VENOUS ANATOMY OF THE SPINE (under construction)
 
The man benhind the scene, par excellance!   The venous system holds the strings of understanding such vascular diseases affecting the spinal column as our imperfect knowledge has, up to date, painfully acquired.  The arterial system, in its elegant complexity, can seldom claim pre-eminence in primary injury to the spine.  Arterial strokes of the cord are rare, and their morbid anatomy relatively straightforward.  Far more important, and intriguing, are the varied and imperfectly understood injuries to the cord inflicted by dysfunction of the venous apparatus  — hypertension, high-flow venopathy, and outflow obstruction, for example.  The prototypical vascular condition of the cord — dural AV fistula – manifests its clinical and anatomical consequences as a function of progressive venous failure.   In the brain, where arterial pathology seems to predominate, we do not hold the venous system in the same regard.  I hope that this is at least partially justified by our relatively superior understanding of the intracranial venous apparatus in comparison with the spinal one, though I personally consider the current state of affairs to be unjustified.
 
The study of spinal venous system presents a number of difficulties, accounting for our quite limited understanding of the problems at hand.   We do not have ANY reliable noninvasive method of visualizing this system.  Postcontrast MRI affords frequent glimpses, but it is not optimized; any future gains in resolution would be meaningless in view of physiologic motion that so profoundly influences even the catheter angiogram – an undisputed queen of spatiotemporal resolution.  Moreover, catheter angiography is seldom perfomed in absence of pathology and, anyway, is suboptimal for visualization of the venous system of the spine, diluted by multiple adjacent unopacified contributors.  Spinal venograms, a sourse of excellent anatomical and even functional material, are now performed with the frequency of a pneumoencephalogram.  The best anatomical demonstrations, therefore, come from post-mortem studies, which produce exquisite detail but offer no direct hemodynamic information and are subject to multiple technical artifacts.  Perhaps most importantly, basic research in this area is painfully rare.  The last comprehensive treatment on the subject of venous anatomy was provided by Armin Thron, in his brilliant work “Vascular Anatomy of the Spinal Cord“, published in 1988.  It was soon therearfer heavily referenced in the Lasjaunias and Berenstein’s “Surgical Neuroangiography”, first released in 1990.  I do not think that any important contribution to the subject was made since that time. (please feel free to correct me if you are in the field, and come accross this section — contact us page has the required info). What we do know comes from a synthesis of existing anatomical works and a far less perfect understanding of the functional anatomy.  The latter is based principally on pathophysiologic material, and therefore may be subject to erroneous inferences.
 
Broadly speaking, the vertebral venous system can be divided into three components — the intradural (extramedullary and intramedullary veins), extradural (epidural) and intraossesous/paraspinal veins.  Classically, the names are anterior external, anterior internal, posterior internal, and posterior external venous plexuses, corresponding to intradural and extradural networks.
The veins are a capacitance network — about 75% of intracranial blood pool at any given time is situated in the veins.  The same probably goes for the spinal cord, if not more.  The intramedullary and extramedullary systems are highly redundant and therefore fail only under extreme circumstances.  The weak link, and threfore site of pathology, are the radiculomedullary veins bridging the two systems across the dura.  These veins are relatively fewer, and have no seconary support network accross the thick dura mater.  Thrombosis of these veins is the immediate proximal cause of venous hypertension experienced by patients with spinal dural fistulas.
 
 
 
 
a –centripetal network of veins, predominantly draining the gray matter into: b – central (sulcal) veins of the intrinsic system; c – peripheral (radial, a.k.a. marginal) centrifugal veins of the intrinsic system; d – venous anastomosis between the centripetal and centrifugal systems; e – anterior (ventral) median vein; f – posterior (dorsal) median vein; g – transmedullary anastomosis between dorsal and ventral median venous systems; h – extrinsic surface anastomosis between dorsal and ventral median veins; i – vein of filum terminale; j – dominant radicular vein of the cauda equina; k – radicular vein (this is the weak link between the cord venous system and the extradural space); l – nerve root sleeve; m – shallow angle of radicular vein piercing the dura of the nerve root sleeve; n – intervertebral vein; o – radicular veins of the cauda equina; p – anterior epidural (a.k.a. ventral intrinsic) venous plexus; q – posterior epidural (a.k.a. dorsal intrinsic) venous plexus; r – ascending spinal (lumbar) vein; s – basivertebral vein, draining the intravertebral body venous plexus (t); u – anterior extrinsic venous plexus surrounding the surface of the vertebral body; v – posterior extrinsic venous plexus on the surface of the lamina /posterior elements, also participating in drainage of the paraspinal muscles
 
