“It is not suffering that diminishes man, but
suffering without meaning.”
Asked to describe their pain, especially chronic pain, patients
often appear perplexed, stating, “I don’t know.
It just hurts.” Pain is a subject of deceptive complexity.
It means different things to different people. Algology is the study
of pain, and this chapter discusses a number of important points
relevant to its clinical application, pain medicine.
Before we focus on the patient with chronic pain and a general
approach to pain management, basic nosology and terminology need
There are a number of ways to classify pain. Some pain specialists
separate it into malignant (cancer) and nonmalignant pain. Others
divide it into acute, recurrent acute, and chronic pain. Acute pain
is short-lived and follows injury or near injury to tissue. Recurrent
acute pain is similar in duration but tends to recur. It need not
involve injury. Examples are migraine headache and sickle cell vasoocclusive
episodes. Depending on the injury, chronic pain is variably defined
as that persisting 1 to 6 months after the tissue has healed. One
example of chronic pain is postherpetic neuralgia.
Pain can also be classified in terms of mechanism. Nociceptive
pain denotes pain arising from tissue injury, and the degree of
pain is usually somewhat proportional to the degree of injury. Nociceptive
pain itself may be subcategorized into visceral pain, a dull, crampy,
and poorly localizable discomfort—as might be experienced
in gastroenteritis—or somatic pain, a sharper and more localizable
sensation of the body wall, as might be felt after a laceration.
Each type of pain may be mild or intense.
Neuropathic pain is not nociceptive, and the degree of pain is
not proportional to the degree of injury; it is caused by disordered
sensory processing of the nervous system and is a pathologic persistence
of a normal sensitizing mechanism that can be useful in the setting
of acute pain. Neuropathic pain can be subcategorized into central
neuropathic pain, which can originate at any level of the central
nervous system, and peripheral neuropathic pain, which is generated
at the level of a nerve or nerve root. The most famous example of
central pain is poststroke thalamic pain; common examples of peripheral
neuropathic pain are neuromas, diabetic neuropathy, and complex regional
pain syndrome, types 1 and 2 (previously known as reflex sympathetic
dystrophy and causalgia). As with central pain, a number of different
mechanisms may be involved. Even when the damage occurs in the periphery,
such as injury to a nerve, the constant bombardment of sensory neurons
in the spinal cord with pain signals from the periphery renders
the neurons hypersensitive to all input, even to non-noxious stimuli.
Neurons then almost continuously “fire up” the pain
pathway. Although this is a normal sequence of events in acute injury
(i.e., sensitizing injured areas so that they may be protected from