Visceral pain is the most common form of pain produced by disease and one of the most frequent reasons for patients to seek medical attention. Yet much of what we know about the basic mechanisms of pain derives from experimental studies of somatic nociception. This would be justified if the mechanisms of somatic and visceral pain were similar so that information obtained by studying one form of pain could be extrapolated to interpret the mechanisms of the other. However, the more we know about the mechanisms of somatic and visceral sensation the more we realise that these two processes, while having many common features, also have important differences. We seldom have any sensory experiences from our internal organs other than pain and discomfort and even when other sensations occur, such as bladder or stomach fullness, these can easily evolve towards pain if the stimulus persists.

The five main characteristics of visceral pain—that is, those clinical features that make visceral pain unique—are that visceral pain1:

(i)

is not evoked from all viscera,

(ii)

is not linked to visceral injury,

(iii)

is referred to other locations,

(iv)

is diffuse and poorly localised, and

(v)

is accompanied by motor and autonomic reflexes.

Properties (i) and (ii) generated the notion that some viscera lacked an afferent innervation. We now know that …