Spinal Extradural Arteriovenous Malformations With Parenchymal Drainage: Venous Drainage Variability and Implications in Clinical Manifestation

Abstract and Introduction

Object. Although nontraumatic spinal arteriovenous malformations and fistulas (AVMs and AVFs) restricted to the epidural space are rare, they can lead to significant neurological morbidity. Careful diagnostic imaging is essential to their detection and the delineation of the pathological anatomy. Aggressive endovascular and open operative treatment can provide arrest and reversal of neurological deficits.
Methods. The authors report on 6 cases of extradural AVMs/AVFs causing progressive myelopathy. Clinical findings, diagnostic evaluation, treatment, and outcome are discussed. Special consideration is given to the anatomy of the lesions and the operative techniques used to treat them. A review of the literature concerning extradural vascular malformations is also presented.
Results. All 6 cases of extradural AVMs had an extradural fistulous location with intradural medullary venous drainage. These cases illustrate progressive myelopathy through cord venous congestion (hypertension) that can be caused by an extradural nidus or fistula. In 4 cases, a large epidural lake was identified on angiography. At surgery, the epidural lake was obliterated and medullary drainage interrupted. All patients had stabilization of their neurological deficits and successful obliteration of the AVM/AVF was obtained.
Conclusions. Extradural AVMs and AVFs are a poorly described entity with published clinical experience limited to sporadic case reports and small series. Although these lesions have a purely extradural location of arteriovenous shunting and early venous drainage, they can be responsible for acute and progressive neurological symptoms similar to those caused by their dural-based intradural counterparts. With careful imaging recognition of the pathological anatomy, surgical and endovascular techniques can be used for the treatment of extradural AVMs affording effective and durable obliteration with stabilization or reversal of neurological symptoms. Venous drainage directly correlates the pathologic mechanisms of presentation. Specific attention must be paid intraoperatively to the epidural lake common to both variants so that recurrence is avoided.

Spinal vascular malformations and fistulas are classified based on their vascular supply, vascular drainage, and nidus location. Although relatively uncommon lesions, they are clinically important because they can produce neurological symptoms caused by hemorrhage, ischemia, and mass effect.^[12] Selective spinal angiography has led to a better understanding of this pathological entity.

Arteriovenous shunting occurs with direct communication between the arterial and venous system. These can be plexiform AVMs (collections of dysplastic vessels supplied by feeding arteries and drained by venous channels) or AVFs (feeding artery and draining vein without an intervening nidus). The many shunt types and lesions locations have led to a number of classification systems.

Oldfield and Doppman^[10] classified spinal vascular malformations into 4 types. Type 1 is a dAVF, usually arising near the nerve root sleeve. Typically, retrograde drainage occurs via the spinal medullary veins, resulting in intramedullary venous hypertension and engorged veins on the surface of the cord. Type 2 lesions are glomus AVMs made up of a distinct mass of dysmorphic arteries and veins without a capillary bed. These may be partially or entirely intramedullary. Type 3, juvenile AVMs, are congenital malformations consisting of arterial and venous networks without a distinct nidus. Often extensive, these variably involve the spinal cord, and vertebral and paraspinal tissues (metameric AVMs). Type 4 AVMs, or perimedullary fistulas, are intradural AVFs located on the spinal cord surface with medullary venous drainage.

Spinal AVMs and AVFs confined to the epidural (also termed extradural) space and combined intraduralextradural lesions are rare and do not fall into the above classification system, yet they represent a neurologically relevant location of arteriovenous shunting that is anatomically distinct from typical spinal dAVFs. Spetzler and colleagues^[13] proposed a reclassification of spinal cord vascular lesions to include extradural variants and to designate such lesions as extradural rather than epidural to hopefully minimize confusion. These extradural lesions may cause myelopathy through a combination of the potential mechanisms including compression by venous channels, venous congestion, "vascular steal," and conceivably hemorrhage.^[8]

Current understanding of extradural AVMs is derived from case reports and small case series that elaborate on their potential to cause devastating epidural hemorrhages or progressive myelopathy due to venous hypertension or cord compression. Our experience leads us to believe that 2 distinct types of extradural AVMs exist: pure extradural AVMs as described by Spetzler et al.,^[13] and extradural AVMs and AVFs with retrograde medullary venous drainage with the fistula site located at some distance from the dural penetration site. Additionally, there are mixed- or juvenile-type true vascular malformations that involve both intradural and extradural compartments extensively. Our discussion is limited to extradural AVMs with medullary venous drainage.

Neurosurg Focus. 2009;26(1):E5 © 2009  American Association of Neurological Surgeons

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Cite this: Spinal Extradural Arteriovenous Malformations With Parenchymal Drainage: Venous Drainage Variability and Implications in Clinical Manifestation - Medscape - Jan 01, 2009.