What is the investigation of choice for lower back pain radiating to the left leg?

Investigation of Choice for Lower Back Pain Radiating to the Left Leg

MRI lumbar spine without IV contrast is the investigation of choice for lower back pain radiating to the left leg, but only after 6 weeks of failed conservative therapy in patients who are surgical or intervention candidates. 1

Initial Management: No Imaging Required

For most patients presenting with lower back pain and radiculopathy (leg pain), imaging is NOT appropriate initially and provides no clinical benefit. 1 The evidence-based approach is:

• Conservative management for at least 6 weeks before considering any imaging, as the majority of disc herniations show reabsorption or regression by 8 weeks after symptom onset. 2
• Routine imaging in uncomplicated radiculopathy leads to increased healthcare utilization without improving patient outcomes. 1, 2
• Disc abnormalities (protrusions, bulges) are present in 29-43% of completely asymptomatic individuals, meaning imaging findings often do not correlate with symptoms. 2

When MRI Becomes Appropriate

MRI lumbar spine without IV contrast should be ordered only when ALL of the following criteria are met: 1

• Symptoms have persisted for ≥6 weeks despite optimal conservative therapy
• Patient is a candidate for surgery or epidural steroid injection
• The goal is to identify actionable pain generators that could be targeted for intervention

Why MRI Over Other Modalities

MRI is superior to all other imaging options because: 1

• Superior soft-tissue contrast that accurately depicts disc degeneration, herniation, the thecal sac, and neural structures
• No ionizing radiation exposure (a single lumbar spine X-ray delivers gonadal radiation equivalent to daily chest X-rays for over one year)
• High sensitivity for nerve root compression: In symptomatic patients, 65% with radiculopathy have demonstrable disc herniation on MRI 3

CT lumbar spine has poor visualization of intradural and spinal cord pathologies and is generally not recommended for initial evaluation. 1 Plain radiography requires at least 50% bone erosion before abnormalities become visible, making it inadequate for soft tissue pathology. 3

Red Flags Requiring Urgent Imaging

Proceed immediately to MRI (bypassing the 6-week waiting period) if any of these red flags are present: 1, 2

• Cauda equina syndrome: New urinary retention, overflow incontinence, saddle anesthesia, or bilateral leg weakness 3
• Progressive or severe neurologic deficits: Worsening motor weakness or sensory loss 4
• Suspected malignancy: History of cancer 3
• Suspected infection: Fever, immunosuppression, recent spinal procedure 3
• Significant trauma relative to age (fracture concern) 4

For suspected cauda equina syndrome specifically, MRI lumbar spine without IV contrast is the imaging study of choice because of its ability to accurately depict soft-tissue pathology, assess vertebral marrow, and assess spinal canal patency. 3

When to Add IV Contrast

MRI lumbar spine without AND with IV contrast should be considered when: 3

• Underlying malignancy is clinically suspected (to evaluate intramedullary, intradural-extramedullary, and extradural disease)
• Infection is suspected (to identify epidural/paraspinal abscess and distinguish abscess from phlegmon)
• Inflammatory conditions are being evaluated

The addition of IV contrast with fat suppression is invaluable in identifying epidural and paraspinal abscess. 3

Common Pitfalls to Avoid

• Do not order imaging without completing 6 weeks of conservative therapy unless red flags are present—this leads to unnecessary healthcare costs and does not improve outcomes. 1, 2
• Do not assume imaging abnormalities explain symptoms—disc protrusions are common in asymptomatic individuals and clinical correlation is essential. 5
• Do not use plain radiography as initial imaging for radiculopathy—it cannot visualize soft tissue pathology causing nerve root compression. 3
• Do not order CT as first-line imaging unless MRI is contraindicated or unavailable—CT has poor soft tissue resolution for neural structures. 1

Alternative Imaging When MRI Contraindicated

If MRI cannot be performed (pacemaker, severe claustrophobia, metallic implants): 1

• CT myelography of the lumbar spine can assess spinal canal patency, subarticular recesses, and neural foramina, though it requires lumbar puncture and intrathecal contrast injection 3
• CT lumbar spine without IV contrast can predict significant spinal stenosis (≥50% thecal sac effacement) and exclude cauda equina impingement, but has limited soft tissue detail 3, 1