23.1 Pain Theory: Nociception, Sensitization & Chronic Pain Pathophysiology
Key Takeaways
- A-delta fibers are thinly myelinated and conduct at 5 to 30 m/s producing fast sharp first pain, while unmyelinated C fibers conduct at 0.5 to 2 m/s producing slow burning second pain.
- Wind-up occurs when repetitive C-fiber input expels the magnesium plug from the NMDA receptor, which is why ketamine, magnesium, and methadone rather than additional opioid interrupt central sensitization.
- Primary hyperalgesia is peripheral and occurs at the injury site, whereas secondary hyperalgesia and allodynia are central and occur in surrounding uninjured tissue.
- Gate control theory holds that A-beta input activates inhibitory interneurons in the substantia gelatinosa, providing the physiologic basis for TENS and spinal cord stimulation.
- Chronic postsurgical pain affects 50 to 80 percent after amputation, 30 to 50 percent after thoracotomy, and 20 to 50 percent after mastectomy, and is predicted by catastrophizing, preoperative opioid use, and severe acute postoperative pain.
Why This Topic Matters on the NCE
Domain III.L of the content outline separates pain theory — anatomy, physiology, pathology, and psychodynamics, for both acute and chronic pain — from pain management. Theory questions are not decoration: the reason ketamine works for the opioid-tolerant patient, the reason a transcutaneous stimulator relieves pain, and the reason a thoracotomy patient still hurts a year later all come directly from the physiology below.
1. The Four Processes of Nociception
| Process | Where | What happens | Drugs acting here |
|---|---|---|---|
| Transduction | Peripheral nociceptor | Noxious stimulus converted to an action potential | NSAIDs, local anesthetics, topical agents |
| Transmission | Peripheral nerve to dorsal horn to thalamus to cortex | Propagation of the signal | Local anesthetics, alpha-2-delta ligands |
| Modulation | Dorsal horn | Amplification or inhibition of the incoming signal | Opioids, alpha-2 agonists, ketamine, SNRIs |
| Perception | Cortex and limbic system | Conscious, emotional experience of pain | General anesthetics, opioids, psychological therapy |
2. Primary Afferent Fibers
| Fiber | Myelination | Conduction velocity | Sensation |
|---|---|---|---|
| A-alpha / A-beta | Heavily myelinated, large | 30 to 120 m/s | Proprioception, light touch, vibration |
| A-delta | Thinly myelinated | 5 to 30 m/s | First pain: fast, sharp, well localized |
| C fibers | Unmyelinated | 0.5 to 2 m/s | Second pain: slow, dull, burning, poorly localized |
The two-wave experience of stepping on a tack — a sharp jab followed a second later by a spreading ache — is the A-delta and C-fiber conduction velocity difference made conscious.
Local anesthetics block small, myelinated fibers most readily, which produces the clinical differential blockade sequence: sympathetic (B fibers) first, then pain and temperature, then touch, then motor. This is why a spinal sympathectomy extends two to six dermatomes above the sensory level.
3. Gate Control Theory
Melzack and Wall proposed in 1965 that large-diameter A-beta (touch) input activates inhibitory interneurons in the substantia gelatinosa (Rexed lamina II) of the dorsal horn, "closing the gate" on C-fiber nociceptive transmission. Rubbing an injury genuinely does reduce pain.
Gate control is the direct physiologic basis for:
- Transcutaneous electrical nerve stimulation (TENS)
- Dorsal column spinal cord stimulation for neuropathic and failed back surgery syndrome pain
4. Ascending and Descending Pathways
Ascending. First-order neurons synapse in the dorsal horn; second-order neurons decussate within one to two segments and ascend in the contralateral spinothalamic tract. The lateral (neospinothalamic) division carries discriminative information — location and intensity — to the ventral posterolateral thalamic nucleus and then to the somatosensory cortex. The medial (paleospinothalamic) division projects to the reticular formation and limbic system and carries the affective, unpleasant dimension of pain.
Descending inhibition. The periaqueductal gray projects to the rostral ventromedial medulla and then via the dorsolateral funiculus to the dorsal horn, where norepinephrine and serotonin inhibit transmission. This pathway explains why alpha-2 agonists (clonidine, dexmedetomidine) and serotonin-norepinephrine reuptake inhibitors (duloxetine) are analgesic, and why endogenous opioids act supraspinally as well as spinally.
5. Sensitization: How Acute Pain Becomes Chronic
Peripheral sensitization
Tissue injury releases an inflammatory soup — bradykinin, prostaglandins, substance P, calcitonin gene-related peptide, histamine, serotonin, hydrogen ions, adenosine triphosphate, and nerve growth factor — that lowers the nociceptor threshold. The result is primary hyperalgesia: exaggerated pain at the site of injury. NSAIDs act here.
