Transmedullary Venous Anastomoses: Anatomy and Angiographic Visualization Using Flat Panel Catheter Angiotomography

FPCA Acquisitions

All FPCA acquisitions were successful, with no intra- or postprocedural complications. FPCA is primarily performed in our practice to complement angiograms with negative findings in patients with progressive myelopathies and MR imaging findings suggesting venous hypertension, which explains the high ratio of studies with normal findings (5 of 8). 3D spinal DSA is the preferred technique for the evaluation of fast-flow lesions such as arteriovenous malformations or perimedullary arteriovenous fistulas, while FPCA is occasionally performed for slow-flow lesions (eg, spinal epidural AVFs). Finally, the spinal venous system is not specifically studied when FPCA is performed to investigate an arterial condition (crus compression syndrome, for example).

The principal advantage of FPCA over other vascular imaging techniques such as CTA or MRA is its higher spatial resolution.³ As a drawback of its acquisition mode, FPCA will display arteries and veins simultaneously. These can, however, be differentiated on the basis of their course, branching pattern, and correlation with the corresponding 2D spinal DSA anatomy.³

Anatomy of the Perimedullary and Intramedullary Venous Systems

The perimedullary venous system includes the AMSV and PMSV, which are 2 anastomotic chains respectively coursing over the anteromedian and posteromedian fissures. The smaller paired PLSVs lie along the dorsal nerve rootlets (Fig 9A). ^9,10 The principal longitudinal venous axes are interconnected by the coronary plexus of the pia mater.¹¹ The coronary plexus generally includes a few larger perimedullary anastomoses that can be visualized during spinal DSA, if the venous phase is of sufficient quality. However, perimedullary anastomoses are not clearly distinguished from TMVAs without the multiplanar assistance of FPCA (Fig 1).³

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Fig 9.

Artistic representation of the normal anatomy of the spinal venous system, emphasizing the 2 classic types of transmedullary venous anastomoses. A, The central veins establish longitudinal connections within the central gray matter (paracentral longitudinal anastomosis) and in the depth of the anteromedian fissure (sulcal longitudinal anastomosis). B, A centrodorsolateral TMVA is formed by a central vein connecting to a paracentral vein and continuing as a long oblique venous channel joining the ipsilateral PLSV. C, A median anteroposterior TMVA is formed by a large connection established between a central vein and a septal vein, with a typical loop around the central canal. Copyright 2015 Lydia Gregg.

The intramedullary venous network is divided into central and peripheral compartments (Fig 9A). The central veins drain the anterior and median portions of the spinal parenchyma¹² and terminate into the AMSV. Neighboring central veins can establish longitudinal anastomotic chains, both within the central gray matter (paracentral longitudinal anastomosis) and the anteromedian fissure (sulcal longitudinal anastomosis).¹² Paracentral longitudinal anastomoses can participate in aberrant intramedullary drainage patterns similar to the developmental venous anomalies seen in the brain or brainstem.⁴ When prominent, sulcal or paracentral longitudinal anastomoses can be visualized with FPCA (Fig 7E).

The peripheral veins course radially to end in the coronary plexus. Those located within the anterior and posterior radicular fasciculi are generally more developed.¹³ The septal veins drain into the PMSV. All intrinsic veins drain centrifugally into the perimedullary venous system.^14,15

Transmedullary Venous Anastomoses

Two types of TMVAs are classically recognized. The centrodorsolateral type was described by Kadyi¹¹ in 1889 (Fig 10): A large anastomotic connection is established between one of the central veins and one from the posterior surface of the spinal cord. The small trunk designated by “v” does not course within the median septum, but leftward to reach the surface of the spinal cord in the vicinity of the line of emergence of the left posterior nerve rootlets…. The venous anastomosis “a” forms a loop, both legs of which run parallel to the central canal: one goes down, the other one goes up and follows a S-shaped curve before reaching the trunk of the corresponding central vein “c” (p 146, P.G. translation). In 1939, Herren and Alexander¹⁴ further defined the centrodorsolateral anastomosis as being formed by a central vein connecting to a paracentral vein, itself continuing as a long oblique venous channel that terminates in the ipsilateral PLSV (Fig 9B). They specifically named this last oblique portion “centrodorsolateral anastomosis,” while Crock and Yoshizawa¹⁶ applied the term to the whole anastomotic chain, from the AMSV to the PLSV.

