What is the most likely diagnosis and recommended work‑up and management for an adult with new‑onset low back pain radiating down the left leg, suggestive of left‑sided exiting nerve root compression from a disc protrusion?

Diagnosis: Lumbar Disc Herniation with Left-Sided Radiculopathy

The most likely diagnosis is lumbar disc herniation causing left nerve root compression, presenting as radiculopathy with characteristic radiating leg pain below the knee in a dermatomal distribution. 1

Clinical Confirmation

The clinical picture strongly suggests disc herniation based on:

• Radicular pain pattern: Pain radiating down the leg below the knee in a sciatic nerve distribution has 91% sensitivity for herniated disc 1
• Anatomic likelihood: More than 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels 1, 2
• Mechanical nature: The pathophysiology involves both compression sensitizing the nerve root to mechanical stimulation and a chemically mediated inflammatory reaction, not just mass effect alone 3

Essential Physical Examination Findings to Document

Perform a focused neurological examination to identify the specific nerve root involved 1:

• L4 nerve root compression: Test knee extension strength and patellar reflex; assess for sensory changes in anterior/medial thigh 1, 2
• L5 nerve root compression: Test great toe and foot dorsiflexion strength; check for sensory loss on dorsal foot 1, 2
• S1 nerve root compression: Test foot plantarflexion strength and ankle reflexes; assess posterior leg/lateral foot sensation 1, 2
• Straight-leg-raise test: Perform bilaterally (91% sensitivity, though only 26% specificity for disc herniation) 1
• Crossed straight-leg-raise: More specific (88%) but less sensitive (29%) 1

Critical Red Flags Requiring Immediate MRI and Urgent Evaluation

Before proceeding with conservative management, immediately assess for these emergent conditions 1, 2:

• Cauda equina syndrome: Urinary retention (90% sensitivity), fecal incontinence, saddle anesthesia—requires urgent MRI and surgical consultation 4, 1
• Malignancy: History of cancer, unexplained weight loss, age >50 years, or failure to improve after 1 month 1, 2
• Infection: Fever, IV drug use, or recent infection 1, 2
• Compression fracture: Older age with osteoporosis or corticosteroid use 1, 2
• Progressive neurologic deficit: Rapidly worsening motor weakness or multilevel deficits 1

Initial Management Algorithm

If NO Red Flags Present:

Begin with 4-6 weeks of conservative therapy before considering imaging 4, 1:

1. Pharmacologic management:

   • NSAIDs as first-line (good evidence for moderate pain relief in radicular pain) 1
   • Add skeletal muscle relaxant for acute symptoms (good evidence for short-term effectiveness) 1
   • Acetaminophen as adjunctive therapy (fair evidence) 1
   • Avoid opioids as first-line; use lowest dose for shortest time only if NSAIDs insufficient 1

2. Non-pharmacologic interventions:

   • Advise remaining active (more effective than bed rest) 1
   • Superficial heat (good evidence for moderate benefits) 1
   • Spinal manipulation (fair evidence for small to moderate benefits in acute presentations) 1

3. Patient education:

   • Most patients improve within the first 4 weeks with noninvasive management 4, 1
   • The majority of disc herniations show reabsorption or regression by 8 weeks after symptom onset 4
   • Early imaging does not improve outcomes and incurs unnecessary expense 1

Imaging Indications:

Order MRI lumbar spine WITHOUT IV contrast if 4, 1:

• Symptoms persist beyond 4-6 weeks despite adequate conservative therapy AND patient is a surgical/interventional candidate 4, 1
• Severe or progressive neurologic deficits develop at any time 1
• Any red flags are present 1

MRI is the imaging study of choice because it accurately depicts soft-tissue pathology, assesses nerve root compression, and evaluates spinal canal patency 4, 1

Interventions to AVOID

Do NOT offer epidural steroid injections: The most recent high-quality BMJ guideline provides a strong recommendation AGAINST epidural injection of local anesthetic, steroids, or their combination for chronic radicular spine pain 1. This is echoed by NICE guidelines recommending against spinal injections for managing low back pain 1.

Specialist Referral Timing

• Non-severe radicular pain: Refer within 3 months if symptoms persist 1
• Severe disabling radicular pain: Refer within 2 weeks if preventing normal daily activities 1
• Surgical consideration: Guidelines suggest referring after minimum 3 months to 2 years of failed conservative therapy, though trials of surgery included only patients with at least 1 year of symptoms 4

Common Pitfalls to Avoid

• Do not image prematurely: Early imaging in patients without red flags leads to increased healthcare utilization, higher rates of unnecessary injections and surgery, and worse disability outcomes 4
• Do not assume all leg pain is disc herniation: Only 4% of primary care patients with low back pain have symptomatic disc herniation 1, 2
• Do not confuse with spinal stenosis: Stenosis presents with pseudoclaudication improving with sitting, while disc herniation pain typically worsens with sitting and improves with standing/walking 2
• Do not rush to surgery: Most disc herniations resolve with conservative management, and premature surgery exposes patients to unnecessary risks 1