Spinal Arteriovenous Malformations (AVM's) are rare disorders.
These defects of the circulatory system consist of an
abnormal connection between the normally higher intravascular
pressure arterial system and the lower pressure venous pathways. Arteries
carry oxygen-rich blood away from the heart to the body's cells; veins
return oxygen-depleted blood to the lungs and heart. Under ordinary
circumstances there is a pressure regulatory system in place that
reduces the arterial blood pressure through progressively smaller vessels
until the capillary portion is reached. This capillary
network is vital in order to permit the transfer of nutrients
and oxygen to the surrounding tissue as well as reducing the
intravascular pressure before the blood is transferred to the relatively
thin walled veins.
An AVM can occur when the intervening capillary system is absent.
There are several potential consequences to the presence of an
Haemorrhage into surrounding tissue is a real threat since the
thin walled vein portion of this malformation may be
unable to sustain the higher intravascular blood pressure that is
directly shunted from the thicker muscular walled arteries. Haemorrhage
can be as little as a "leak" or as destructive as an
explosive disruption within the tissue that surrounds it.
The precise cause of Spinal Cord Arteriovenous Malformations
has never been clearly defined. Malformations that arise within the
Dural Sheath and within the parenchyma (substance) of the Spinal Cord are
usually found in younger patients. These are generally believed to
be Congenital in origin.
Ischemia (lack of blood born oxygen) is a frequent problem in
the absence of the capillary system which is designed to release
oxygen and nutrients to these tissues. As pressure within the venous
system increases, the vein dilates resulting in
slowing of blood flow. Slow blood flow leads to ischemia.
Additionally, the surrounding tissue (the Neurons and Fibre Tracts)
is subjected to increased localized pressure from these dilated
veins. This local pressure produces ischemia as well. As a
result, the tissue suffers from several sources of decreased blood
perfusion and is then also predisposed to hemorrhage.
When Arteriovenous Malformations are found in older
patients, they are usually associated with some form of
trauma. The term used for this malformation is an Arteriovenous
Fistula (AVF). These AVF malformations develop when a Spinal
Radicular (Nerve Root) Artery forms an abnormal arteriovenous
communication (known as a "fistula") with the venous
Medullary Vein circulation of the Spinal Cord. This is a
relatively "slow-flow" malformation that usually
develops over a period of months to years.
The high-pressure arterial blood flow from the Radicular Artery causes
a dilatation of the Spinal Cord's venous system which is weaker and
thinner walled compared to the artery. This results in slower flow
within the veins, a condition known as venous stasis. In turn
this venous outflow obstruction then causes ischemic (lack of
oxygen and nutrient supply) changes to the surrounding and adjacent
Spinal Cord tissue. This hypo-perfusion results in impaired
neural function. The progressive venous engorgement, as well as
the Spinal Cord ischemia, appears to conspire to produce Neurologic
compromise of Spinal Cord function.
Congenital intradural Spinal AVMs consist of a tangle of abnormal
vessels. There appears to be a progressive recruitment of
arterial blood vessels and thin-walled venous vessels. Once the
high-flow arterial system overcomes the capacity of the abnormal venous
vessels, hemorrhage occurs.
Spinal Vascular Malformations have been classified into four
Type 1 (AVF) patients with are usually over 40 years
of age and occur more frequently in males than in females. Symptoms
tend to increase over an extended period of months and years
manifested by progressive weakness in the lower extremities as well
as bowel or bladder control difficulties. Pain, located in the
distal posterior thoracic spine with or without a significant
radicular component, can be a prominent feature. Symptoms may be
worsened by strenuous activity or a change in body position.
- Type 1 is the most common type of
malformation. This is a Dural AVF arising at the dural nerve
root sleeve. These malformations are created when a
single arterial feeder develops a fistula to the spinal venous
circulation. Patients with type 1 malformations
become symptomatic because the AVF causes hypo-perfusion of
the Spinal Cord from the venous congestion and hypertension within
- Type 2 is called a Glomus AVM which consists of
a tightly compacted abnormal group of arterial and venous
vessels (referred to as a "nidus") residing
inside a short segment of the substance of the Spinal Cord
- Type 3 malformations are called "Juvenile"
Arteriovenous Malformations. They are extensive lesions
fed by multiple vessels with abnormal vessels located within the
Spinal Cord substance (intramedullary) as well as on the surface
of the Spinal Cord (extramedullary.)
- Type 4 malformations are intradural
extramedullary AVFs on the surface of the Spinal Cord.
