There are many clinical features of visceral pain but the one that makes it more distinctive is the frequent referral of the painful sensation to areas of the body away from the diseased organ. This is what Henry Head called "referred pain,'' a painful sensation reported in a region of the body remote from the originating lesion (1). Referred pain is a useful diagnostic tool in the clinic because the patterns of referral produced by a lesion in a given internal organ are constant across subjects (2). It is also a phenomenon of considerable neurobiological interest, which implies that the brain can attribute a painful sensation to the wrong location and generate a mismatch between the real and the perceived sites of injury.
Pain is usually regarded as a protective sensation, the "psychical adjunct of a protective reflex'' (3) and a reasonably accurate location of the sensation would be necessary to produce an effective protection of the injured site. However, visceral pain shows a number of clinical and neurobiological properties that question a purely defensive role for the organism (4). Many internal organs are insensitive to pain and there is often no relation between the extent of damage to internal organs and the intensity of the resulting pain (5). The sensory innervation of the viscera is sparse and the functional characteristics of the sensory receptors that innervate internal organs also differ from those of cutaneous or muscle nociceptors (4) (Chapter 5) Visceral pain is often dull and persistent in its early phases and, although it is subsequently projected to more superficial areas of the body, it follows temporal and spatial patterns that cannot be confounded with the kinds of superficial pain felt by somatic injuries (6,7). In this way, pain from internal organs often forces the patient to take rest or seek help and in doing so can contribute to the defense of the organism.
In this chapter, we review the phenomenon of referred visceral pain from a variety of perspectives. We describe its clinical features and properties, the experimental models—both in animals and in humans—that are being used in its study and the neurobiological and molecular mechanisms that may be involved in its generation. In this way, we hope to present the reader with a multidisciplinary approach to this unique feature of visceral pain, combining clinical observations with potential neurological mechanisms.
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