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Fig 10.

Illustration reproduced from Kadyi H. Über die Blutgefässe des Menschlichen Rückenmarkes. Lemberg: Gubryonowisz & Schmidt; 1889 (Fig 9, page 146) shows the first TMVA depiction known to the authors (ie, a sagittal view of a centrodorsolateral TMVA). The vein, designated as v, courses laterally and posteriorly from the midsagittal plane to a PLSV. The segment a connects to a central vein c after looping around the central canal.

Crock and Yoshizawa¹⁶ described a second type of TMVA linking the AMSV to the PMSV, the median anteroposterior anastomosis (Fig 9C). This type of TMVA probably also derives from a connection established between a central vein and a paracentral vein, which then continues dorsally as a septal vein rather than adopting a dorsolateral course.¹⁶

Case 7 revealed 2 previously unreported TMVA configurations. The first was a median anteroposterior anastomosis with a duplicated origin from the AMSV and from the surrounding venous network (Fig 7B); the second combined median anteroposterior and centrodorsolateral characteristics, draining both dorsally into the PMSV and dorsolaterally into a PLSV (Fig 7D).

While initially believed to be rare,¹² both types of TMVAs were consistently documented in a microangiographic study of postmortem material, though with irregular sizes and topographic distributions.¹⁵ The highest prevalence was noted in the upper cervical and cervicothoracic regions (up to 2 TMVAs per centimeter). The largest channels were also found in the cervicothoracic region.¹⁵ The distance separating 2 TMVAs was increased in the lower thoracic region, while no TMVAs were noted in the lumbar region. They were occasionally documented at the level of the conus medullaris, either supplementing or replacing the anastomotic venous circle of the conus.¹⁵ Centrodorsolateral TMVAs were more frequent but generally smaller (0.1–0.2 mm) than median anteroposterior TMVAs (0.3–0.7 mm).¹⁵ Both types can be sinuous or can adopt a relatively straight intramedullary path.¹⁵ The relation between the size, number, and course of TMVAs and factors such as age and sex is currently unknown.

TMVAs receive relatively few intramedullary tributaries even though they are generally comparable in size with the principal longitudinal venous axes.^14,15 Herren and Alexander¹⁴ proposed that rather than having a drainage function, TMVAs might help regulate pressure and flow by rapidly transferring blood from one longitudinal axis to another.¹⁴ This hypothesis was later supported by Thron and Rossberg,¹⁵ who suggested a correlation between the higher cervical and cervicothoracic prevalence of TMVAs and the greater motion range of the spine and spinal cord at these levels. TMVAs might thus protect the principal venous axes from pressure variations related to compression occurring during hyperflexion and hyperextension of the cervical spine.¹⁵ Our last example (case 8) suggests that under abnormal circumstances leading to impaired venous flow, TMVAs may indeed act as alternate drainage pathways limiting the risk of venous engorgement and ischemia (Fig 8). In this patient, a sulcal longitudinal anastomosis also appeared to play the role of a collateral pathway in the area compressed by protruding disk material.

Recognizing TMVAs as normal structures carries both diagnostic and therapeutic implications. For example, TMVAs were recently used as an access route for successful obliteration of a perimedullary arteriovenous fistula.⁸ Median anteroposterior TMVAs have been documented by contrast-enhanced MRA and by DSA,^6,7,17 while centrodorsolateral TMVAs have, to our knowledge, not been imaged clinically until now, probably because of their smaller size.¹⁵ New imaging methods such as spinal SWI also require a sound understanding of the spinal venous system anatomy and its variants, for example, to differentiate TMVAs from small intraparenchymal hemorrhages.¹⁸

In summary, the introduction of novel imaging techniques, such as spinal FPCA or SWI, can strengthen our still deficient understanding of medullary venous pathology. This improved imaging ability creates the need for a more intricate knowledge of the anatomy of the spinal venous system. This article reports several types of TMVAs documented by FPCA. Some of these vessels were previously known, in general from postmortem investigations, while others had not yet been described. Appreciating the existence of TMVAs is clinically important because these channels may be confused with intramedullary hemorrhages or vascular malformations on noninvasive imaging.