L4–L5 is the most common level for lumbar disc herniation. A posterocentral disc herniation at this level compresses which nerve root in the lateral recess, and what dermatomal/myotomal deficit results?

• A L4 root — loss of knee jerk, weakness of quadriceps
• B S1 root — loss of ankle jerk, weakness of calf/plantar flexion
• C L4 root — loss of ankle jerk, sensory loss medial aspect of foot
• D L5 root — weakness of extensor hallucis longus, sensory loss over dorsum of foot and first web space; knee and ankle jerks preserved ✓

Explanation

At the L4–L5 level, the L5 nerve root travels in the lateral recess and exits at the L5–S1 foramen. A posterolateral disc herniation at L4–L5 compresses L5 root, causing weakness of extensor hallucis longus (EHL — great toe extension is the signature sign), tibialis anterior, and peronei. Sensory loss is over the dorsum of the foot and first web space (L5 dermatome). Knee jerk (L3-L4) and ankle jerk (S1) are both preserved. L3–L4 herniation compresses L4 root, affecting knee jerk; L5–S1 herniation affects S1 root and ankle jerk.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.