S1, L5 radicular syndromes; ancillary investigations

1. Big picture

L5 and S1 radiculopathies are the classic lower lumbar root syndromes tested in neurology exams. They are most often caused by lumbar disk herniation, especially:

• L4–L5 disk herniation → usually compresses the L5 root
• L5–S1 disk herniation → usually compresses the S1 root

The examiner expects you to localize the lesion clinically from:

1. Pain distribution
2. Sensory loss
3. Motor weakness
4. Reflex abnormality
5. Provocation signs, especially Lasègue sign
6. Ancillary investigations, mainly magnetic resonance imaging and neurophysiology

The most important exam contrast is:

L5 = dorsum of foot / big toe / foot dorsiflexion weakness S1 = posterior calf / little toe / Achilles reflex loss / plantar flexion weakness

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