What lumbar spine MRI (including the lumbosacral region) should be ordered for a patient with left lower back pain and foot drop?

MRI Lumbar Spine Without IV Contrast

Order an MRI lumbar spine without IV contrast immediately—foot drop represents a progressive neurologic deficit that requires urgent imaging to identify the cause of nerve root compression and guide potential surgical intervention. 1

Clinical Reasoning

Foot drop is a motor deficit indicating significant nerve root compromise, most commonly affecting the L5 nerve root (though L4 or S1 can also be involved). This is not simple low back pain—it's a red flag that demands immediate evaluation regardless of symptom duration.

Why MRI Without Contrast is the Correct Choice

• MRI is the imaging study of choice for evaluating nerve root compression, disc herniation, and spinal canal pathology 1
• Contrast is not necessary for initial evaluation of radiculopathy with motor deficit—the standard noncontrast sequences accurately depict:
  • Disc herniations and their relationship to nerve roots
  • Spinal canal and neural foraminal stenosis
  • Soft tissue pathology
  • Vertebral marrow abnormalities 1

When to Consider Adding Contrast

MRI with and without IV contrast becomes relevant only if 1:

• You suspect underlying infection (epidural abscess, discitis/osteomyelitis)
• There's concern for malignancy or metastatic disease
• The patient has prior lumbar surgery (to distinguish scar from recurrent disc herniation)
• The noncontrast study is nondiagnostic or indeterminate

Important Clinical Distinctions

This is NOT Cauda Equina Syndrome (Yet)

While foot drop is serious, it doesn't meet criteria for cauda equina syndrome unless accompanied by 1:

• Bladder/bowel dysfunction
• Saddle anesthesia
• Bilateral lower extremity weakness
• Sexual dysfunction

If any of these features are present, this becomes a surgical emergency requiring stat MRI (with or without contrast are considered equivalent alternatives for CES) 1.

Timing Matters

The ACR guidelines recommend imaging after 6 weeks of conservative management for uncomplicated radiculopathy 1. However, progressive neurologic deficit (like foot drop) overrides this waiting period—these patients should be imaged immediately as they are potential surgical candidates 2, 3.

Common Pitfalls to Avoid

1. Don't wait for conservative management to fail when motor deficits are present
2. Don't order contrast unnecessarily—it adds cost, time, and potential adverse reactions without improving diagnostic yield for straightforward radiculopathy
3. Don't order CT as the initial study unless MRI is contraindicated (pacemaker, severe claustrophobia, metallic foreign body). While CT can detect spinal stenosis, it's inferior for visualizing disc herniations and nerve root compression 1
4. Don't forget to assess for bilateral symptoms—unilateral foot drop with back pain is typically radiculopathy, but bilateral findings raise concern for cauda equina syndrome

Alternative Imaging (If MRI Unavailable)

If MRI is contraindicated or unavailable 1:

• CT myelography can assess neural compression but requires lumbar puncture with intrathecal contrast injection
• CT without contrast can exclude cauda equina compression but is inferior for characterizing the specific pathology

The evidence strongly supports MRI lumbar spine without IV contrast as the single best initial imaging study for your patient with left lower back pain and foot drop.