Abdominal pain pathophysiology
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The pain associated with the abdomen in cases of inflammation of the parietal peritoneum (the part of the peritoneum lining the abdominal wall) is steady and aching and is worsened by changes in the tension of the peritoneum caused by pressure or positional change. This pain is often accompanied by tension of the abdominal muscles contracting in an effort to relieve such tension. The pain associated with the obstruction of a hollow viscus (as opposed to peritoneal and solid organ pain) is often intermittent or "colicky," coinciding with the peristaltic waves of the organ. Such cramps are exactly what is experienced in early acute appendicitis and gastroenteritis and are somewhat relieved by writhing and massages. Pain that is felt in the abdomen may be "referred" from elsewhere (e.g., a disease process in the chest may cause pain in the abdomen), and abdominal processes can cause radiated pain elsewhere (e.g., gall bladder pain—in cholecystitis or cholelithiasis—is often referred to the shoulder). The pain associated with abdominal vascular disturbances (thrombosis or embolism) can be sudden or gradual in onset and can be severe or mild. Pain associated with the rupture of an abdominal aortic aneurysm may radiate to the back, flank, or genitals.
Ischemic Acute Abdomen
Arterial supply to the intestines is provided by the superior and inferior mesenteric arteries, SMA and IMA respectively, both of which are direct branches of the aorta.
The superior mesenteric artery supplies:
The inferior mesenteric artery supplies:
Of note, the splenic flexure, or the junction between the transverse and descending colon, is supplied by the most distal portions of both the inferior mesenteric artery and superior mesenteric artery. It is referred to medically as a watershed area, or an area especially vulnerable to ischemia during periods of systemic hypoperfusion, such as in shock (medical).
Acute abdomen of the ischemic variety is usually due to:
- A thromboembolism from the left side of the heart, such as may be generated during atrial fibrillation, occluding the SMA.
- Nonocclusive ischemia, such as that seen in hypotension secondary to heart failure may also contribute, but usually results in a mucosal or mural infarct, as contrasted with the typically transmural infarct seen in thromboembolus of the SMA.
- Primary mesenteric vein thromboses may also cause ischemic acute abdomen, usually precipitated by hypercoagulable states such as polycythemia vera.
Chronic Functional Abdominal Pain
CFAP is characterized by chronic pain, with no physical explanation or findings (no structural, infectious, or mechanical causes can be found). It is theorized that CFAP is a disorder of the nervous system where normal nerve impulses are amplified "like a stereo system turned up too loud," resulting in pain. This visceral hypersensitivity may be a stand-alone cause of CFAP, or CFAP may result from the same type of brain-gut nervous system disorder that underlies IBS. As with IBS, low doses of antidepressants have been found useful in controlling the pain of CFAP.