1. What is pain?

The International Association for the Study of Pain defines pain as: “An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Some dictionaries define pain as: “An unpleasant sensation, occurring in varying degrees of severity as a consequence of injury, disease, or emotional disorder.” Inherent in both definitions is the concept that pain always has a subjective component. It is both a physiologic sensation and an emotional reaction to that sensation. In some cases, there may be no tissue injury, but the pain is no less “real.” In clinical terms, Margo McCaffrey (an internationally regarded expert on pain) has defined pain most succinctly and appropriately: “Whatever the patient says hurts.”

2. What is the difference between pain and suffering?

Pain is a sensation plus a reaction to that sensation. Suffering is a more global concept—an overall negative feeling that impairs the sufferer’s quality of life. Both physical and psychological issues are actively involved with suffering, and the pain itself may be only a small component. In some instances, pain may be an expression of suffering (see “Somatoform Disorders” in Chapter 29, Psychological Syndromes).

3. What is the difference between impairment and disability?

Impairment is a medical concept; disability is a legal or societal concept. Impairment is any loss or abnormality of psychological, physiologic, or anatomic structure or function. According to the World Health Organization (WHO) definition, disability results from impairment; it is any restriction or lack of ability to perform an activity in the manner or within the range considered normal for a human. In governmental terms, disability is sometimes called a functional limitation. Another definition of disability is a disadvantage (resulting from an impairment or functional limitation) that limits or prevents the fulfillment of a role that is normal for an individual (depending on age, sex, and social and cultural factors). This definition corresponds to the WHO classification of handicap.

4. What is meant by inferred pathophysiology?

We can rarely define with certainty the pathophysiologic mechanisms underlying a specific pain syndrome. However, a specific set of symptoms may lead us to believe that a pain syndrome is more likely due to nerve injury (neuropathic pain), lesions of muscle or bone (somatic nociceptive pain), or disease of the internal organs (visceral nociceptive pain). This inferred pathophysiology implies that we understand the basic mechanisms underlying a pain syndrome, and leads to the pathophysiologic classification of pain syndromes (see Chapter 2, Classification of Pain). This pathophysiologic classification may be overly self-serving, because we can only infer, and rarely verify, the true mechanism.

5. What is the definition of nociception?

Nociception is the activation of a nociceptor by a potentially tissue-damaging (noxious) stimulus. It is the first step in the pain pathway.

6. What is a nociceptor?

A nociceptor is a specialized, neurologic receptor that is capable of differentiating between innocuous and noxious stimuli. In humans, nociceptors are the undifferentiated terminals of A-delta fibers and C fibers, which are the thinnest myelinated and unmyelinated fibers, respectively. A-delta fibers are also called high-threshold mechanoreceptors. They respond primarily to mechanical stimuli of noxious intensity.

7. What is the difference between pain threshold and pain tolerance?

Pain threshold refers to the lowest intensity at which a given stimulus is perceived as painful; it is relatively constant across subjects for a given stimulus. For example, most subjects will define a thermal stimulus as painful when it reaches about 50° C. Similarly, barring disease states, mechanical pressure produces pain at approximately the same amount of pressure across subjects. Pain threshold as it relates to sensitivity to pressure is measured with an algometer.

Pain tolerance, on the other hand, is the greatest level of pain that a subject is prepared to endure. Tolerance varies much more widely across subjects and depends on prescribed medications. Clinically, pain tolerance is of much more importance than pain threshold. (More detailed discussions of threshold and tolerance are found in Chapter 6, Pain Measurement.)

8. You touch an apparently normal area of skin and the patient jumps with pain. Why?

This reaction is an example of allodynia, an abnormal circumstance in which an innocuous stimulus is perceived as painful. It is common in many neuropathic pain conditions, such as postherpetic neuralgia, complex regional pain syndrome, and certain other neuropathies. Two different types of allodynia are described: thermal and mechanical. In thermal allodynia, an innocuous warm or cold breeze may be perceived as painful. With mechanical allodynia, a very light touch (such as the clothes rubbing against the skin) may be extremely painful, while firmer pressure is not.

In cases of allodynia resulting from neurological injury, the skin surface may appear normal. Allodynia is also present in skin sensitized by a burn or inflammation, but in these patients the affected skin is visibly abnormal.

9. What is meant by analgesia?

Analgesia is the absence of pain despite the presence of a normally painful stimulus. Analgesia can be produced peripherally (at the site of tissue damage, receptor, or nerve) or centrally (in the spinal cord or brain). In general, the nonsteroidal antiinflammatory drugs and other minor analgesics act primarily at the site of tissue damage, whereas opioids and so-called adjuvant drugs act primarily at the spinal cord or cerebral level.

10. What is the difference between analgesia and anesthesia?

Anesthesia implies loss of many sensory modalities, leaving the area “insensate.” Analgesia refers specifically to the easing of painful sensation.

11. What is meant by paresthesia?

A paresthesia is any abnormal sensation. It may be spontaneous or evoked. The most common paresthesia is the sense that a limb “falls asleep” when a nerve in the limb is compressed; also known as “pins and needles.” Paresthesias are not always painful.