Spinal dural arteriovenous fistula

Spinal dural arteriovenous fistulas (SDAVF) are the most common type of spinal vascular malformation, accounting for approximately 70% of all such lesions ⁶.

This article specifically relates to spinal dural arteriovenous fistulas. For a discussion of intracranial dural arteriovenous fistulas refer to the main article: dural arteriovenous fistula.

Epidemiology

There incidence peaks around the 5^th- 6-6^th decades and males are more commonly affected than females ².

Clinical presentation

SDAVFs cause symptoms through venous hypertension and congestion of the cord with oedema. The most common clinical presentations are progressive pain, lower extremity weakness or sensory changes. Sphincter dysfunction may also occur. Onset of symptoms is insidious and progression occurs over several years. There is often a significant delay between presentation and diagnosis.

Haemorrhage is very rare but occasionally encountered and may be a source of unexplained subarachnoid haemorrhage ^2,7.

Pathology

These fistulae are abnormal direct connections between an artery and a vein in the dura of the nerve root sleeve. 85% of SDAVFs consist of a single transdural arterial feeder, however there are cases with many arterial feeders originating from either a single or multiple levels that may be either unilateral or bilateral ². The direct arterial inflow into the venous system raises the pressure within the coronal venous plexus, which is valveless. The coronal venous plexus dilates and venous drainage of the cord decreases, causing venous congestion and intramedullary oedema. Cord ischaemia and infarction may result. Nearly 60% of SDAVFs are spontaneous, with the remainder being caused by trauma ³.

Radiographic features

MRI

There is typically cord enlargement in the lower thoracic region and conus, with signal change involving multiple spinal segments. The segmental level of cord enlargement and signal change does not correlate with the location of the fistula ⁵.

Typical signal characteristics include:

• T1: intramedullary hypointensity and flow voids on the cord surface may be seen
• T2
  • diffuse multilevel intramedullary hyperintensity (oedema). Regardless of the location of the fistula, the T2 hyperintensity involves the conus in up to 90% of cases because of orthostasis. The exception is SDAVFs of the upper cervical spine (C1-C2) which often drain intracranially and present more commonly with subarachnoid haemorrhage ⁵.
  • T2 hypointensity in the periphery of cord may be present. This is thought to represent pial capillaries containing deoxyhaemoglobin secondary to venous hypertension ⁵
  • usually, but not always, prominent serpiginous intradural extramedullary flow voids along the dorsal aspect of the spinal cord. These usually span more than three segments. If there is significant cord swelling, the veins can be compresend by the mass effect and not be detectable on imaging ⁵.
  • the vessels may be large enough to give the surface of the cord a scalloped appearance 
• T1 C+ (Gd)
  • patchy intramedullary enhancement is often seen (due to breakdown of the blood-brain barrier because of either chronic infarction or a capillary leak phenomenon secondary to venous hypertension) 
  • serpentine enhancing veins on the cord surface
• contrast enhanced MRA
  • helps determine the segmental level of the fistula in order to guide selective catheter angiography 

If MRI is contraindicated, the following may be performed:

• CT angiography: may successfully localise the fistula in up to 75% of cases ⁵ 
• CT myelography: may demonstrate tortuous filling defects due to dilated veins ⁵

Angiography (DSA)

Gold standard test for confirming diagnosis and provides option for treatment. It is a time consuming and potentially dangerous investigation as dissection of a vessel can potentially lead to cord ischaemia. 

The site of maximal MRI abnormality is not a reliable indicator of the location of the fistula, which can be many levels away. As such a complete spinal angiogram consists of selective catheterisation of the bilateral ⁸:

• intercostal arteries
• lumbar arteries
• median and lateral sacral arteries
• vertebral arteries
• ascending cervical arteries
• intracranial vessels may also need to assessed if no fistula is found including:
  • ascending pharyngeal artery
  • meningohypophyseal trunk
  • middle meningeal artery
  • occipital artery

Treatment and prognosis

Treatment options include endovascular or surgical occlusion of the shunt.

• endovascular occlusion: 
  • performed with either glue or N-butyl-2-cyanoacrylate (Onyx) after superselective catheterization of the radiculomeningeal artery supplying the fistula
  • Occlusion rates of up to 85% have been reported ⁵.
  • endovascular treatment is contraindicated if the radicular artery also supplies the anterior spinal artery. Embolization of a fistula that supplies a posterior spinal artery remains controversial ⁵
• surgery: surgical occlusion consists of a targeted laminectomy and intradural exploration with coagulation or disconnection of the draining vein. Occlusion rates as high as 98% have been reported ⁵

After treatment of the fistula, the T2 hyperintensity, prominent flow voids, and enhancement should decrease with time, but can persist for up to a year. These postoperative imaging features do not correlate with clinical outcome ⁵.

If treated early, motor and sensory function can be improved or stabilized in most cases. Pain and bowel and bladder dysfunction are only reversed in a minority of patients ⁵.

Differential diagnosis

General imaging differential considerations include:

• intramedullary neoplasm
• CSF flow artefact
  • typically seen in the subarachnoid space dorsal to the cord
  • no signal abnormality within the cord
• spinal cord arteriovenous malformation (SCAVM) 
  • haemorrhage is common
  • acute onset of symptoms
  • males and females affected equally
  • usually present during the 3^rd decade