A patient develops acute low back pain radiating to the right foot (S1 distribution). MRI shows a postero-lateral disc herniation at L5–S1. The posterolateral disc herniation at L5–S1 typically compresses which nerve root?

• A L5 nerve root — it exits above the disc through the L4–L5 foramen
• B L4 nerve root — it exits at the same level through L4–L5 foramen
• C S1 nerve root — it exits below the L5–S1 disc in the lateral recess before exiting through the S1 foramen ✓
• D S2 nerve root — it exits at the sacral hiatus

Explanation

In the lumbar spine, the nerve root exits below the pedicle of the same-numbered vertebra (e.g., L5 exits between L5 and S1 through the L5–S1 foramen). A posterolateral disc herniation (the most common direction, where the posterior longitudinal ligament is thinner laterally) at L5–S1 compresses the traversing root, which is the S1 nerve root. The L5 nerve root has already exited above the L5–S1 disc. A central disc herniation at L5–S1 may compress S1 bilaterally or lower sacral roots. S1 compression causes: loss of plantarflexion, absent ankle reflex, and sensory loss on the lateral foot and heel.

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

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Written and medically reviewed by the StethoPrep medical team.