Visceral pain

Visceral pain is defined as pain that results from the activation of nociceptors of the thoracic, pelvic, or abdominal viscera (internal organs) in the human body. Visceral structures are highly sensitive to distension (stretch), ischemia and inflammation, but relatively insensitive to other stimuli that normally evoke pain such as cutting or burning.

Visceral pain is diffuse, difficult to localize, and often referred to a distant, usually superficial, structure. It may be accompanied by symptoms such as nausea, vomiting, changes in vital signs as well as emotional manifestations. The pain may be described as sickening, throbbing, pulsating, deep, squeezing, and/or dull.

Physiology

Nociceptive innervation

Nociceptive innervation is often the only type of sensory innervation possessed by visceral structures. Nociceptive innervation of visceral structures entails two distinct modalities:

The actual viscera possess sparse nociceptive innervation via "slow" group C nerve fibers which are bundled into autonomic nerves to be conveyed to the spinal cord segments where the organ which they innervate originally arose during embryological development; in the spinal cord, the central branch of the visceral nociceptive neuron then synapses with multiple 2nd-order nociceptive neurons which also receive nociceptive stimuli from 1st-order nociceptive neurons innervating the skin. Consequently, true visceral pain is perceived as dull chronic pain referred to the dermatomes of the body surface innervated by nociceptive neurons from the same spinal cord segments as the embryologic origin of the affected organ. Due to the sparse innervation, highly localised insults (like incision through the visceral surface of a viscera) tends to be relatively or completely painless, whereas insults that cause diffuse activation of visceral nociceptive produce intense suffering.

Parietal surfaces (like the parietal surface of the peritoneum, pericardium, and pleura) possess abundant nociceptive innervation (much like the skin) directly from local spinal nerves.