L4 Nerve Root Compression
Compression of the L4 nerve root causes pain on the lateral aspect of the leg to the lateral knee. 1
Neuroanatomical Distribution
The L4 nerve root follows a specific dermatomal pattern that distinguishes it from other lumbar nerve roots:
- L4 dermatome covers the lateral thigh, lateral knee, and medial lower leg, making lateral leg pain to the lateral knee a characteristic presentation of L4 radiculopathy 1
- This distribution is distinct from L5 (which affects the dorsal foot and big toe) and S1 (which affects the lateral foot and ankle) 2
Clinical Examination Findings
When evaluating suspected L4 nerve root compression, specific examination findings help confirm the diagnosis:
- Knee strength and knee reflexes should be assessed, as these are mediated by the L4 nerve root 1
- Weakness in knee extension (quadriceps weakness) is the motor finding associated with L4 radiculopathy 1
- Diminished patellar reflex (knee jerk) indicates L4 involvement, as this reflex is primarily mediated by L3-L4 nerve roots 2
- The straight leg raise test has 91% sensitivity for detecting lumbar disc herniation causing nerve root compression, though specificity is only 26% 1
Common Pitfalls
A critical caveat to avoid misdiagnosis:
- Do not confuse L4 radiculopathy with L5 or S1 compression, which present with different pain patterns (L5 affects dorsal foot/big toe sensation, S1 affects plantar flexion and ankle reflexes) 1, 2
- The L5 nerve root can be compressed at both L4-5 and L5-S1 levels simultaneously, but this causes different symptoms than isolated L4 compression 3
Imaging Recommendations
When clinical examination suggests L4 nerve root compression:
- MRI lumbar spine without IV contrast is the imaging study of choice for visualizing nerve root compression 4
- Immediate MRI is indicated for severe or progressive neurological deficits 1
- For persistent symptoms without improvement after 4-6 weeks of conservative management, delayed imaging is appropriate 1