Central sensitization and wind-up
Repetitive C-fiber input depolarizes the dorsal horn neuron enough to expel the magnesium ion plug from the NMDA receptor. Glutamate can then act at the now-unblocked NMDA receptor, and each successive identical stimulus produces a progressively larger response. This is wind-up.
Consequences of central sensitization:
- Secondary hyperalgesia — increased pain sensitivity in uninjured tissue surrounding the injury
- Allodynia — pain from a normally non-painful stimulus such as light touch
- Expansion of receptive fields and persistence of pain after the stimulus stops
Ketamine is an NMDA receptor antagonist and is therefore the pharmacologic answer to wind-up. Magnesium, methadone (which has NMDA antagonism in addition to mu agonism), and nitrous oxide share this mechanism. Preemptive and preventive multimodal analgesia aims to prevent central sensitization from becoming established rather than to treat it afterward.
Opioid-induced hyperalgesia
Distinct from tolerance. In tolerance, more opioid is needed for the same effect and increasing the dose works. In opioid-induced hyperalgesia, pain worsens as the opioid dose rises, becomes diffuse and poorly localized, and is often accompanied by allodynia. High-dose intraoperative remifentanil is the classic trigger. Treatment is to reduce the opioid and add ketamine, an alpha-2 agonist, or regional analgesia.
6. Chronic Pain: Definitions and Pathology
Chronic pain is pain persisting beyond 3 months or beyond the expected period of tissue healing. It is not simply prolonged acute pain — it involves neuroplastic change in the dorsal horn and brain.
| Pain type | Character | Typical responsiveness |
|---|---|---|
| Nociceptive (somatic) | Sharp, well localized, aching | Opioids, NSAIDs, regional blocks |
| Nociceptive (visceral) | Dull, cramping, poorly localized, referred | Opioids, sympathetic blocks |
| Neuropathic | Burning, shooting, electric, with allodynia and numbness | Poorly opioid responsive; gabapentinoids, TCAs, SNRIs |
| Nociplastic | Widespread, disproportionate to identifiable injury (e.g., fibromyalgia) | Non-opioid, multimodal, exercise, cognitive therapy |
Chronic postsurgical pain
Pain persisting more than 3 months after surgery, not explained by other causes, is common and frequently neuropathic from intraoperative nerve injury:
| Procedure | Approximate incidence |
|---|---|
| Limb amputation (phantom limb pain) | 50 to 80 percent |
| Thoracotomy | 30 to 50 percent |
| Mastectomy | 20 to 50 percent |
| Inguinal hernia repair | about 10 percent |
| Coronary artery bypass grafting | 30 to 50 percent |
Psychodynamics
The NCE outline explicitly names psychodynamics as part of pain theory. Pain catastrophizing, preoperative anxiety and depression, high preoperative opioid use, female sex, younger age, and severe acute postoperative pain are the strongest predictors of chronic postsurgical pain. This is why psychological preparation and aggressive control of acute pain are preventive, not merely comforting.
Complex regional pain syndrome
- Type I (formerly reflex sympathetic dystrophy): no identifiable nerve lesion
- Type II (formerly causalgia): a definable peripheral nerve injury
Both are diagnosed clinically by the Budapest criteria, requiring symptoms and signs in sensory (hyperalgesia, allodynia), vasomotor (temperature and color asymmetry), sudomotor/edema, and motor/trophic categories, with no better explanation. Early mobilization and physical therapy are central to treatment; sympathetic blockade is diagnostic and sometimes therapeutic.
Exam Traps
- A-delta = first, fast, sharp. C = second, slow, burning. Reversing these is the most common error.
- Wind-up is an NMDA phenomenon, so ketamine — not more opioid — is the answer.
- Secondary hyperalgesia is central; primary hyperalgesia is peripheral.
- Escalating opioid with worsening diffuse pain is opioid-induced hyperalgesia; reduce the opioid.
A patient reports that after a needle stick they feel an immediate sharp jab followed about one second later by a diffuse burning ache. Which fiber types account for the first and second sensations respectively?
A patient who received a high-dose remifentanil infusion for a five-hour spine fusion reports severe diffuse pain in the recovery unit with allodynia extending well beyond the incision, and pain worsens after each additional hydromorphone bolus. What is the most appropriate management?
Transcutaneous electrical nerve stimulation relieves pain primarily by which mechanism?
Which patient has the highest expected incidence of chronic postsurgical pain?