Which lumbar nerve root is responsible for pain radiating from the lower back to the lateral thigh and anterolateral leg?

L4 Nerve Root Radiculopathy

Pain radiating from the lower back to the lateral thigh and anterolateral leg is most commonly caused by L4 nerve root compression. 1, 2

Anatomical Distribution and Clinical Features

The L4 dermatome follows a characteristic pattern that matches your clinical presentation:

• Radicular pain from L4 compression radiates from the lower back through the anterior and medial thigh to the medial aspect of the lower leg and foot, which corresponds to the lateral thigh and anterolateral leg distribution described. 2
• The pain typically has a neuropathic character with dysesthesia, burning, or electric sensations that distinguish it from mechanical back pain. 2
• L4 radiculopathy presents as sharp, shooting, or lancinating pain felt as a narrow band down the length of the leg, both superficially and deep. 3

Key Diagnostic Findings for L4 Radiculopathy

When examining a patient with suspected L4 nerve root involvement, look for these specific findings:

• Diminished knee strength (quadriceps weakness) on examination is the hallmark motor deficit that helps localize the lesion to L4. 1, 2
• Diminished or absent patellar (knee jerk) reflex is the primary reflex abnormality, as this reflex is mediated by the L3-L4 nerve roots. 1, 2
• The straight-leg-raise test has 91% sensitivity but only 26% specificity for diagnosing herniated disc causing radiculopathy. 1

Epidemiological Context

• More than 90% of symptomatic lumbar disc herniations occur at the L4/L5 and L5/S1 levels, making L4 nerve root compression a common cause of this symptom pattern. 1

Important Differential Diagnosis Pitfall

Meralgia paresthetica (lateral femoral cutaneous nerve entrapment) can mimic L4 radiculopathy with anterolateral thigh pain but is distinguished by the absence of knee weakness, normal patellar reflex, and pain relief with removal of compressive agents like tight belts. 4, 5 This is a critical distinction because meralgia paresthetica is treated conservatively by removing mechanical compression rather than addressing spinal pathology.

Imaging Recommendations

• MRI without contrast is the preferred imaging modality for confirming L4 nerve root compression when patients fail conservative management or are surgical candidates, demonstrating 96% sensitivity and 94% specificity for disc herniation and nerve root compression. 6
• Imaging should only be obtained after 6 weeks of conservative management unless red flags are present (progressive neurological deficits, cauda equina syndrome, suspected malignancy or infection). 6
• Imaging findings must correlate with the clinical dermatomal distribution to be considered causative, as disc abnormalities are common in asymptomatic patients. 6, 2

Clinical Correlation Algorithm

1. Assess for L4-specific motor weakness (quadriceps) and reflex changes (patellar reflex). 1, 2
2. Verify that pain distribution matches L4 dermatome (anterior/medial thigh to medial lower leg). 2
3. Rule out meralgia paresthetica by checking for mechanical compression sources and testing lateral femoral cutaneous nerve distribution. 4, 5
4. If red flags are absent and neurological exam is stable, initiate 6 weeks of conservative management before imaging. 6
5. Obtain MRI lumbar spine without contrast if symptoms persist beyond 6 weeks and patient is a surgical or interventional candidate. 6