REHAB · EP 01 · PAIN
Before You Listen
Episode Setup
- Topic in one line: the foundational pain taxonomy, four-stage pain neuroanatomy, dorsal horn laminae and gate control theory, descending inhibition, central sensitization, and the non-opioid analgesic pharmacology (nonsteroidal anti-inflammatory drugs (NSAIDs), acetaminophen, gabapentinoids, tricyclic antidepressants (TCAs), serotonin-norepinephrine reuptake inhibitors (SNRIs)) that every physiatrist needs cold on the American Board of Physical Medicine and Rehabilitation (ABPMR) Part I examination.
- Prerequisites: basic peripheral and central neuroanatomy, the spinothalamic tract, pre- and postsynaptic membrane physiology, voltage-gated calcium and sodium channels, and glucuronide versus oxidative drug metabolism.
- Runtime: approximately 40 minutes for Part 1.
- Scope boundary: Part 1 maps pain taxonomy, the neuroanatomy of pain signaling, the dorsal horn, descending inhibition, central sensitization, and the non-opioid pharmacology pillar (NSAIDs, acetaminophen, gabapentinoids, TCAs, SNRIs). Part 2 turns to opioid pharmacology (mu/kappa/delta receptor subtypes, equianalgesic conversion, morphine metabolites in renal failure, methadone, buprenorphine, opioid-induced hyperalgesia) and local anesthetic safety (amides vs esters, maximum doses, local anesthetic systemic toxicity (LAST), lipid emulsion rescue).
Vignette. A 58-year-old woman with a 5-year history of painful diabetic peripheral neuropathy returns to clinic. She describes constant burning in both feet at rest, intolerance of bedsheets brushing her toes, and a sharp shooting jolt that travels up her calf when she steps onto a cold tile floor. Her current regimen is acetaminophen 1 gram every 6 hours and oxycodone 10 mg three times daily; she still rates her pain 7 out of 10 and has gained 4 kg from inactivity. Glycated hemoglobin is 7.4 percent. Estimated glomerular filtration rate (eGFR) is 38 mL/min/1.73 m². On exam light touch on the dorsum of the foot is reported as painful, pinprick produces an exaggerated and prolonged response, and ankle reflexes are absent.
What is the mechanism-based pain category, which two terminology terms apply to her exam findings, what three first-line non-opioid agents target the underlying mechanism, and which currently prescribed agent should be tapered first and why?
(Answer at the end of this chapter)
Section 1: Pain Epidemiology and the Rehabilitation Burden
Bottom line: chronic pain affects roughly 20–21% of US adults with high-impact pain (limiting daily activity) in 7–8%, total economic cost in the $560–635 billion range when lost productivity is included; sex skew is striking in selected conditions (fibromyalgia female-to-male ratio ~7:1); the rehabilitation problem set sits atop this pain backbone — globally ~12 million people live with spasticity, ~15 million with dysphagia, ~2.5 million with pressure injuries, and ~14 million falls per year in older US adults — placing pain at the center of every PM&R subspecialty.
Chronic pain is a population-level disease. The CDC’s most recent NHIS analysis estimates 20.9% of US adults met criteria for chronic pain (pain on most days or every day for the past 3 months) and 7.4% met criteria for high-impact chronic pain (chronic pain that limits life or work activities). Translated to incidence, that is approximately 51.6 million Americans with chronic pain and 17.1 million with high-impact pain at any point in time — a population larger than diabetes, heart disease, or cancer prevalence in the same cohort.
Economic burden. Total annual cost of chronic pain in the United States is conservatively estimated at $560 billion to $635 billion, including direct medical spending plus lost productivity. This makes chronic pain the costliest condition in US healthcare by population, exceeding the combined cost of cardiovascular disease and cancer.
Sex distribution. Chronic pain prevalence is higher in women across nearly every diagnosis tested. Fibromyalgia has the most striking skew at roughly 7:1 female-to-male ratio in clinical series (population-based 2010 ACR criteria narrow this closer to 2–3:1, but the clinical population remains overwhelmingly female). Migraine, temporomandibular dysfunction, complex regional pain syndrome (CRPS), and irritable bowel syndrome all show 2–4× female predominance. The neurobiologic basis (estrogen modulation of nociception, sex differences in microglial activation, differential descending inhibition) is active research; for the boards, the direction of the sex skew is what is tested.
Pain anchors the rehabilitation problem set. Three companion epidemiology numbers worth memorizing in parallel: globally an estimated ~12 million people live with spasticity (the leading post-stroke, SCI, MS, and cerebral palsy sequela), ~15 million with dysphagia (post-stroke, neurodegenerative, head-and-neck cancer), and ~2.5 million pressure injuries are managed annually in US inpatient care alone. Falls in older adults account for ~14 million reported falls per year in the United States with downstream consequences including TBI, hip fracture, and the fear-of-falling cycle. Each of these endpoints carries pain as a primary symptom, secondary driver, or rehabilitation barrier — which is why PM&R training centers pain expertise as a horizontal skill across every body system.
High Yield — Pain epidemiology numbers worth memorizing
- US adults with chronic pain ≈ 21% (≈ 52 million people); high-impact chronic pain ≈ 7.4% (≈ 17 million).
- Annual US economic cost ≈ $560–635 billion — the costliest single condition by population.
- Fibromyalgia F:M ≈ 7:1 in clinical series; 2–3:1 with strict ACR 2010 criteria.
- Migraine, TMD, CRPS, IBS all 2–4× female-predominant.
- Spasticity ≈ 12 million globally; dysphagia ≈ 15 million; pressure injuries ≈ 2.5 million/year US inpatient; falls ≈ 14 million/year US older adults.