Posted by: faithful | March 20, 2007

the nature of pain

Persis Mary Hamilton, BSN, MS, EdD


  • Explain the nature of pain, its definitions, characteristics, types, and sources.
  • Trace a pain stimulus from tissue damage to modulation.
  • Differentiate somatic, visceral, neuropathic, and psychogenic pain.
  • Discuss factors that influence the perception of pain.
  • Contrast standards of care for pain management of the JCAHO and AAP.


Pain is a universal human experience and the most common reason people seek medical care. Pain tells us something is wrong, that tissue in our body has been damaged, and we need to do something to change the situation. Because pain is such a strong motivator for action, it is considered one of the body’s most important protective mechanisms.

Definitions of Pain

The International Association for the Study of Pain defined pain as “an unpleasant, subjective, sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage (1979).” Pain, however, is much more than a physical sensation caused by a specific stimulus. It is a complex mechanism with physical, emotional, and cognitive components. It is subjective, and highly individual.

Pain cannot be objectively measured in the same way as, for example, the chemical content of urine or the oxygen content level of blood. Only the person who is suffering knows how the experience feels. For these reasons, McCaffery defined pain as “whatever the experiencing person says it is and whenever he says it does (1979).” The American Pain Society goes further by stating that it is “not the responsibility of clients to prove they are in pain; it is the nurse’s responsibility to accept the clients report of pain (2005).”

Comfort: The Absence of Pain

Pain alters the quality of life more than any other health-related problem. It interferes with sleep, mobility, nutrition, thought, sexual activity, emotional well-being, creativity, and self-actualization. Surprisingly, even though pain is such an important obstacle to comfort, it is one of the least understood, most under-treated and oft-discounted problems of healthcare providers and their clients. For this reason, some nurses add “comfort” to Maslow’s hierarchy of basic human needs (1968). The American Pain Foundation goes further, declaring the relief of pain a “basic human right (2001)” and the American Bar Association called it a “basic legal right (2000).”

Characteristics of Pain

Traditionally, pain was considered merely a physical symptom of illness or injury, a simple stimulus-response mechanism. Though the historic role of nurses has been to relieve pain and suffering, there has been little understanding of the complexity of pain and there have been only limited ways to manage it. Recent research shows pain to be a distinct disorder, with physical, emotional, and cognitive components. This view of pain has broadened our understanding of pain and given us new ways to understand its characteristics, as described in Box 1-1.

Algesia: Sensitivity to pain.

Breakthrough pain: Transitory increase in pain to a level greater than the client’s well-controlled baseline level (McCaffery & Pasero, 2003).

Hyperalgesia: Excessive sensitivity to pain.

Idiopathic pain: Chronic pain for which there is no identifiable psychological or physical cause.

Intractable pain: Pain that is highly resistant to relief, such as an in advanced cancer.

Pain threshold: Amount of pain required before individuals feel the pain. The lower the threshold, the less pain they can endure; the higher the threshold, the more pain they can endure.

Pain tolerance: Maximum amount and duration of pain a person is able to endure. Tolerance varies widely among people and is influenced by emotions and cultural background.

Pain syndrome: A group of symptoms of which pain is the critical element, such as headaches and post-herpetic neuralgia.

Phantom limb pain: Pain that occurs in a limb after it is removed or as a result of severe damage to the affected nerve plexus due to perceptual disruption in the brain.

Psychogenic pain: Chronic pain with no identified organic explanation.

Radiating pain: Pain that begins at its source and extends to nearby tissues.

Referred pain: Pain that is felt at a different location than where tissue was damaged. This phenomenon occurs because pain fibers in the damaged area synapse near fibers from other areas of the body, for example, a myocardial infarction may create referred pain in the left shoulder.

Types of Pain

Pain is classified as acute and chronic. Acute pain has an identifiable cause and occurs soon after an injury to tissues in the body, such as bone, skin, or muscle. Acute pain is protective in that it motivates a person to take action. Its onset may be sudden or slow and its intensity may vary from mild to severe. Acute pain is temporary, lasts less than 6 months, and subsides as healing takes place. Severe acute pain activates the sympathetic nervous system, causing diaphoresis, increased respiratory and pulse rates, and elevated blood pressure.

