Pain Physiology & Assessment
Pain assessment High-yield Verified · Sep 2026Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (IASP, as quoted by Open RN); nociception is the detection and transmission of the noxious signal that usually, but not always, produces it.
Pain is what the patient says it is, but it is also physiology: nociceptors, A-delta and C fibers, the dorsal horn, the thalamus and cortex, and descending pathways that turn the signal up or down. Knowing the type (nociceptive, neuropathic, nociplastic) and the time course (acute, subacute, chronic) points to the right treatment. Knowing the assessment tools, and the order in which sedation and respiratory depression appear, keeps opioid therapy safe.
What’s going wrong in the body
The normal physiology, where it fails, and why the body’s compensation backfires.
01 From stimulus to experience: nociception, perception and modulation
Nociception is not the same as pain. StatPearls: nociception is "the detection of noxious stimuli by nociceptors, followed by transduction and transmission of the sensory nervous information from the periphery to the brain," while pain "entails the actual unpleasant emotional and sensory experience generated from nervous signals." "Nociception can occur without subsequent awareness of pain, and pain can be present without a measurable underlying noxious stimulus."
Transduction. Nociceptors in "the skin, joints, viscera, bone, and muscle" respond to heat or cold, excess pressure and chemicals. "Noxious stimuli are detected by nociceptors and transduced into electrical energy." Damaged tissue releases mediators — including histamine, substance P, nerve growth factor and arachidonic acid — an inflammatory mix that "is secreted at the site of injury to stimulate nociceptor activation."
Transmission and perception. "A-Delta fibers are fast-conducting fibers and associated with the initial sharp, stinging, or pricking pain sensation. C fibers are slower-conducting fibers and are associated with the secondary sensation of diffuse, dull, burning, and aching pain." The impulse travels to the dorsal horn of the spinal cord, then to the thalamus and on to the cortex. "In the cerebral cortex, pain impulses are perceived, and the conscious awareness of pain occurs."
Modulation. Brainstem centres (the rostral ventral medulla and periaqueductal gray) engage descending pathways that release endogenous opioids, GABA and glycine to limit pain, while substance P, glutamate and aspartate excite it. Touch fibers can also damp the signal at the spinal cord through inhibitory interneurons: "This is the primary mechanism behind rubbing a wound in an effort to dull the sharp pain." Thoughts and feelings modulate too — "Attentional orienting to the painful sensation and its source can serve to heighten the painful experience," and expectations, emotion and coping skills feed back on perception. (Transduction → transmission → perception → modulation is a teaching framework built from these steps.)
02 Types of pain — mechanism decides treatment
Three mechanisms (CDC 2022). Pain has traditionally been sorted as nociceptive (for example, osteoarthritis and muscular back pain) or neuropathic (for example, diabetic neuropathy and postherpetic neuralgia). Nociplastic pain, as in fibromyalgia, has been proposed as a third category, with "augmented central nervous system pain and sensory processing and altered pain modulation."
Neuropathic pain is, in IASP’s definition as relayed by Open RN, "pain caused by a lesion or disease of the somatosensory nervous system," often described as burning or like pins and needles. It follows diabetes, stroke and HIV, and "is generally undertreated because it typically does not respond to analgesics" — adjuvants such as tricyclic antidepressants and gabapentin are used instead. StatPearls: "Sharp or throbbing sensations typically indicate acute nociceptive pain, whereas burning, shooting, pins-and-needles, or electric shock–like qualities suggest a neuropathic origin."
Where nociceptive pain comes from. "Visceral pain is diffuse, difficult to locate, and often referred to a distant, usually superficial, structure," and may come with nausea and vomiting. Deep somatic pain (ligaments, tendons, bones, muscles) is dull, aching and poorly localized; superficial pain from the skin is sharp and clearly located. Referred pain is felt away from the stimulus — retained colon gas felt in the shoulder, or neck, shoulder and back pain with a myocardial infarction.
Sensitization. "Pain resulting from a typically harmless stimulus is referred to as allodynia," and hyperalgesia is an exaggerated response to a painful stimulus. HHS adds opioid-induced hyperalgesia: "Hyperalgesia is a condition where patients have a hypersensitivity to pain caused by pain medications."
03 Acute, subacute and chronic pain
Time course (CDC 2022, adult outpatient guideline). Pain is classed as "acute (duration of <1 month), subacute (duration of 1–3 months), or chronic (duration of >3 months) pain." Acute pain is sudden and time-limited, often from injury, trauma or surgery; unresolved acute or subacute pain can evolve into chronic pain. HHS: "Acute pain and chronic pain are often interlinked, with most cases of chronic pain beginning as acute pain."
