The bowel has its own constant rhythmic flow called peristalsis, that moves its contents (stool) from the stomach to the rectum (see Diagram — Gastrointestinal System). If this gets stuck (for various uncommon reasons), it’s bad. Pain can begin either gradually or abruptly, gets progressively worse, with eventual vomiting (with or without nausea). The main danger is rupture of the bowel, when stool leaks out into the abdomen, causing Peritonitis which can lead to fatal Sepsis. Obstruction can happen in either the small or large intestine.
We suspect bowel obstruction when a patient has abdominal pain and nausea / vomiting, always with the pain having begun first. We may hear abnormal bowel sounds with our stethoscope, not just lots of gurgles (which can be normal), but “tinkles” (like tapping crystal) or “rushes” (like short bursts of rushing water that come & go). The abdomen is clearly tender wherever we gentle palpate (press on it).
We mainly think of bowel obstruction in patients at risk for it. These include anyone who has ever had abdominal surgery or radiation treatment (like for cancer), because scars (called “adhesions”) can wrap around the bowel & interfere with peristalsis. Obstructions also occur in people with diseases that can damage the bowel, like Ulcerative Colitis, Crohn’s Disease, or Colon Cancer (but not Irritable Bowel Syndrome). And sometimes it just happens.
Diagnosis is easily made by a simple abdominal x-ray (which is not useful for any other condition). But in an ER, they’ll usually do a CT scan, which can find a variety of conditions causing the pain. Some of those conditions might even be causing the obstruction.
Treatment involves resting the bowel completely, i.e. nothing by mouth, not even water. So patients are admitted to the hospital for IV fluids, to keep them hydrated, & also provide pain medicine. Usually the obstruction gets better on its own, but sometimes surgery is necessary. Also, it’s crucial to identify the cause of the obstruction, if none is obvious.