Spinal vascular malformations include a heterogeneous group of pathological/anatomical entities with congenital and acquired etiologies that induce spinal cord dysfunction, primarily through intradural extramedullary drainage and enlargement of the coronal venous plexus, resulting in venous hypertension and/or a direct mass effect. These lesions are rare, comprising approximately 5% of neurovascular disorders. Clinical manifestations are pain or venous congestive myelopathy, usually in the thoracolumbar spine. This may progress to hemorrhage from vascular thrombosis and necrotizing myelopathy (Foix–Alajouanine syndrome).1 Due to the differing hemodynamics, pathophysiology, and treatment considerations, a thorough knowledge of the various types of spinal dural arteriovenous fistulas (dAVFs) and arteriovenous malformations (AVMs) is essential.

Types

The first descriptions of the single coiled dural vessel form of spinal dAVF with intradural extramedullary venous drainage were published by Kendall and Logue in 1977.2 Type I spinal dAVFs (also known as angioma racemosum venosum) are the most common and comprise 56% of all spinal lesions in the spinal vascular malformation database at the University of Toronto.3 They occur nearly 3–4 times as frequently in men. These generally low flow lesions may be further divided into type I-A (single feeder dAVFs) and I-B (≥2 arterial feeders).4 Although they are typically supplied via radicular arteries, anterior spinal artery feeders have also been reported.5 Type II spinal AVMs (also known as angioma racemosum arteriovenosum) are characterized by a compact, glomus-type, totally intramedullary nidus. They have no gender predominance but are symptomatic in younger patients. Type III (juvenile or metameric) spinal AVMs are highly complex intramedullary lesions that frequently extend into the extramedullary, epidural, or even extraspinal compartments. Vascular supply includes radiculo-medullary, radiculo-pial, radiculo-meningeal, and segmental arteries. They are fortunately quite rare.

Type IV (perimedullary) spinal AVFs are intradural extramedullary lesions, usually on the pial surface of the cord or filum terminale. They are divided into three groups based on their flow: single feeder, low flow (type IV-A); moderate flow with ≥1 feeders (type IV-B); and very high flow, multiple feeders (type IV-C). Type IV-C dAVFs are supplied by the anterior and occasionally posterior spinal arteries.6

Although the majority of spinal vascular malformations can be classified into one of the above categories, purely extradural vascular lesions have been described.7–10 Intraspinal cavernous malformations are discrete intramedullary lesions comprised of low flow, thin walled sinusoids without intervening normal parenchyma. Approximately 3–5%11 of cavernous malformations occur in the spinal cord, usually in the thoracic and cervical cord.12 As cavernous malformations are angiographically occult, treatment of these lesions is outside the scope of this report.

Planning

MRI has replaced myelography as the firstline imaging modality for a suspected spinal dAVF3 due to its utility in demonstrating anterior/posterior nidal components with respect to cord anatomy and revealing epidural, vertebral, paraspinal, and muscular tissue involvement in metameric AVMs. A high quality diagnostic spinal angiogram with thin focal spot to identify the artery of Adamkiewicz, locate all arterial feeders/draining veins, any intra- and para-nidal components such as aneurysms, fistulas, and venous ectasias, and identify the subtype of AVM/dAVF is crucial. The fistula site is often remote from the draining veins, which can extend over several segments. Prolonged mean transit time in the spinal cord (ie, >20 s) is consistent with venous congestion. Provocative testing with amytal sodium and/or lidocaine with simultaneous neurophysiological monitoring for motor/somatosensory evoked potentials may increase the safety of embolization, particularly when rapid flow in a pedicle pre-embolization obscures clear visualization of all of the distal branches.13

Endovascular treatment

Endovascular treatment of spinal AVMs largely consists of superselective injection of permanent liquid embolic agents, which at present consists of n-butyl cyanoacrylate (Codman Neurovascular, Raynham, Massachusetts, USA) and Onyx liquid embolic agent (ev3, Irvine, California, USA). Particle embolization is not recommended and is viewed as palliative, necessitating multiple treatments for even that goal due to inevitable revascularization14 ,15 and recruitment of smaller, less accessible feeders. Although the initial goal is parent artery occlusion, the endovascular treatment goal should include nidal penetration and, in the case of a type I or IV dAVF, penetration of the draining vein to obliterate the fistula and eliminate the possibility of recruiting new feeders.16 Cure of complex epidural AVMs17 with excellent recovery18 has been reported with Onyx. Treatment of juvenile spinal AVMs is largely palliative due to nidal size, intramedullary infiltration, intricate vascular supply and drainage, and steadily progressive neurological morbidity associated with these lesions. When embolizing high flow AVF components of a metameric AVM, use of ‘coil nests’ for occlusion of distal arterial feeders may minimize unwanted venous embolization of liquid embolic agents.

The majority of patients experience some clinical improvement on closure of the fistula19 although the rapidity and extent of improvement directly correlates with the initial duration of symptoms and degree of disability.20 Spectacular improvement, such as reversal of paraplegia, is occasionally seen.21 Motor function responds first, followed by sensation, and bowel/bladder incontinence is the last to demonstrate improvement. Patients with an angiographic cure who experience clinical deterioration likely have venous thrombosis with an increased mass effect on the cord. For this reason, some practitioners utilize a heparin infusion for 24–48 h following trans-arterial embolization, particularly when venous outflow restriction is present.16

Spinal dAVFs

Perimedullary AVFs

Spinal AVMs

Extradural vascular malformations

Spinal axis tumor embolization