Causes of True Visceral Pain
Any stimulus that excites pain nerve endings in diffuse areas of the viscera can cause visceral pain. Such stimuli include ischemia of visceral tissue, chemical damage to the surfaces of the viscera, spasm of the smooth muscle of a hollow viscus, excess distention of a hollow viscus, and stretching of the connective tissue surrounding or within the viscus. Essentially all visceral pain that orig-inates in the thoracic and abdominal cavities is trans-mitted through small type C pain fibers and, therefore, can transmit only the chronic-aching-suffering type of pain.
Ischemia. Ischemia causes visceral pain in the same waythat it does in other tissues, presumably because of the formation of acidic metabolic end products or tissue-degenerative products such as bradykinin, proteolytic enzymes, or others that stimulate pain nerve endings.
Chemical Stimuli. On occasion, damaging substances leakfrom the gastrointestinal tract into the peritoneal cavity. For instance, proteolytic acidic gastric juice often leaks through a ruptured gastric or duodenal ulcer. This juice causes widespread digestion of the visceral peritoneum, thus stimulating broad areas of pain fibers. The pain is usually excruciatingly severe.
Spasm of a Hollow Viscus. Spasm of a portion of the gut,the gallbladder, a bile duct, a ureter, or any other hollow viscus can cause pain, possibly by mechanical stimula-tion of the pain nerve endings. Or the spasm might cause diminished blood flow to the muscle, combined with the muscleâ€™s increased metabolic need for nutrients, thus causing severe pain.
Often pain from a spastic viscus occurs in the form of cramps, with the pain increasing to a high degree ofseverity and then subsiding. This process continues intermittently, once every few minutes. The intermittent cycles result from periods of contraction of smooth muscle. For instance, each time a peristaltic wave travels along an overly excitable spastic gut, a cramp occurs. The cramping type of pain frequently occurs in appen-dicitis, gastroenteritis, constipation, menstruation, par-turition, gallbladder disease, or ureteral obstruction.
Overdistention of a Hollow Viscus. Extreme overfilling of ahollow viscus also can result in pain, presumably because of overstretch of the tissues themselves. Overdistention can also collapse the blood vessels that encircle the viscus or that pass into its wall, thus perhaps promoting ischemic pain.
Insensitive Viscera. A few visceral areas are almost com-pletely insensitive to pain of any type. These include the parenchyma of the liver and the alveoli of the lungs. Yet the liver capsule is extremely sensitive to both direct trauma and stretch, and the bile ducts are also sensitive to pain. In the lungs, even though the alveoli are insen-sitive, both the bronchi and the parietal pleura are very sensitive to pain.