 
The intramedullary venous network 
The spinal cord is drained by a redundant, centripetally arranged veinous network which extends to the cord surface in a semi-organized fashion.  These intramedullary veins (also known as radial veins) are angiographically invisible in normal state.  Once on the surface, the spinal veins are organized in a loose network that is much more distributive than the arterial system.  An anterior and posterior longitudinal spinal veins are described; these run along the length of the cord in an interconnecting fashion, and go by various names such as “anterior and posterior coronal veins”; the anterior vein may pass for a discrete entity and may be called “anterior median spinal vein”; but variability is the rule.  Basically, there are veins that run along the spinal cord (extramedullary intradural) and collect blood from intramedullary radial and other veins. The longitudinal veins can be angiographically visualized in the cervical cord segment with relative consistency in the non-patholgic state.  Why not thoracic or lumbar, as same venous network exists throughout the cord?  This one is for you to figure out…
 
Below is a series of lateral digital subtraction angiography projections of the cervical spinal cord segment in arterial, early venous, and late venous phases.  A certain illusion of anterior (light blue) and posterior (dark blue) venous systems may be formed by looking at the 3-D cyllinder of the cord in two dimensions end-on.  The real situation may be different but the end-on effect serves to accentuate intravenous contrast at the edges of the cord.  In general, though, the anterior veins, like the arteries, are more constant.  Of course, visualization of spinal cord veins assumes that the spinal artery (red)  was injected — in this case coming from the vert.  This is not always the case (see above).  Notice also that even parenchymal contrast can be seen within the cord (yellow); this infrequent occurence may be ascribed to a combination of prominent ASA, dense contrast, and thin neck (the answer to the above paragraph) 
 
Contrast-enhanced MRI will often show a normal anterior median spinal vein, particularly in the sagittal plane where it weaves in and out of the field of view.  Little use can be made of this observation under normal conditions, except not to confuse it for leptomeningeal disease.  Occasionally, spinal venous thrombosis may be suspected on MRI in the appropriate clinical setting (see below).  Spinal cord venous infarcts, although likely underdiagnosed, are still quite rare, while awareness of their very existence is rarer still.  Literature on subject understandably consists of individual cases.  One good one is by: Niino M, Isu T, and Tashiro K.  Nonhemorrhagic venous infarction of the spinal cord without spinal vascular malformation. Journal of Neurology 1999; 246 (9) 852-4 Link: http://www.springerlink.com/content/l412g0t09r8djl2g/ 
 
Spinal venous network
Injection of a dominant (Adamkiewitz, in this case) radiculomedullary artery should allow, with use of modern equipment and appropriate technique (meaning general anesthesia, full paralysis, apnea during image acquisition, and glucagon-induced bowel paralysis, if necessary) for visualization of the spinal veins at the appropriate time point (perhaps around 5 seconds artery to vein circulation, with variability depending on conditions).  Failure to visualize spinal veins under these conditions, particularly when looking for a spinal fistula, should be taken as indirect evidence of existence of such fistula elsewhere, with secondary venous congestion.
Images 1-5 in time sequence, from earliest to latest.  In this patient, a particularly dominant “radiculomedulopial” artery (white) supplies both the anterior spinal artery (red) and a small left posterior spinal artery (yellow arrow, image 1).  Two “coronary” arteries (pink) opacify the right posterior spinal network (yellow arrows of images 2, 3, and 5) via the radiculomedullary artery.  These coronary arteries run on the surface of the cord and, by definition, supply the posterior spinal axis via the anterior spinal artery.  Delayed phase images visualize the perimedullary spinal venous network (blue arrows, images 4 and 5).  Image 5 is the same timepoint as 4, with the mask image adjusted to show the arterial phase in white.
 
 
Medullary veins (a.k.a. radiculomedullary veins)
These veins serve as a link between the intramedullary and extramedullary (internal and external) venous plexuses.   They run a similar oblique course as the radiculomedullary arteries.  These can be visualized pretty consistently via catheter angiography following injection of a relatively prominent radiculomedullary artery.  They typically come off at a different level from the radiculomdullary artery.  Only the bigger ones are visible angiographically, and it is likely that even quite functional ones are too small to see in the normal state.   These veins have a critical role in pathophysiology of cord disease in spinal dural fistulas (see below).
 
Below the conus medullaris, normal radicular veins of the individual cauda equina roots may be visible (each nerve root has its own artery and vein).  In the thin, young patient below, these normal veins are unusually well seen.  Images to left and center are in the arterial phase of a large T6 origin radiculomedullary artery injection, imaged at the thoracolumbar junction (level of conus).  The ASA (red), and coronary artery (pink) are visible, as well as a very small normal radicular artery (brown).  Venous phase image on the right shows the dominant spinal vein (blue) and multiple thin radicular (medullary) veins (light blue) paralleling course of the cauda nerve roots.  Again, these are normal veins, particularly well seen due to thin body habitus and young age of this patient.
 