Chronic pain rewires the system. StatPearls: "Chronic pain, typically defined as pain persisting for more than 3 months, often involves maladaptive neuroplasticity and impaired descending modulation" — it "often reflects central nervous system dysfunction rather than peripheral nociception." CDC: about one in five U.S. adults had chronic pain in 2019, and about one in 14 had high-impact chronic pain that limited life or work.
Unrelieved pain is harmful. AHRQ: "It is often actually physiologically unsafe to have pain," with effects on endocrine, metabolic, cardiovascular, gastrointestinal and immune function. StatPearls: "Thorough assessment and treatment of acute pain remain critical to preventing progression to chronic pain." Chronic pain is linked with depression and anxiety and "an increased risk of suicidal ideation."
04 Opioid safety: sedation comes before respiratory depression
The warning sign is sedation. AHRQ: "Opioid-induced respiratory depression is preceded by an increasing level of sedation. An alert patient will not suddenly succumb to respiratory depression." Watching sedation and lowering the opioid prevents it. Nurse checks of sedation and breathing are more reliable than pulse oximetry alone — "decreased oxygen saturation is a later sign of impending respiratory depression."
Who is at highest risk. Open RN: patients receiving their first opioid dose, those receiving an increased dose, and those "taking benzodiazepines or other sedatives concurrently with opioids." A sedation scale is used for opioid-naive patients starting opioids for moderate to severe pain, at least every 2 hours for the first 24 hours (AHRQ, describing the Pasero Opioid-induced Sedation Scale).
Tolerance, physical dependence and opioid use disorder are different things. HHS: "Tolerance is present when the same dose of a drug when given repeatedly produces a reduced biological response." "Physical dependence is not the same as addiction" — it is a physiologic adaptation that causes withdrawal when the drug is stopped suddenly or an antagonist is given. Opioid use disorder is diagnosed when repeated opioid use causes significant problems or distress and at least two of 11 defined criteria occur within a year (CDC, restating DSM-5); CDC: "opioid use disorder is a chronic, treatable disease from which persons can recover and continue to lead healthy lives."
Causes & risk factors
Modifiable factors are teaching targets — the patient can act on them.
Signs & symptoms
Each finding tied to the mechanism that produces it — and the triage it calls for.
Diagnostics
Complications
Medications for this condition
Why each class is prescribed here — every card links to its full pharmacology page.
Nonopioid for mild pain and fever — no anti-inflammatory effect. Open RN: "A potential severe side effect of acetaminophen is hepatotoxicity (severe liver damage)"; calculate the cumulative 24-hour dose before giving another. Part of multimodal analgesia.
Nonopioid for mild to moderate pain that also reduces fever and inflammation by blocking prostaglandin production. Watch older adults and patients on warfarin or corticosteroids for GI bleeding (melena); renal failure and, except with aspirin, heart attack, heart failure and stroke are risks.
For moderate to severe pain. Morphine has no ceiling effect — more analgesia, but also more sedation and respiratory depression. Monitor sedation and respiratory status before each dose; PCA is safe only because the patient must be alert enough to press the button, so no one else presses it. Start a bowel program with the first dose. CDC 2022 (adult outpatients): nonopioid therapies are at least as effective for many common acute pains and preferred for subacute and chronic pain; when opioids are used, prescribe the lowest effective dose of immediate-release opioids for no longer than needed, and offer naloxone.
Adjuvants for neuropathic pain. StatPearls: initial treatment "typically involves gabapentin or pregabalin." Open RN names gabapentin as an adjuvant that adds to, or works independently of, opioid analgesia.
Adjuvant for neuropathic pain. StatPearls: neuropathic pain may respond better to serotonin-norepinephrine reuptake inhibitors such as duloxetine than to NSAIDs like ibuprofen in acute injuries. HHS also lists TCAs and SNRIs.
Beyond medications
- Multimodal care — combine drug and non-drug approaches tailored to the patient; nonpharmacological interventions "often provide tremendous benefits to the client."
- Complementary approaches — acupuncture, massage therapy, meditation, relaxation techniques, spinal manipulation, Tai Chi, yoga and dietary supplements (Open RN).
- Pre-empt pain (Open RN, nursing-intervention list) — give analgesia before pain becomes severe and before pain-inducing activities such as physical therapy; around-the-clock dosing may be used for the first 24–48 hours after surgery or injury unless sedation or breathing says otherwise.
- Pain journal for chronic pain, kept by the patient or a caregiver.
- Avoid the IM route — AHRQ: "The IM route of administration is not recommended for pain management." It is painful, with unreliable absorption and a lag to peak effect.
Nursing considerations
The RN-specific layer — each action paired with the reason it matters.
How a real nurse thinks
The clinical-judgment loop, worked for a typical presentation — reasoning, not reference.
- 1 Recognize cues
Opioid-naive patient, benzodiazepine last night, PCA doses doubled by someone other than the patient, now frequently drowsy and drifting off, RR 10. SpO2 still 95%.
