What investigations are required to exclude a tethered spinal cord?

MRI of the Lumbar Spine is Required to Exclude Tethered Cord

To exclude a tethered cord, MRI of the lumbar spine without IV contrast is the essential investigation, as it has high sensitivity and specificity for detecting the key diagnostic findings: low-lying conus medullaris (below the middle third of L2 vertebra) and fatty filum terminale 1.

Diagnostic Approach

Primary Imaging Modality

• MRI lumbar spine without contrast is the gold standard for excluding tethered cord syndrome 1, 2
  • Provides direct visualization of conus position
  • Identifies fatty filum terminale
  • Detects associated spinal dysraphism (open or closed)
  • Moderate strength of evidence for diagnostic accuracy with medium to high sensitivity and specificity 2

Age-Specific Considerations

In neonates and infants <4 months:

• Spinal ultrasound can be used as initial screening before the posterior spinal elements ossify 1
• Sensitivity of 80%, specificity of 89% for detecting tethered cord 3
• However, MRI should still be performed if ultrasound is abnormal or if clinical suspicion persists despite negative ultrasound 3

In older children and adults:

• MRI is mandatory; ultrasound is not adequate due to ossification of posterior elements 1

What to Look for on MRI

The radiographic diagnosis of tethered cord requires identifying:

1. Conus medullaris position below the middle third of L2 vertebra (the lowest normal level at 95% confidence) 4
2. Thickened or fatty filum terminale 1
3. Associated findings: lipomas, dermal sinus tracts, diastematomyelia, syringomyelia 1

When Contrast is Needed

Gadolinium contrast is NOT typically required for structural evaluation of tethered cord 1.

Add contrast (with pre-contrast images) only when:

• Suspected neoplasm
• Infected dermal sinus tract
• Need to assess enhancement patterns 1

Clinical Context Matters

Perform MRI when any of these clinical features are present:

• Cutaneous stigmata: midline lumbar hemangioma, lipoma, hairy patch, dermal sinus tract, or sacral dimple 4, 5
• Neurologic symptoms: progressive leg weakness, gait disturbances, muscle atrophy, loss of reflexes, sensory changes in sacral dermatomes 4, 6
• Urologic dysfunction: new-onset incontinence, urinary retention, recurrent UTIs, enuresis 4
• Bowel dysfunction: fecal incontinence, constipation 4
• Orthopedic abnormalities: foot deformities, leg length discrepancy, scoliosis, progressive lower back pain 4, 7
• Anorectal malformations: 9% prevalence of tethered cord in this population 3

Common Pitfalls

• Don't rely on normal neurologic exam to exclude tethered cord - studies show physical examination has only 62% accuracy for diagnosing intraspinal anomalies 8
• Don't use CT or plain radiographs - they provide no information about the spinal cord position or intraspinal contents 1
• Don't skip MRI in asymptomatic infants with cutaneous stigmata - symptoms can develop insidiously and may become irreversible if untreated 4, 5
• Don't assume negative ultrasound excludes tethered cord - if symptoms develop later, MRI must be performed 3

Scope of MRI

The decision between targeted lumbar spine MRI versus complete spine MRI depends on clinical presentation 1:

• Lumbar spine MRI is sufficient for isolated concern about tethered cord
• Complete spine MRI if evaluating for associated syringomyelia, Chiari malformation, or extensive spinal dysraphism 1