 
 
Extrinsic Venous Network
It can be a little problematic to visualize this network from catheter injections.  Venograms are a thing of the past, mostly.  A segmental vessel injection can visualize only a part of the network subserving the arterial distribution of the interrogated vessel.  For anatomical demonstration purposes, one can resort to some unusual methods, such as reflux of IV contrast administered into an upper extremity in an individual with distal venous outflow obstruction — such as this patient with subclavian stenosis (orange) status post pacemaker placement (red).
 
The extrinsic network consists of anterior epidural plexus (purple) and a generally smaller posterior epidural plexus (not seen).  The medullary veins drain into a venous network within and around the nerve sheath (dark blue) which is named “emissary veins”, and classically consists of doral and ventral (with respect to the nerve root sheath) emissary vein.  These emissary veins connect the dorsal (posterior) and ventral (anterior) epidural plexi with the longitudinal efferent veins (white and green arrows, not well demonstrated) which run craniocaudally along the vertebral body. At cervical level, the vertebral venous network which envelops the vertebral artery is particularly well seen (light blue).  An extensive paraspinal network subserving the posterior elements and adjacent musculature is present (pink), which is of little clinical consequence.  The longitudinal efferents eventually drain into the azygous system (light green) or ven cava. 
 
 
 

 

 
Anterior extrinsic (Anterior Epidural) venous  plexus — catheter angiography
In this patient, an unusual fistula between branches of the internal iliac artery and anterior epidual venous plexus allows for visualization of the venous anatomy from an arterial injection.  Unlike most spinal dural fistulas, this one does NOT drain via the radicular veins into the intrinsic venous plexus system directly related to the cord, but instead congests the extradural networks and impairs nerve root drainage.
 
 
A 4F RDC catheter (black arrow) is engaged in the proximal right internal iliac artery.  The center image is a labeled version of the otherwise identical left image.  The right “native” image is enhanced with white ovals to mark the pedicles and orange lines to indicate expected boundaries of the nerve root sleeve.  A number of arterial channels belonging to the L5 and S1 nerve root sleeves (red arrows) converge onto a fistula which opens into an epidural venous pouch (pink, part of “G” or anterior external venous plexus) behind the L5 vertebral body.  The resultant congestion of the venous apparatus allows for angiographic visualization of the same venous anatomy which was illustrated on the CT scan above and diagram below.  From the fistulous pouch (pink), contrast opacifies the diamond-shaped anterior extrinsic (ventral epidural) venous plexus (dark blue arrows, letter “G”).  This network drains outside of the spinal column via emissary veins which surround the nerve root sleeve – venous channels superior (light blue, “F”) and inferior (purple, “F”) to the nerve root sleeve are visualized at multiple levels.  The emissary veins empty into longitudinal efferent veins (yellow arrows, “I”), more prominent on the left, and located on the side of the vertebral body, usually anterior to the nerve root.  The spinal cord veins would normally drain into radicular veins on the inside of the nerve root sleeve (not visible from this injection) which would then empty into the emissary veins.
 
 
 
 
The same fistula in time sequence, from left to right, corresponding to earlier to later phases.  The labels in leftmost picture are the same. The images are correlated with the diagram above. Notice particular congestion of the left L5 emissary vein (purple, “F” in diagram).  The second to left image shows early appearance of the inferior vena cava (orange), which is progressively better filled on subsequent images.  The common iliac vein (green) is also labeled on the rightmost injection, but first seen on the second to left image.  Both the IVC and internal iliac vein are opacified from the longitudinal efferents (yellow, “I” in diagram) via dorsoventrally oriented veins (white arrows) which are markedly foreshortened on the AP pojections, and whose depth is better appreciated on the stereoscopic images below.  Blue arrow, or “G”, visualizes the anterior extrinsic venous plexus, into which the fistula drains.
 
 
Stereo pair of the fistula and its recepient venous network, for enhanced appreciation.  An earlier phase image (right) better shows the arterial network (red) leading into the venous pouch (pink).  The arteries seem to converge on a single point (brown), which is the fistula itself.
 
 
Stereo pair and corresponding labeled image, helping demonstrate the nature of the fistula, which consists of innumerable arterial channels (red) running predominantly towards the right L5 foramen, but also with significant contribution via the right S1 radicular arteries (white).  The fistula itself is therefore located distal to both of these structures, in the ventral epidural space.
 
Finally, a microcatheter injection of the dominant L5 contributor elegantly demonstrates how innumerable arterialized channels (red) ultimately converge onto several common arterial conduits (green), which traverse the neural foramen to supply what is, here and almost ALWAYS, a single hole fistula (brown).  For embolization to be curative, the n-BCA (or Onyx) must completely fill the fistula (which, in this case, would be evidenced by visualization of the large epidural venous pouch (pink) on the glue shot