Spinal Arteriovenous Fistulas & Malformations (SAVF / SAVM)

You were diagnosed with a spinal arteriovenous lesion — what does that mean?

A diagnosis of SAVF or SAVM means there is an abnormal “shortcut” between an artery and a vein near or within the spinal cord. Blood is driven under high pressure into veins, which become dilated and are unable to drain the spinal cord effectively.

The spinal cord—like the brain—is exquisitely sensitive to blood flow. Chronic venous congestion or vascular steal can gradually lead to irreversible myelopathy if not treated.

What are spinal arteriovenous lesions (SAVF / SAVM)?

In simple terms, these are arteriovenous fistulas or arteriovenous malformations located in the spinal canal and/or spinal cord.

• SAVF (Spinal Arteriovenous Fistula): a direct connection between an artery and a vein without an intervening abnormal vascular “network.” Most commonly superficial (intradural or extramedullary).
• SAVM (Spinal Arteriovenous Malformation): there is a nidus—an abnormal tangle of vessels—between feeding arteries and draining veins, often intramedullary.

Both entities can damage the spinal cord through hemorrhage, venous hypertension/congestion, or vascular steal.

Types & classification — why does it matter?

Multiple classification systems exist, but for patients the key questions are where the lesion is located and what kind of shunt it is:

• Extradural fistulas — located in the epidural space, fed by a radicular/segmental arterial branch and draining into epidural veins. Symptoms arise when drainage is redirected toward perimedullary veins.
• Intradural dorsal fistulas — the most common SAVF, often in the thoracic spine. A physiologic vein at the nerve root becomes arterialized and produces venous hypertension.
• Intradural ventral fistulas — midline connections between the anterior spinal artery and an enlarged venous network. Subtypes A–B (smaller) and C (giant, multiple pedicles, high-flow).
• Intramedullary SAVMs — a nidus within the spinal cord parenchyma, supplied by spinal arteries and frequently associated with microaneurysmal dilatations.
• Conus medullaris lesions — a distinctive category with multiple feeders and large draining veins, often combining myelopathy and radiculopathy.
• Metameric / combined extradural–intradural lesions — extensive malformations that may involve bone, muscle, skin, spinal canal, and spinal cord (e.g., Cobb syndrome).

Definitive classification relies on dedicated angiography and determines whether the optimal approach is endovascular, surgical, or combined.

Causes, genetics & associated syndromes

Most spinal AVMs are congenital, whereas most spinal AV fistulas (SAVFs) are considered acquired (often presenting later in life).

• Hereditary syndromes — most notably Hereditary Hemorrhagic Telangiectasia (HHT / Osler–Weber–Rendu), where patients may develop AVMs in the brain, lungs, liver, and spinal cord.
• Genetic basis — in HHT, endoglin (a TGF-β pathway protein) deficiency has been described in endothelial cells, with primary involvement of the venous limb.
• Metameric lesions — multiple shunts arising within the same developmental segment, explaining the coexistence of cutaneous, osseous, and intramedullary abnormalities.
• Non-hereditary contributors — trauma, prior surgery, or venous thrombosis/occlusion may trigger or modify a fistula in select cases.

In practice, the most important point is that this is a structural vascular problem and does not resolve with medication. When technically feasible, definitive treatment requires disconnection/occlusion of the shunt.

How does it present — what symptoms should I watch for?

Most spinal arteriovenous lesions begin with insidious, progressive symptoms:

• Heaviness or fatigue in the legs
• Progressive weakness (paraparesis), often symmetric
• Numbness, burning sensations, or sensory disturbance in the trunk/limbs
• Gait instability or worsening balance
• Bladder/bowel dysfunction (urgency, retention)
• Radicular pain in certain lesion types

Less commonly—especially with high-flow intramedullary SAVMs—the first presentation may be sudden hemorrhage with severe pain, acute paraparesis, or cauda equina syndrome.

