## Discriminating L5 from S1 Root Lesions ### Comparison Table: L5 vs S1 Dermatome and Myotome | Feature | L5 Root | S1 Root | | --- | --- | --- | | **Dermatome (Sensory)** | Dorsum of foot, lateral leg, dorsal first web space | Lateral foot, sole, posterior leg, lateral heel | | **Myotome (Motor)** | Foot dorsiflexion (tibialis anterior), hip abduction, knee extension | Plantarflexion (gastrocnemius/soleus), hip extension, knee flexion | | **Key Reflex** | Absent L5 reflex (no specific reflex) | Absent ankle reflex (Achilles) | | **Gait Pattern** | Foot drop (high-stepping gait) | Heel-walking difficulty | **Key Point:** The **most reliable discriminator** between L5 and S1 lesions is the combination of sensory distribution and plantarflexion strength. S1 root injury causes plantarflexion weakness (via S1-innervated gastrocnemius and soleus), while L5 root injury does NOT affect plantarflexion. ### Why This Patient Has S1 Lesion 1. **Sensory loss:** Lateral foot and sole = S1 dermatome (NOT L5) 2. **Motor loss:** Plantarflexion weakness = S1 myotome (NOT L5) 3. **Both findings align** with a single S1 root lesion **High-Yield:** Plantarflexion strength testing is the **single best bedside discriminator** between L5 and S1 radiculopathy. Loss of plantarflexion = S1. Preserved plantarflexion with foot drop = L5. **Clinical Pearl:** S1 radiculopathy also causes loss of the Achilles reflex (ankle jerk), whereas L5 radiculopathy does not affect any major reflex. This reflex finding is a quick confirmatory test. ### L5 Root Lesion (Contrast) - Sensory: Dorsum of foot, lateral leg, first web space - Motor: Foot dorsiflexion weakness (foot drop), hip abduction weakness - Reflex: Achilles reflex preserved - Gait: High-stepping gait to clear the drooping foot 
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