What are the clinical features and appropriate investigations for spinal dural arteriovenous fistula (SDAVF)?

Spinal Dural Arteriovenous Fistula: Clinical Features and Investigations

MRI of the complete spine without and with IV contrast is the initial imaging modality of choice for suspected spinal dural arteriovenous fistula (SDAVF), followed by digital subtraction angiography (DSA) for definitive diagnosis and treatment planning.

Clinical Features

Demographics and Presentation

SDAVF predominantly affects elderly men (77-84% male) in their sixth and seventh decades (mean age 59-65 years) 1, 2, 3. The disease presents with progressive myelopathy due to venous congestion, not hemorrhage in most cases.

Cardinal Symptoms (in order of frequency)

• Leg weakness (48-52% of patients) - typically symmetric and ascending from lower extremities 2, 4
• Sensory disturbances (27-30%) - paresthesias, often symmetric and ascending 1, 2
• Back or leg pain (20-24%) 1, 2
• Sphincter dysfunction (4-6% at presentation, but develops later in disease progression) 2, 4

Pathognomonic Clinical Sign

Exercise-induced weakness worsening occurs in 43% of patients and strongly correlates with thoracic fistula location 2. This occurs due to worsening venous hypertension in the arterialized draining vein during exertion - a highly specific feature that should raise immediate suspicion for SDAVF.

Physical Examination Findings

• Sensory level identified in 37% of patients, most commonly at L1 (23%) or T10 (19%) 2
• Critical caveat: The sensory level corresponds to actual fistula location (±2 levels) in only 40% of cases 2
• The fistula level does NOT reliably predict the sensory level - therefore image the entire spine, not just the symptomatic region

Disease Course

• Mean symptom duration before diagnosis: 7 months 1
• Most patients (44%) have more than 2 symptoms before hospitalization 1
• Progressive myelopathy is the rule if untreated, with poor prognosis 5

Anatomic Distribution

Fistulas occur most commonly in:

1. Lower thoracic spine (51%) 1
2. Lumbar spine (23%) 1
3. Upper thoracic spine (20%) 1

Investigations

Initial Imaging: MRI Complete Spine

MRI complete spine without and with IV contrast is the preferred initial study 6. The protocol should be optimized with:

T2-Weighted Sequences (Non-contrast Component)

• 3D T2-weighted fat-saturated sequences increase sensitivity for detecting epidural fluid collections 7
• T2 hyperintensity in spinal cord: present in 78% of cases 1
• T2 flow voids on spinal surface: present in 88% of cases - highly suggestive finding 1
• Conus involvement: T2 signal abnormality involves the conus in 95% of patients 2

Contrast-Enhanced Component

• Demonstrates dural enhancement and engorged epidural venous plexus 7
• These features support the diagnosis of venous hypertension

Why MRI First?

• Avoids lumbar puncture (unlike CT myelography or cisternography) 7
• Superior spatial resolution for lesion localization 7
• Detects epidural collections with equal sensitivity to CT myelography 7

Definitive Diagnosis: Digital Subtraction Angiography

DSA remains the gold standard for confirming SDAVF diagnosis and is mandatory before treatment 8, 3.

DSA Protocol

• Complete spinal angiography with selective catheterization 3
• Include 2D, 3D, and reformatted cross-sectional views 8
• DSA provides superior spatial and temporal resolution compared to non-invasive imaging 8
• Identifies the exact fistula location, feeding arteries, and venous drainage pattern 3

Why DSA is Essential

• MRI/MRA cannot reliably identify the precise fistula point 3
• Treatment planning requires detailed angioarchitecture 8
• Distinguishes SDAVF from other spinal vascular malformations 9

Advanced/Adjunctive Imaging (When Initial Studies Are Negative)

If clinical suspicion remains high despite negative initial MRI:

1. Dynamic CT myelography - for subtle CSF-venous fistulas or slow leaks 7
2. MR myelography with intrathecal gadolinium (off-label use) - increases sensitivity for slow leaks 7
3. Time-resolved 4D-MRA - provides temporal resolution for hemodynamic assessment 6

Imaging Modalities to Avoid

• CT spine without contrast: inadequate for cord assessment 6
• Plain radiographs: not adequate for diagnosis 6
• CTA spine: not comprehensive, doesn't evaluate spinal cord adequately 6
• Standard MRA without DSA: insufficient for treatment planning 3

Critical Diagnostic Pitfalls

1. The sensory level misleads: Only 40% correlation with fistula location - always image the entire spine 2
2. Delayed diagnosis is common: Mean 7-month delay due to nonspecific symptoms mimicking spinal stenosis or peripheral neuropathy 1, 2
3. Normal initial MRI doesn't exclude SDAVF: Up to 46-67% of initial spine imaging may be normal in clinically suspected cases 7
4. Exercise-induced weakness is key: This symptom should trigger immediate consideration of SDAVF, especially in elderly men 2

Prognostic Factors

Preoperative Aminoff-Logue Scale (ALS) scores are the strongest predictor of postoperative functional improvement 1. Earlier diagnosis and treatment (lower ALS scores) correlate with better outcomes, emphasizing the importance of rapid diagnosis when SDAVF is suspected.