Natural history & risks if left untreated

Without treatment, most spinal arteriovenous lesions are progressive:

• Venous hypertension and congestion drive progressive myelopathy, leading to gradual but permanent loss of function.
• Over time, Foix–Alajouanine syndrome may develop—subacute necrotizing myelopathy with irreversible injury.
• The longer symptoms persist (particularly > 3 years), the worse the prognosis—even after successful shunt disconnection.
• High-flow SAVMs carry an additional risk of hemorrhage.

For this reason, international practice strongly favors not “watching and waiting” in symptomatic disease. Timely treatment is central to preserving function.

How is it diagnosed? MRI, MRA & angiography

Modern diagnosis relies on a combination of imaging studies:

• Spinal MRI — may show spinal cord edema/enlargement, signal abnormalities, and characteristic serpiginous flow voids from dilated perimedullary veins on T2/FLAIR sequences.
• MRA / MR myelography — helpful for visualizing vascular structures, but not always sufficient to localize the shunt precisely.
• Digital subtraction angiography (DSA) — the gold standard. Selective catheterization of relevant vessels (segmental/radicular arteries) is performed to:
  • Define lesion type (SAVF vs SAVM)
  • Identify the feeding artery/arteries
  • Map venous drainage patterns
  • Plan an endovascular and/or surgical strategy

Sometimes MRI reveals “only” dilated veins without obvious edema. Even so, if clinical suspicion is present, a comprehensive angiographic evaluation by an experienced team is essential.

When is treatment needed? Is there any role for “observation”?

In most specialized centers, current practice is:

• Symptomatic lesions (progressive myelopathy, radicular pain, hemorrhage) — indication for prompt definitive treatment (endovascular, surgical, or combined).
• Asymptomatic lesions — individualized decision-making. Many experts still recommend treatment given the generally progressive natural history. Rarely, small fistulas with purely extradural drainage may be monitored closely.
• Hemorrhage — considered an urgent indication for complete angiographic evaluation and definitive disconnection of the responsible shunt, when technically safe.

Endovascular treatment — when is it sufficient?

Endovascular embolization (using liquid embolic agents such as NBCA or Onyx, and/or coils) is a core therapeutic tool:

• Extradural fistulas (SAVFs) — often treated almost exclusively via an endovascular approach, with excellent rates of definitive occlusion.
• Giant ventral fistulas — frequently best approached endovascularly due to complex angioarchitecture and high-flow physiology.
• Intramedullary malformations (SAVM) — embolization may be used to reduce flow, occlude high-risk aneurysms, and facilitate subsequent microsurgical treatment.

Despite its advantages, in intradural dorsal fistulas (dorsal SAVF) endovascular-only treatment can carry higher recurrence rates in some series. For that reason, the final strategy is individualized and determined within a specialized neurovascular conference.

Microsurgical treatment — when is it the best option?

Microsurgery remains the “gold standard” for many intradural lesions—especially when the shunt can be safely disconnected:

• Intradural dorsal fistulas (dorsal SAVFs) — typically treated via a posterior approach (laminectomy/laminoplasty), identification of the arterialized vein at the nerve root, and division at the fistulous point.
• Small ventral fistulas — when technically accessible, microsurgical disconnection may be preferred.
• Intramedullary malformations (SAVM) — the standard of care remains microsurgical resection of the nidus, often after targeted preoperative embolization to reduce flow.
• Conus medullaris AVMs — commonly require a combined (endovascular + surgical) strategy to decompress and control pathological flow.

Intraoperative angiography, ICG videoangiography, and continuous neuromonitoring (SSEP/MEP) enhance safety and reduce the risk of new neurological deficits.

Recovery, prognosis & follow-up

Outcomes after definitive shunt occlusion depend primarily on:

• The duration and severity of symptoms prior to treatment
• The lesion type and anatomical location
• Whether permanent spinal cord changes are already present on MRI (myelopathy)

In general:

• Many patients experience stabilization and, in some cases, partial improvement in strength and gait following disconnection.
• Long-standing deficits may persist even when angiographic cure is complete.
• Long-term postoperative follow-up is recommended with clinical assessment and MRI/MRA—particularly after endovascular therapy— to monitor for recurrence.

Rehabilitation (physical therapy, occupational therapy, neuro-urologic follow-up) plays a central role in function and quality of life.