These lesions can have the same or similar clinical presentations as
Spinal Stenosis although confusion between Spinal Stenosis
and neurogenic claudication can usually be differentiated by
some of the characteristics unique to each entity.
There is a particularly rare form of Spinal Dural AVF known
as Foix-Alajouanine syndrome. In this case there is a rapidly
progressive myelopathy believed to be due to venous thrombosis from spinal
Types 2-4 patients are usually younger than 30 years
of age. They are often identified as a result of a hemorrhage
(either subarachnoid or intra-parenchymal), vascular steal
phenomenon (a form of ischemia), or, more rarely, pressure
effect by their "mass" upon the Spinal Cord.
Patients with Subarachnoid Hemorrhage usually experience
sudden onset of a severe headache, stiff neck (meningismus), and
photophobia (pain associated light). A Spinal AVM should be considered
in the differential diagnosis of any patient with a Subarachnoid Hemorrhage
who has negative results from a cerebral angiogram.
Haemorrhage within the Spinal Cord substance (intraparenchymal) is
usually associated with sudden neurologic deterioration, sudden onset
of pain, and a distinct spinal level of neurologic dysfunction.
More rarely, patients come to attention because of
"vascular steal" phenomenon in which oxygenated arterial
blood that is shunted through the AVM causes hypo-perfusion of the
surrounding normal Spinal Cord tissue.
"Mass effect" caused by growth and progressive enlargement of
the intradural AVM can cause compression of the surrounding
neural tissue which results in impairment in neurologic function.
Since Types 2-4 intradural spinal vascular malformations develop
during embryogenesis they can be located in any region of
the Spinal Cord.
Patients with intradural AVMs may present with upper or lower
extremity difficulties, whereas patients with Type 1 dural AVFs,
typically have lower limb extremity involvement only.
An accurate diagnosis is important since these lesions often represent
a reversible cause of Spinal Cord dysfunction (myelopathy.)
There are some potentially distinguishing features related to each type of
malformation. These include:
MRI (including MR Angiography) as well as CT Angiography
are among the initial neuroimaging investigations that help to identify
these problems. Ultimately Spinal Cord Angiography is prerequisite
to accurately and definitively identify the precise anatomy, configuration
of both the lesion as well as the feeding and draining vessels.
Myelography and Post-myelographic CT scanning are additional
Neuroimaging techniques that are occasionally used to identify the extent
of the lesion.
There are no effective pharmacological means available to treat
Spinal Vascular Malformations.
It is common to use "Steroids" in an attempt to acutely improve the
patient's neurologic function since steroids can decrease
the swelling (edema) that often accompanies an injury to the Spinal
Cord. There is no place for the long-term use of steroids
since they do nothing to treat the underlying pathology of the disorder
and have deleterious side effects such as gastric ulceration, elevated
blood glucose levels, and suppression of the immune system.
Treatment must be custom tailored to each patient since each Spinal
Vascular Malformation is a unique lesion. The present surgical
treatment options include open microsurgical ligation and/or
resection of the malformation. Some can be treated using
Minimally Invasive Microendoscopic techniques.
Endovascular occlusion using various methods of "embolization"
is a common and often preferential treatment method for many of these
Radiosurgery is also an important method of treatment for some
lesions. Spinal radiation is not without risk however the advances in
radiosurgery are making it increasingly safer.
It is common for Neurosurgeons to recommend a combination of the techniques
described above. The use of endovascular embolization followed by
microsurgical resection is frequently an effective combination.
Type 1 Dural AVFs can be treated with either open or endovascular
ligation. Both techniques yield excellent results, with
successful occlusion rates reported as higher than 80%.
Endovascular technique is generally regarded as being less invasive.
If the patient has multiple sites of fistula formation, direct
microsurgical ligation is more appropriate since all feeding
vessels can be ligated under direct vision.
Direct microsurgery is also necessary in cases where Endovascular
access to the arterial feeding vessel is impossible due to the extreme
tortuosity of the vascular supply to the AVM or where the arterial blood
supply to healthy regions of the Spinal Cord is difficult to spare when
attempting the selective obstruction of the blood supply to the AVM.
Types 2-4 Intradural AVMs are often best treated with
Endovascular therapy; however, microsurgical resection may be
appropriate for some lesions. The "Juvenile" variety is almost only
treated with endovascular therapy. Radiosurgery is appropriate
for some lesions as well.
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This page last edited on 2/19