# Chronic Pain Fundamentals for the Anesthesiology Resident

## Introduction

Chronic pain, defined as pain persisting beyond the normal healing period (typically greater than 3 months), affects an estimated 20% of adults worldwide. As anesthesiologists increasingly practice in pain medicine, a solid foundation in chronic pain mechanisms, assessment, and multimodal treatment is essential. This lecture provides the framework for understanding chronic pain from a residency training perspective.

## Pain Physiology and Classification

### Nociceptive Pain

Nociceptive pain arises from activation of nociceptors (A-delta and C fibers) by tissue-damaging stimuli. Somatic nociceptive pain is well-localized with a sharp or aching quality, arising from skin, muscle, bone, and joints. Visceral nociceptive pain is poorly localized with a deep, cramping quality, arising from internal organs and often referred to somatic dermatomes (for example, cardiac pain referred to the left arm). Nociceptive pain responds to anti-inflammatory agents, acetaminophen, and opioids.

### Neuropathic Pain

Neuropathic pain results from damage or dysfunction of the somatosensory nervous system and is characterized by burning, shooting, or electric shock-like quality. Associated features include allodynia (pain from a normally non-painful stimulus) and hyperalgesia (exaggerated pain from a mildly painful stimulus). Examples include diabetic peripheral neuropathy, postherpetic neuralgia, phantom limb pain, and complex regional pain syndrome (CRPS). This type of pain responds to anticonvulsants (gabapentin, pregabalin), SNRIs (duloxetine), tricyclic antidepressants, and topical lidocaine.

### Nociplastic Pain

Nociplastic pain arises from altered nociceptive processing without clear evidence of tissue damage or a somatosensory lesion. It involves central sensitization with enhanced excitability of central neurons, expanded receptive fields, and loss of descending inhibition. Examples include fibromyalgia, irritable bowel syndrome, tension-type headache, and chronic pelvic pain. Treatment responds to centrally acting agents, physical therapy, cognitive behavioral therapy (CBT), and neuromodulation.

### Mixed Pain States

Many chronic pain conditions involve multiple mechanisms simultaneously. For example, chronic low back pain may have nociceptive (facet arthropathy), neuropathic (radiculopathy), and nociplastic (central sensitization) components.

| Pain Type | Mechanism | Character | Examples | First-Line Pharmacotherapy |
|---|---|---|---|---|
| Nociceptive (somatic) | Nociceptor activation by tissue damage | Sharp, aching, well-localized | Fracture, osteoarthritis, surgical incision | NSAIDs, acetaminophen, opioids |
| Nociceptive (visceral) | Organ nociceptor activation | Deep, cramping, poorly localized, referred | Pancreatitis, renal colic, cardiac ischemia | NSAIDs, opioids, nerve blocks |
| Neuropathic | Somatosensory nerve damage/dysfunction | Burning, shooting, electric | DPN, postherpetic neuralgia, CRPS, phantom limb | Gabapentinoids, SNRIs, TCAs, topical lidocaine |
| Nociplastic | Altered central processing; central sensitization | Diffuse, widespread, amplified | Fibromyalgia, IBS, tension headache, chronic pelvic pain | SNRIs, gabapentinoids, CBT, exercise, neuromodulation |

![Diagram of pain transmission pathways from peripheral nociceptor to cortex](images/pain-pathways.png)

## Chronic Pain Assessment

### History

The history should address location, quality, and intensity (using a numeric rating scale of 0 to 10 or a visual analog scale), as well as the temporal pattern (constant versus intermittent, duration, aggravating and alleviating factors). Functional impact on sleep, work, activities of daily living, mood, and relationships should be assessed. Previous treatments including medications tried (with doses and duration), interventional procedures, physical therapy, and psychological treatments are reviewed. Red flags warranting further investigation include unexplained weight loss, fever, progressive neurologic deficit, bowel or bladder dysfunction, and a history of malignancy.

### Psychosocial Evaluation

Depression and anxiety are comorbid in 30 to 50% of chronic pain patients. Screening with validated tools such as the PHQ-9 for depression and the GAD-7 for anxiety is recommended. Catastrophizing, assessed with the Pain Catastrophizing Scale (PCS), is a strong predictor of poor outcomes. Secondary gain, disability status, and litigation are important contextual factors. Substance use history covering current and past opioid, benzodiazepine, alcohol, and illicit drug use should be obtained.

### Physical Examination

A focused neurologic exam includes sensory testing (light touch, pinprick, temperature), motor strength, and deep tendon reflexes. Provocative tests specific to the pain syndrome (straight leg raise, Spurling test, Tinel sign) are performed. Myofascial trigger points are assessed. Observation of pain behaviors, guarding, and antalgic gait provides additional information.

## Pharmacologic Management

### Non-Opioid Analgesics

Acetaminophen is a first-line agent with a ceiling dose of 3 to 4 g/day (2 g/day in hepatic impairment) and minimal anti-inflammatory effect. NSAIDs are effective for inflammatory and musculoskeletal pain but carry GI, renal, and cardiovascular risks with chronic use. COX-2 selective inhibitors such as celecoxib have reduced GI risk but similar cardiovascular risk.