- 2 Analyze cues
Sedation is increasing — the step that comes before opioid respiratory depression. She has three high-risk features: first opioid exposure, increased dose, and a concurrent sedative. The normal SpO2 is falsely reassuring; desaturation is a later sign.
- 3 Prioritize
Breathing first: stop further opioid, keep her roused and breathing, and get help before sedation deepens. Pain control and teaching come after she is safe.
- 4 Generate solutions
Pause the PCA, stay with her and stimulate her, monitor sedation and respiratory rate closely, notify the prescriber, have naloxone ready per protocol if she becomes unresponsive, stop the daughter pressing the button, and plan multimodal non-opioid analgesia with the prescriber.
- 5 Take action
The nurse pauses the PCA, rouses the patient, notifies the surgeon with sedation level, RR and PCA use, and explains to the daughter that only the patient may press the button because needing to be awake enough to press it is the safety check.
- 6 Evaluate
Thirty minutes later she is awake and talking in full sentences, RR 16. After a scheduled nonopioid dose and a reduced PCA setting, she rates pain 3/10 and walks to the chair; sedation is rechecked before each further opioid dose.
Scenario — you’re the nurse
Work each question before revealing the answer. Rationales explain the wrong options too.
A 70-year-old woman, 18 hours after open abdominal surgery, has an IV opioid patient-controlled analgesia (PCA) pump. She had never taken opioids before surgery. Her home medicines include a nightly benzodiazepine for sleep, which she took last night. Her daughter is at the bedside.
Q1 Multiple choice Which finding is the earliest warning that this patient is developing opioid-induced respiratory depression? - A. SpO2 95%
- B. Falling asleep in the middle of a sentence
- C. Pain 8/10 at 0700
- D. Small pupils
Reveal answer & rationale
Q2 Multiple choice What should the nurse do first? - A. Stop further opioid delivery and stay with the patient to rouse her and monitor sedation and breathing
- B. Give a full rapid IV push of naloxone
- C. Reassess pain in 45–60 minutes
- D. Teach the daughter to press the button only once an hour
Reveal answer & rationale
Q3 Select all that apply Which factors increased this patient’s risk of respiratory depression? Select all that apply. - A. She had never taken opioids before
- B. The dose delivered increased
- C. She took a benzodiazepine last night
- D. She reports her pain accurately
- E. Someone other than the patient pressed the PCA button
Reveal answer & rationale
Q4 Select all that apply The nurse cares for three other patients who cannot self-report pain. Match the correct tool. Select all that apply. - A. FLACC for a 3-year-old after tonsillectomy
- B. PAINAD for an older adult with advanced dementia
- C. CPOT for an intubated ICU adult
- D. The nurse selects the FACES picture that matches the child’s expression
- E. A behavior score of 6 on a behavior tool equals a pain intensity of 6/10
Reveal answer & rationale
Q5 Multiple choice A patient taking opioids for months needs a higher dose for the same relief and has withdrawal symptoms when a dose is late. The nurse recognizes that this describes: - A. Opioid use disorder
- B. Tolerance and physical dependence, which are expected adaptations
- C. Opioid-induced hyperalgesia
- D. Neuropathic pain
Reveal answer & rationale
Common questions
What are the four steps of nociception?
When does acute pain become chronic?
Are normal vital signs proof that a patient is not in pain?
Why do nurses check sedation rather than just oxygen saturation on opioids?
How soon should pain be reassessed after a dose?
Is needing more opioid over time the same as addiction?
Sources
- Nursing Fundamentals 2e — Chapter 11: Comfort — Open RN — Open RN, Chippewa Valley Technical College (NCBI Bookshelf, CC BY 4.0)
- Physiology, Pain — StatPearls — StatPearls (NCBI Bookshelf)
- Pain Assessment — StatPearls — StatPearls (NCBI Bookshelf)
- Chronic Pain — StatPearls — StatPearls (NCBI Bookshelf)
- CDC Clinical Practice Guideline for Prescribing Opioids for Pain — United States, 2022 (MMWR) — Centers for Disease Control and Prevention
- Improving the Quality of Care Through Pain Assessment and Management — Patient Safety and Quality: An Evidence-Based Handbook for Nurses, Ch. 17 — Agency for Healthcare Research and Quality
- Pain Management Best Practices Inter-Agency Task Force Report (2019) — U.S. Department of Health and Human Services
- Evaluating pain in non-verbal critical care patients: a narrative review of the critical care pain observation tool (Afenigus 2024) — Frontiers in Pain Research (PMC, CC BY)
Reviewed by Hae Suk Lee, RN
Educational summary for nursing students. Scenarios are fictional teaching cases. Always verify against current guidelines and your institution's protocols before acting in practice. Not medical advice.