Referred pain is pain that is felt in a different place to the source of the pain. In this broad definition, visceral pain described above is referred to the midline anteriorly. There are other referred pains that are important when considering abdominal complaints. A good example is pain felt over the shoulder due to diaphgragmatic irritation.
The diaphragm is supplied by the phrenic nerve which contains nerve fibers from C 3, 4 and 5 (3, 4 and 5 keeps the diaphragm alive). Along with the motor fibers in the phrenic nerve, there are sensory fibers which supply the diaphragm and enter the cord at the C 3, 4 and 5 segments. This segment of the cord also supply's the supraclavicular nerves (medial, intermediate and lateral) via the cervical plexus. The lateral supra-clavicular nerve supply's the skin directly over the acromium process. Irritation of the diaphragm from either above or below may be experienced as pain over the acromium process. 2
For example, following laparoscopy patients often describe discomfort in the shoulders, presumably due to stretching of the diaphragm by the insufflation of carbon dioxide to maintain the pneumo-peritoneum. Patients with a perforated duodenal ulcer may complain of right acromial pain. Ladies with a ruptured ectopic pregnancy may develop shoulder pain on depressing the head of the bed, presumably due to irritation of the diaphragm by blood and fluid when the patient is tilted.
The importance of shoulder tip/ acromial pain in a surgical patient is that the diaphragm is being irritated, perhaps due to a perforation.
Referred pain is usually felt as heavy, dull or boring.