The nervous system has highly developed responses to different types of noxious stimuli within the abdomen. Highly specialized sensory neurons or nociceptors are located in the capsules of the abdominal viscera, parietal peritoneum, and mesentery. Stimulation of these nociceptors by mechanical stretching, inflammation, or ischemia causes abdominal pain. Abdominal pain can be classified as dull and generalized or sharp and localized. Noxious stimuli are transmitted by two types of nociceptors: C-polymodal (CPM) fibers and Aδ fibers. CPM fibers transmit pain arising from hollow or parenchymal organs of the abdomen via the sympathetic nerves to the spinal cord and on to the brain for conscious perception. This pain is “dull” and generalized. Pain mediated by Aδ fibers is piercing and localized. These receptors are stimulated by inflammatory cytokines (as occurs in peritonitis) and a rapid signal is transmitted to the thoracolumbar spinal cord.

Abdominal pain can be further classified as visceral or somatic. Visceral pain is generally mediated by CPM fibers and is generated with gastrointestinal obstructive disease or organomegaly. Somatic pain, which can be the result of abdominal wall injury, generally emits a painful response via CPM and Aδ fibers. Somatic pain is generally more painful and arises from conditions such as pancreatitis and peritonitis

Physical Examination

Animals presented with abdominal pain may have life-threatening diseases such as hypovolemic or endotoxic shock, intra-abdominal hemorrhage, or serious injury to abdominal organs. These conditions must be immediately addressed before a lengthy physical examination is performed. In stable animals, a careful physical examination is systematically performed. Abnormalities include a stilted gait, arched back, and reluctance to move. Some patients will adopt a “praying” posture in which the rear quarters are elevated in a standing position and the forelimbs and chest are in contact with the floor. Presumably this removes pressure on intra-abdominal organs. Once palpation of the abdomen is initiated, the patient should be carefully observed for any responses. Although often “less is more” in regard to physical restraint, some patients need to be muzzled for a thorough examination as they may be fractious in response to pain. The exact physical examination is tailored to each individual patient. For example, a middle-aged dachshund with a tense abdomen may have intervertebral disk disease and should be thoroughly examined for back pain before zeroing in on the abdomen. A middle-aged Great Dane with a tense abdomen is a likely candidate for gastric bloating and subsequent volvulus. Puppies and kittens more commonly have infectious diseases or dietary indiscretion as the cause. Geriatric patients have a higher risk for neoplasia.