What is the likely diagnosis and first‑line management for severe low back pain radiating to the left posterior thigh in a patient without red‑flag symptoms?

Lumbar Radiculopathy: Diagnosis and First-Line Management

This presentation is consistent with lumbar radiculopathy, most likely from L5-S1 or L4-L5 disc herniation, and first-line management consists of NSAIDs, remaining active (avoiding bed rest), and patient education about the favorable natural prognosis—with imaging and specialist referral reserved only for red-flag symptoms or failure of 6 weeks of conservative therapy. 1

Likely Diagnosis

Lumbar disc herniation with radiculopathy is the primary diagnosis when severe low back pain radiates down the leg in a typical nerve root distribution. 1 The posterior thigh distribution suggests involvement of the L5 or S1 nerve root, as more than 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels. 1

Key Diagnostic Features to Assess

• Perform a straight-leg-raise test, which has 91% sensitivity for herniated disc (though only 26% specificity). 1
• Assess for motor weakness: check knee extension strength (L4), great toe and foot dorsiflexion (L5), and foot plantarflexion (S1). 1
• Check reflexes: knee reflex (L4) and ankle reflex (S1). 1
• Map sensory distribution to determine which nerve root is affected. 1

Critical Red Flags Requiring Immediate Imaging and Referral

Screen for cauda equina syndrome by asking about urinary retention (90% sensitivity for established cauda equina), fecal incontinence, and saddle anesthesia—these require urgent MRI and surgical evaluation. 1, 2

Additional red flags that mandate immediate imaging include: 1, 3, 2

• Progressive motor weakness or sensory deficits
• History of cancer (raises probability of malignant cause from 0.7% to 9%)
• Unexplained weight loss or age >50 years
• Fever, IV drug use, or recent infection (suggesting spinal infection)
• Significant trauma or chronic steroid use (suggesting compression fracture)

First-Line Management (Absence of Red Flags)

Pharmacologic Treatment

NSAIDs are the primary medication with the strongest evidence for moderate pain relief in acute radicular pain from disc herniation. 1, 3

• Add a skeletal muscle relaxant for short-term effectiveness in acute low back pain. 1
• Acetaminophen can be used as adjunctive therapy, though evidence shows only fair efficacy. 1
• Avoid opioids as first-line therapy; reserve for severe pain unresponsive to NSAIDs, using the lowest dose for the shortest time. 1

Non-Pharmacologic Interventions

Advise patients to remain active rather than bed rest, which is more effective for acute low back pain. 1, 3, 2

• Apply superficial heat, which has good evidence for moderate benefits in acute low back pain. 1
• Consider spinal manipulation, which has fair evidence for small to moderate benefits in acute presentations. 1
• Refer to physical therapy immediately for exercises such as the McKenzie method, which may decrease recurrence and healthcare utilization. 4, 3, 2

Patient Education

Inform patients that most improve within the first 4 weeks with noninvasive management, emphasizing the favorable natural prognosis. 1, 4

• Explain that the majority of disc herniations show reabsorption or regression by 8 weeks after symptom onset. 4
• Reassure that early routine imaging does not improve outcomes and incurs unnecessary expenses. 1

When to Escalate Care

Imaging Indications

Do NOT order MRI or CT initially unless red flags are present. 1, 5, 4 Routine imaging for acute low back pain without red flags provides no clinical benefit and may lead to unnecessary interventions. 1, 4

Order MRI lumbar spine without contrast only after 6 weeks of failed conservative therapy in patients who are potential candidates for surgery or epidural steroid injection. 1, 5, 4

Specialist Referral Timeline

Refer to spine specialist within 2 weeks if radicular pain is severe, disabling, and prevents normal daily activities. 4

Refer within 3 months for less severe radicular pain that persists despite conservative management. 4

Common Pitfalls to Avoid

• Do not prescribe bed rest—patients should remain active as tolerated. 1, 4, 3
• Do not rush to epidural steroid injections—the most recent BMJ guideline strongly recommends against epidural injections for chronic radicular pain. 1
• Do not order imaging before completing 6 weeks of conservative therapy unless red flags are present, as disc abnormalities are common in asymptomatic individuals (29-43%) and do not always correlate with symptoms. 1, 4
• Do not delay urgent evaluation if progressive motor deficits develop—this constitutes a red flag requiring immediate MRI. 1, 4

Evidence Quality Note

The recommendations above are based primarily on American College of Physicians and American College of Radiology guidelines synthesized in recent high-quality systematic reviews. 1, 5, 4 The natural history of lumbar disc herniation strongly favors conservative management initially, with approximately 4% of primary care patients with low back pain having symptomatic herniated discs. 1