A 52-year-old man from Delhi presents to the pain clinic with chronic lower back pain for 8 months following a disc herniation. He reports constant burning pain in the left leg with numbness and tingling. Physical examination reveals diminished sensation to pinprick over the L5 dermatome. MRI confirms L4–L5 disc herniation compressing the left L5 nerve root. The patient has already failed conservative management and three epidural steroid injections. He is now prescribed pregabalin 300 mg daily and referred for spinal cord stimulation (SCS). Which of the following best explains the mechanism by which spinal cord stimulation provides pain relief in this neuropathic pain syndrome?

Explanation

## Mechanism of Spinal Cord Stimulation in Neuropathic Pain **Key Point:** Spinal cord stimulation (SCS) works via the **Gate Control Theory of Pain**, proposed by Melzack and Wall in 1965. Large-diameter Aβ fibres (touch, pressure, vibration) synapse in the dorsal horn and activate inhibitory interneurons in the substantia gelatinosa, which suppress the transmission of pain signals from small-diameter C-fibres and Aδ fibres (nociceptors) to the spinothalamic tract. ### Gate Control Theory — The Mechanism ```mermaid flowchart TD A[Electrical stimulation of dorsal columns]:::action --> B[Activation of large-diameter Aβ fibres]:::outcome B --> C[Excitation of inhibitory interneurons<br/>in substantia gelatinosa]:::action C --> D[Presynaptic inhibition of nociceptive<br/>C-fibre and Aδ-fibre terminals]:::action D --> E[Reduced pain signal transmission<br/>to spinothalamic tract]:::outcome E --> F[Pain relief]:::outcome ``` ### Why SCS Works in Neuropathic Pain 1. **Neuropathic pain pathophysiology:** Nerve root compression (as in this case) causes: - Ectopic discharge from injured axons - Central sensitization (increased excitability of dorsal horn neurons) - Loss of normal inhibitory tone 2. **SCS restores inhibition:** By activating Aβ fibres, SCS recruits the "gate" mechanism, flooding the dorsal horn with inhibitory signals that compete with nociceptive input. 3. **Clinical efficacy:** SCS is FDA-approved and highly effective in failed back surgery syndrome (FBSS) and radiculopathy when conservative management fails. **High-Yield:** The substantia gelatinosa is the **critical gating zone** — it contains GABA-ergic and enkephalin-releasing interneurons that mediate presynaptic inhibition of nociceptive terminals. **Clinical Pearl:** SCS is particularly effective in neuropathic pain (radiculopathy, FBSS, complex regional pain syndrome) because it directly addresses the loss of inhibitory control; it is less effective in pure nociceptive pain (e.g., cancer pain) where the pain pathway is intact. **Mnemonic:** **GATE** = **G**ABA-ergic interneurons **A**ctivated by **T**ouch (Aβ) **E**xcitation → pain suppression.