A 52-year-old man from Delhi presents with chronic lower back pain radiating to the left leg. Imaging reveals a left-sided disc herniation at L4–L5. Which is the most common site of spinal cord lesion causing referred pain in the lower limb?

Explanation

## Spinal Pain Pathways and Referred Pain Mechanisms **Key Point:** Dorsal root ganglion (DRG) compression is the most common structural lesion causing radicular pain and referred pain in the lower limb, particularly with disc herniation. ### Anatomical Sites of Spinal Lesions and Pain Mechanisms | Lesion Site | Anatomical Structure | Pain Type | Clinical Presentation | Frequency in Disc Herniation | | --- | --- | --- | --- | --- | | **Dorsal root ganglion** | Sensory nerve root cell bodies | Radicular, referred | Sharp, dermatomal, radiating | **Most common** | | Anterior horn cells | Motor neurones (ventral horn) | Muscle weakness | Flaccid paralysis, atrophy | Rare (motor, not pain) | | Dorsal columns | Fasciculus gracilis, cuneatus | Proprioception loss | Loss of vibration/position sense | Rare (non-nociceptive) | | Ventral commissure | Crossing spinothalamic fibres | Bilateral pain/temperature loss | Suspended sensory level | Rare (central cord syndrome) | **High-Yield:** Disc herniation at L4–L5 most commonly compresses the **L5 nerve root** (not L4), causing L5 dermatomal pain radiating down the lateral leg and dorsum of the foot. ### Mechanism of Referred Pain from DRG Compression ```mermaid flowchart TD A[Disc herniation at L4-L5]:::outcome --> B[Mechanical compression of L5 nerve root]:::action B --> C[Dorsal root ganglion irritation]:::action C --> D{Nociceptive signal transmission}:::decision D -->|Via dorsal root| E[Dorsal horn synapse<br/>Substantia gelatinosa]:::action E --> F[Spinothalamic tract crossing]:::action F --> G[Thalamus and sensory cortex]:::action G --> H[Perceived radicular pain<br/>L5 dermatomal distribution]:::outcome ``` **Clinical Pearl:** The dorsal root ganglion is located in the **intervertebral foramen** and is particularly vulnerable to compression by posterolateral disc herniations. This is why L4–L5 disc herniations classically present with L5 radiculopathy (not L4). **Mnemonic:** **DRG = Dermatome Radicular Gateway** — the dorsal root ganglion is the sensory gateway where mechanical compression translates into nociceptive signalling. ### Why Other Sites Are Incorrect 1. **Anterior horn cell damage** → motor weakness (lower motor neurone signs), not pain 2. **Dorsal column disruption** → loss of vibration and proprioception, not nociceptive pain 3. **Ventral commissure ischaemia** → bilateral pain/temperature loss (central cord pattern), not unilateral radicular pain