A 52-year-old woman from Mumbai presents with a 3-week history of progressive weakness and sensory loss in the right lower limb. She reports pain in the right buttock and lateral thigh, with numbness extending down the lateral leg to the dorsum of the foot and great toe. On neurological examination, she has weakness of hip abduction and knee extension. Reflexes show diminished patellar reflex on the right. Imaging reveals a large disc herniation at the L3 level compressing the right L4 nerve root. Which myotome is primarily affected?

Explanation

## L4 Myotome and Radiculopathy **Key Point:** The L4 myotome innervates knee extension (quadriceps), ankle dorsiflexion (tibialis anterior), and hip adduction. L4 radiculopathy presents with weakness of these muscles and sensory loss in the L4 dermatome (lateral thigh, lateral leg, dorsum of foot, and great toe). **High-Yield:** L4 nerve root compression characteristically causes: 1. **Motor deficit:** Knee extension weakness (quadriceps) — diminished or absent patellar reflex 2. **Sensory loss:** Lateral thigh, lateral leg, dorsum of foot, great toe (L4 dermatome) 3. **Pain distribution:** Buttock, lateral thigh, lateral leg **Clinical Pearl:** The diminished patellar reflex is a hallmark of L4 radiculopathy because the patellar reflex arc (L3–L4) is disrupted. This is a key clinical sign that distinguishes L4 from L5 radiculopathy (which affects the Achilles reflex, S1–S2). ## Lumbosacral Myotome and Dermatome Comparison | Nerve Root | Motor (Myotome) | Sensory (Dermatome) | Reflex | | --- | --- | --- | --- | | L2 | Hip flexion, adduction | Upper medial thigh | — | | L3 | Knee extension (quadriceps) | Lower medial thigh | Patellar (L3–L4) | | L4 | Knee extension, ankle dorsiflexion | Lateral thigh, lateral leg, dorsum foot, great toe | Patellar (L3–L4) | | L5 | Hip abduction, foot eversion, ankle dorsiflexion | Lateral leg, dorsum foot, lateral toes | — | | S1 | Hip extension, ankle plantarflexion | Lateral foot, sole, heel | Achilles (S1–S2) | **Mnemonic:** **L4 = Lateral thigh + Leg + Lumbosacral** (remember "L4 = Lateral"). **Quad = L3–L4** (Quadriceps reflex = patellar reflex, mediated by L3–L4). ## Pathophysiology The L3–L4 disc herniation compresses the L4 nerve root as it exits the intervertebral foramen. This disrupts both motor and sensory fibers of L4, leading to quadriceps weakness and sensory loss in the L4 dermatome. The patellar reflex diminishment confirms L4 involvement.