### Adjuvant Analgesics

Gabapentinoids (gabapentin, pregabalin) are first-line for neuropathic pain and should be titrated slowly; side effects include sedation, dizziness, and peripheral edema. Tricyclic antidepressants (amitriptyline, nortriptyline) are effective for neuropathic pain and fibromyalgia at sub-antidepressant doses of 10 to 75 mg/day, though anticholinergic side effects limit use in the elderly. SNRIs (duloxetine, venlafaxine) provide dual serotonin and norepinephrine reuptake inhibition that enhances descending pain inhibition and are first-line for diabetic neuropathy and fibromyalgia. Topical agents including lidocaine 5% patch, capsaicin 8% patch, and compounded topicals minimize systemic side effects. Muscle relaxants such as cyclobenzaprine, tizanidine, and baclofen are used short-term for musculoskeletal spasm. Ketamine, an NMDA receptor antagonist, is used for refractory neuropathic pain, central sensitization, and opioid-induced hyperalgesia, available as IV infusion or intranasal formulation.

### Opioid Therapy for Chronic Non-Cancer Pain

Opioids are reserved for moderate-to-severe pain that has not responded to non-opioid therapies. Treatment should start at the lowest effective dose, preferring long-acting formulations with short-acting opioids for breakthrough pain. A morphine equivalent daily dose (MEDD) exceeding 90 mg is associated with significantly increased overdose risk, which is the CDC guideline threshold. Mandatory components of chronic opioid therapy include a written treatment agreement (opioid contract), regular urine drug testing, prescription drug monitoring program (PDMP) review at each visit, periodic reassessment of the 4 A's (Analgesia, Activity, Adverse effects, Aberrant behavior), and naloxone co-prescription for patients at elevated overdose risk.

![WHO analgesic ladder modified for chronic pain with adjuvant therapies](images/chronic-pain-analgesic-ladder.png)

## Interventional Pain Management

### Epidural Steroid Injections (ESIs)

ESIs are indicated for radicular pain from herniated disc or spinal stenosis. Approaches include interlaminar, transforaminal, and caudal. They provide short-to-intermediate term relief (weeks to months) and are most effective when combined with physical therapy. Risks include dural puncture, epidural hematoma, infection, and rare catastrophic neurologic injury from particulate steroid arterial injection.

### Facet Joint Interventions

Medial branch blocks are diagnostic blocks of the nerves innervating the facet joints. Radiofrequency ablation (RFA) provides thermal denervation of medial branch nerves after positive diagnostic blocks, yielding 6 to 12 months of relief. Evidence is strongest for lumbar facet pain, with cervical facet pain also amenable to treatment.

### Sacroiliac Joint Interventions

Intra-articular injection serves both diagnostic and therapeutic purposes. Lateral branch RFA is an emerging technique for prolonged relief.

### Neuromodulation

Spinal cord stimulation (SCS) uses implanted electrodes in the epidural space to deliver electrical pulses that modulate pain signals. Indications include failed back surgery syndrome, CRPS, peripheral neuropathy, and angina. Modern paradigms include high-frequency stimulation (10 kHz), burst stimulation, and dorsal root ganglion (DRG) stimulation. Intrathecal drug delivery systems use implanted pumps for refractory pain, delivering agents such as ziconotide, baclofen, morphine, or hydromorphone.

### Sympathetic Blocks

Stellate ganglion block is used for CRPS of the upper extremity and facial pain syndromes. Lumbar sympathetic block addresses CRPS of the lower extremity and peripheral vascular disease. Celiac plexus block or neurolysis targets pancreatic cancer pain. Superior hypogastric plexus block is used for pelvic pain.

## Non-Pharmacologic Therapies

Physical therapy and exercise have the strongest evidence base for chronic low back pain, osteoarthritis, and fibromyalgia. Cognitive behavioral therapy (CBT) addresses catastrophizing, fear-avoidance, and maladaptive coping. Acceptance and commitment therapy (ACT) promotes psychological flexibility and values-based living despite pain. Mindfulness-based stress reduction (MBSR) has evidence for fibromyalgia and chronic low back pain. Acupuncture has moderate evidence for chronic low back pain, osteoarthritis of the knee, and headache.

![Multimodal chronic pain management framework integrating pharmacologic, interventional, and behavioral approaches](images/multimodal-pain-management.png)

## Key Clinical Pearls

Chronic pain is a disease of the nervous system, not merely a symptom; central sensitization and neuroplastic changes maintain pain long after the original injury has healed. Depression, anxiety, and catastrophizing should always be assessed in chronic pain patients because these psychosocial factors are among the strongest predictors of treatment outcomes. Multimodal therapy combining pharmacologic, interventional, physical, and psychological approaches is more effective than any single modality alone. Opioids have a limited role in chronic non-cancer pain and carry significant risks; they should never be the sole treatment and must be monitored rigorously.

## References

1. Cohen SP, Vase L, Hooten WM. Chronic pain: an update on burden, best practices, and new advances. *Lancet*. 2021;397(10289):2082-2097.
2. Dowell D, Ragan KR, Jones CM, Baldwin GT, Chou R. CDC Clinical Practice Guideline for Prescribing Opioids for Pain. *MMWR Recomm Rep*. 2022;71(3):1-95.
3. Deer TR, Pope JE, Hayek SM, et al. The Polyanalgesic Consensus Conference (PACC): recommendations on intrathecal drug infusion systems. *Neuromodulation*. 2017;20(2):96-132.
4. Fitzcharles MA, Cohen SP, Clauw DJ, Littlejohn G, Usui C, Hauser W. Nociplastic pain: towards an understanding of prevalent pain conditions. *Lancet*. 2021;397(10289):2098-2110.