Chronic pain lasts 6 months or longer and may limit normal functioning. It is nonprotective in that it serves no purpose and may not have an identifiable cause. It is described as nonmalignant (noncancerous) and malignant (cancerous). Nonmalignant, noncancerous chronic pain typically accompanies such conditions as osteoarthritis and peripheral neuropathy. The lack of purpose, and uncertainty of duration, of chronic pain may lead to depression, fatigue, insomnia, anorexia, apathy, and learned helplessness. If it is severe, chronic pain—like prolonged stress—activates the parasympathetic nervous system, resulting in muscle tension, decreased heart rate and blood pressure, and failure of body defenses.

Malignant, cancerous chronic pain may be due to tumor progression, invasive procedures, toxicities of treatment, infection, and physical limitations. Such pain may be felt at the tumor site or some distance from it. Since clients with cancer may experience both chronic and acute pain, caregivers need to investigate any new pain in these clients immediately.

Sources of Pain

The sources of pain are divided into three main categories: nociceptor, non-nociceptor, and psychogenic, as shown in Table 1-1, below.

Nociceptor pain results when tissue damage produces a pain-producing stimulus that sends an electrical impulse across a pain receptor (nociceptor) by way of a nerve fiber to the central nervous system. Nociceptor pain in further divided into visceral and somatic pain. Visceral pain results from stimulation of nociceptors in the abdominal cavity, and thorax. Somatic pain is divided into deep somatic and cutaneous pain. Deep somatic pain arises from bones, tendons, nerves, and blood vessels. Cutaneous pain originates in the skin or subcutaneous tissue. Some body tissues, such as the brain and lung, have no nociceptors and some tissues have many.

Non-nociceptor (neuropathic) pain is caused by direct injury to structures of the nervous system.

Psychogenic pain is pain for which there is little or no physical evidence of organic disease; however, lack of evidence does not mean the client is malingering and not suffering.

Nociceptor: visceral
Physiologic structures Organs and linings of body cavities
Mechanism Activation of nociceptors
Characteristics Poorly localized, diffuse, deep, cramping or splitting
Sources of acute pain Chest tubes, abdominal tube drains, bladder and intestinal distention
Sources of chronic pain syndromes Pancreatitis, liver metastases, colitis
Nociceptor: somatic
Physiologic structures Cutaneous: skin and sub-cutaneous tissues
Deep somatic: blood, muscle, blood vessels, connective tissue
Mechanism Activation of nociceptors
Characteristics Well-localized, constant and achy
Sources of acute pain Incisional pain, insertion sites of tubes and drains, wound complications, orthopedic procedures, skeletal muscle spasms
Sources of chronic pain syndromes Bony metastases, osteo- and rheumatoid arthritis, low-back pain, peripheral vascular disease
Non-nociceptor: neuropathic
Physiologic structures Nerve fibers, spinal cord, and central nervous system
Mechanism Non-nociceptive injury to nervous system structures
Characteristics Non-nociceptive injury to nervous system structures
Sources of acute pain Poorly localized: shooting, burning, fiery, shock-like, sharp, painful numbness
Sources of chronic pain syndromes Nervous tissue injury due to diabetes, HIV, chemotherapy, neuropathies, postherpetic neuralgia
Physiologic structures No organic structures
Mechanism Emotional
Characteristics Variable, often numerous
Sources of acute pain Nonorganic
Sources of chronic pain syndromes Nonorganic psychological factors
Source: Adapted with permission from Ignatavicius et al., 1999.


Though a person is not consciously aware of the process, the experience of pain involves a complex sequence of biochemical and electrical events or processes beginning with tissue damage, followed by transduction, transmission, perception, and modulation. See Figure 1-1.

Illustration of pain pathways

Figure 1-1     Neurological transmission of pain stimuli. Illustration by Jason McAlexander. Copyright © Wild Iris Medical Education.

Tissue Damage

Typically, when tissue damage occurs it releases inflammatory chemicals, called excitatory neurotransmitters, such as histamine and bradykinin, a powerful vasodilator. These substances cause the injured area to swell, redden, and become tender. Bradykinin also stimulates the release of prostaglandins and substance P, a potent neurotransmitter that enhances the movement of impulses across nerve synapses.


Transduction occurs as the energy of the stimulus is converted to electrical energy.


Transmission of the stimulus takes place when energy crosses into a nociceptor at the end of an afferent nerve fiber. Two types of peripheral nerve fibers conduct painful stimuli: the fast, myelinated A-delta fibers and the very small, slow, unmyelinated C-fibers. A-fibers send sharp, distinct sensations that localize the source of the pain and detect its intensity. C-fibers relay impulses that are poorly localized, burning, and persistent. For example, after burning a finger, a person initially feels a sharp localized pain as a result of A-fiber transmission.

