In lumbar disc herniation at L4-L5, the disc compresses the traversing (descending) nerve root. Which nerve root is affected, and what is the characteristic clinical finding?

• A L4 root — weak knee extension and reduced knee reflex
• B S1 root — weakness of plantar flexion and reduced ankle reflex
• C L4 root — weakness of hip flexion and reduced knee reflex
• D L5 root — weakness of ankle dorsiflexion and great toe extension, normal reflexes ✓

Explanation

At the L4-L5 disc level, the traversing root is L5. The L4 root exits through the L4-L5 foramen (it has already exited above the disc level). L5 root compression causes: weakness of ankle dorsiflexion (tibialis anterior), great toe extension (EHL), foot eversion (peronei), and hip abduction (gluteus medius). Sensation is impaired over the dorsum of the foot and first web space. Reflexes are typically normal since L5 has no major reflex. L4 root compression (typically from L3-L4 disc) reduces the knee jerk. S1 root (L5-S1 disc) reduces the ankle jerk and weakens plantar flexion.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.