Acute pain, a phenomenon with which we are all familiar, is an experience with sensory

and affective components that serves to warn the individual about potentially harmful

stimuli in the environment. The sensory component of pain allows the individual to

localize the sensation to a site on the body and identify to some extent the nature of the

inciting insult. The affective component of pain, mediated by structures in the brainstem

and limbic lobes of the brain, lends pain the unpleasant emotional content that is so

important in making individuals withdraw from and avoid painful stimuli. Chronic pain is

defined as pain persisting more than one month beyond the resolution of an acute tissue

injury, or pain that persists or recurs for more than three months associated with a tissue

injury that is expected to continue or progress. Unlike acute pain, chronic pain has few

redeeming features. Chronic pain has been estimated to affect more than 60 million people

in the United States, and the cost, to individuals in terms of suffering and lost income, and

the cost to society in terms of lost productivity and requirement of care is enormous.