Within a few seconds the pain becomes more diffuse and widespread, as a result of C-fiber transmission. Pain stimuli travel quickly to the substantia gelatinosa in the dorsal horn of the spinal cord where the “gating” mechanism (discussed later) occurs. Pain impulses then cross over to the opposite side of the spinal cord and ascend to the higher centers in the brain via the spinothalamic tracts and on to the thalamus and higher centers of the brain, including the reticular formation, limbic system, and somatosensory cortex.


When pain stimuli reach the cerebral cortex, the brain interprets the signal, processes information from experiences, knowledge, and cultural associations, and perceives pain. Thus, perception is the awareness of pain. The somatosensory cortex identifies the location and intensity of pain, and the associated cortex determines how an individual interprets its meaning.


Once the brain perceives the pain, the body releases neuromodulators, such as endogenous opioids (endorphins and enkephalins), serotonin, norepinephrine, and gamma aminobutyric acid. These chemicals hinder the transmission of pain and help produce an analgesic, pain-relieving effect. This inhibition of the pain impulse is called modulation. The descending paths of the efferent fibers extend from the cortex down to the spinal cord and may influence pain impulses at the level of the spinal cord.

Gate-Control Theory

Melzack and Wall proposed the gate-control theory to explain the relationship between pain and the emotions (1982). According to the theory, a gating mechanism occurs when a pain impulse travels to the substantia gelatinosa in the dorsal horn of the spinal cord. There, trigger cells (T-cells) influence the transmission of pain impulses. When their activity is inhibited, the gate closes and impulses are less likely to be transmitted to the brain. This mechanism is controlled by descending nerve fibers from the thalamus and cerebral cortex, areas of the brain that regulate thought, beliefs, and emotions. The gate-control theory helps explain how the thoughts and emotions of a person modify the perception of pain and why interventions such as imagery and distraction help relieve it.


The perception of pain is influenced by physiologic, psychological, and cultural factors, all of which must be considered by caregivers of persons in pain.

Physiologic Factors

Age affects the way people respond to pain. It influences both the development and decline of the nervous system. Aging affects the whole body, exposing the older adult to painful degenerative disorders such as osteoarthritis, painful secondary injuries such as skin abrasions and fractures, and painful surgical procedures such as cataract and hip replacement. Age also affects the way nurses and families respond to a complaint of pain. For example, a nurse may be more sympathetic toward a toddler than an adolescent or a young adult. Table 1-2 gives a brief overview of the perception of pain relative to age.

Age Pain perception
Pre-term infants Have anatomical and functional ability to process pain by mid to late gestation; seem to have greater sensitivity to pain than term infants or children
Newborn infants Response to pain is inborn and does not require prior learning; respond to pain with behavioral cues: facial, crying, body movement
Infants 1 month Infants can metabolize analgesics and anesthesia effectively; can increasingly recognize caregiver as comforter
Toddlers / Preschoolers Can describe pain, its location and intensity; respond to pain by crying, anger, and sadness; may consider pain a punishment; may hold someone accountable for pain and remember experiences in a certain location such as a clinic
School-age children May try to be brave when facing a painful procedure; may regress to earlier stage of development; seek to understand reasons for pain
Adolescents May be slow to acknowledge pain; may consider showing signs of pain a weakness; with persistent pain may regress to earlier stages of development
Adults Fear of pain may prevent some adults from seeking care; may believe admission of pain is a weakness and inappropriate for age or sex; may consider pain a punishment for moral failure
Older adults May have decreased sensations or perceptions of pain; may consider pain an inevitable part of aging; chronic pain may produce anorexia, lethargy, and depression; may not report pain due to fear of expense, possible treatment, and dependency; often describe pain in nonmedical terms such as “hurt” or “ache”; may fear addiction to analgesics; may not want to bother nurses or be a “bad client”


Fatigue decreases coping abilities and heightens the perception of pain. When people are exhausted from physical activity, stress, and lack of sleep, their perception of pain is heightened. Thus, rest from physical, emotional, and social demands as well as sleep are important measures that reduce pain.


Recent research suggests that sensitivity to and tolerance for pain may a genetically linked trait (Ruda et al., 2000). This factor does not negate the need for adequate pain management of all clients.


