mirror image fashion, the left C4 radiculomedullary artery (yellow) supplies the “left-sided” portion of the anterior spinal system (pink) which was also seen from the right vert injection. The odontoid arcade (blue) and its C3 radiculodural artery (green) are also well-seen.
Deep Cervical origin of the radiculomedullary artery — second most common after the vert. At our institution, all cases of posterior fossa subarachnoid hemorrhage with no intracranial cause REQUIRE identification of the anterior spinal artery, as in ~10% of cases (in-house experience) the pathology turns out to be in the cervical spine.
Anterior spinal artery (Q) origin from deep cervical artery, P= radiculomedullary artery; notice collateral opacification of the vertebral artery (long white arrow) via the C2 segmental artery (short white arrow).
Another deep cervical origin — any longitudinal system can give origin to the radiculomedullary artery — in this case the radiculomedullar artery (orange) originates from the deep cervical branch (red). Notice also injection of supreme intercostal artery (pink, lower two images) with extensive deep servical artery anastomoses (yellow) through which the anterior spinal artery can be inadvertently embolized. The catheter, barely engaged in the supreme intercostal, is labeled in blue.
Same patient, contralateral side, demonstrating tumor blush (hemangiopericytoma) from the right subclavian injection supplied by costocervical (purple) and thyrocervical (orange) branches. An ipsilateral supreme interconstal (red) injection demonstrates extensive additional tumor, which is not apparent from the subclavian injection. The vert is labeled in light blue.
Supreme Intercostal Origin of Cervical Spinal Artery — occasionally seen as well, and important to know. The supreme intercostal and upper thoracic arteries can be difficult to catheterize sometimes, especially in patients with capacious dilated atherosclerotic aortas. We use a 4F or 5F RDC (which can be too small for the upper thoracic spine); if that does not work, one can try an appropriately-sized Cobra, or perhaps a Simmons 1. Sometimes, hand-shaping an RDC to produce a bigger curve (so as to push against the contralateral aortic wall) is more helpful than another catheter. In this case, the supereme intercostal was visualized via the T4 segmenal injection through a prominent paraspinal anastomosis (I)
stereo pair, supreme intercostal arteyr origin of the anterior spinal artery (same legends as above), visualized via T4 injection through a prominent post-transverse anastomosis (I). Notice transient contrast reflux into a cervical radiculomedullary branch (P); another longitudinal anastomosis (white arrow) between adjacent T3, T4, and T5 segmental arteries
Supreme intercostal artery (redP origin from the vertebral artery — another example of homology between various longitudinal anastomoses. Notice multiple intercostal arteries (yellow)
Stereo pairs, demonstrating posterior course of the supreme intercostal artery at the level of dorsal ribs
Thoracic region: The artery of thoracic enlargement (Adamkiewicz) usually comes of T9 through T12 region. There is often a region of thoracic cord (mid-lower, depending on the Adamkiewitz origin, which is rather small in caliber, relative to the more well-developed cervical region vessel. A “watershed” of sorts (yellow) therefore exists which occasionally may correspond to cord infarction in states of hypotension. This double catheter injection (done for evaluation of cord infarction in the region of the basket, below the watershed) demonstrates the slender size of mid-to-lower thoracic ASA. Red=ASA; Purple=radiculomedullary arteries
The artery of Adamkiewicz. Typical appearance. Another patient, with stereo views of the radiculomedullary artery. The radiculomedullary artery (pink) often demonstrates a small segment of narrowing at the point where it pierces the dura (white arrow). The intradural segment (blue) opacifies the anterior spinal artery (red). RDC (catheter) is labeled in green.
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