Memory of painful experiences, especially experiences that occur when a child was very young, may increase sensitivity and decrease tolerance to pain. This may be due to anticipation and fear of a specific painful event, such as repeated immunization injections. Sensitivity to pain is increased when there is nonspecific memory of a painful event, such as newborn circumcision without anesthesia. Fortunately, modern medicine no longer condones such draconian practices.


Research has shown that severe, unrelieved pain can cause an overwhelming stress response in both pre-term and full term infants which can lead to serious complications and even death (Pasero, 2004).


Any factor that interrupts or interferes with normal pain transmission affects the awareness and response of clients to pain and places them at risk for injury. Analgesics, sedatives, and alcohol depress the functioning of the central nervous system and some diseases (such as leprosy, or Hansen’s disease) damage peripheral nerves, decrease sensitivity to touch and pain, and render affected individuals more vulnerable to injury.

Psychological Factors


The relationship between pain and anxiety is complex and difficult to manage. Fear tends to increase the perception of pain and pain increases feelings of fear. This connection occurs in the brain because painful stimuli activate portions of the limbic system believed to control emotional reactions. People who are seriously injured or critically ill often experience both pain and heightened levels of anxiety due to their helplessness and lack of control. Nurses need to address both pain and anxiety, using all appropriate measures to relieve suffering.


People manage pain and other stressors of life in different ways. Some see themselves as self-sufficient, internally controlled, and independent. As a result, they may deny or be slow to admit they are in pain. Others may see themselves as insufficient, externally controlled, and dependent on others to treat their pain. No matter what coping style a person may have, it is the responsibility of nurses to relieve pain. Self-sufficient, internally controlled people may do better with client-controlled analgesia, whereas dependent, externally controlled individuals may prefer nurse-administered analgesia. In the interest of alleviating pain, people in both categories may need to modify their coping style.

Cultural Factors

Cultural beliefs and values affect the way people respond to pain. As children they learn what is and what is not acceptable behavior when in pain. In some cultures, any expression of pain is considered cowardly and shameful. In others, noisy demonstrations of pain are expected and acceptable. The meaning of pain itself may be markedly different in different cultures. Some ethnic groups see pain as a punishment for wrongdoing. Others see pain as a test of faith, and still others view pain as a challenge to be overcome. Recent immigrants to America are more likely to view pain from the perspective of their place of birth. Regardless of language, religion, or situation, nurses respect every individual and strive to alleviate pain and suffering.


Because the pain is so important to the provision of healthcare today, many organizations have developed standards by which those who provide healthcare can measure their practice. Two such organizations are the Joint Commission on Accreditation of Healthcare Organizations (JCAHO, Box 1-2) and American Academy of Pediatrics (AAP, Box 1-3). These standards indicate how seriously these national organizations view the reduction of pain and provision of comfort for people of all ages.

To meet the Joint Commission on Accreditation of Healthcare Organizations standards, accredited facilities must have policies in place to meet the following requirements:

  1. Recognize the right of clients to appropriate assessment and management of pain.
  2. Screen for pain and assess the nature and intensity of pain in all clients.
  3. Record assessment results in a way that allows regular reassessment and followup.
  4. Determine and ensure that staff is competent in assessing and managing pain.
  5. Establish policies and procedures that support appropriate analgesic medications.
  6. Ensure that pain does not interfere with participation in rehabilitation.
  7. Educate clients and their families about effective pain management.
  8. Address client needs for symptom management in discharge planning process.
  9. Establish a way to collect facility-wide data to monitor the appropriateness and effectiveness of pain management.
Source: Berry & Dahl, 2001.

The American Academy of Pediatrics policy statement on the assessment and management of acute pain in infants, children, and adolescents concludes with the following recommended strategies:

  1. Expand knowledge about pediatric pain and pediatric pain management principles and techniques.
  2. Provide a calm environment for procedures in order to reduce distress-producing stimulation.
  3. Use appropriate pain assessment tools and techniques.
  4. Anticipate predictable painful experiences; intervene, and monitor accordingly.
  5. Use a multimodal (pharmacologic, cognitive behavioral, and physical) approach to pain management and use a multidisciplinary approach when possible.
  6. Involve families and tailor interventions to the individual child.
  7. Advocate for child-specific research in pain management and Food and Drug Administration evaluation of analgesics for children.
  8. Advocate for the effective use of pain medication for children to ensure compassionate and competent management of pain.
Source: American Academy of Pediatrics, 2001.

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