Evidence for this comes from history, examination and investigation. Proximal obstructions often have frequent colic, and abdominal distension may be minimal. Plain radiology will show only a few distended loops. Vomiting is frequent and bilious.
Obstruction to the mid gut, gives colic every 20 minutes or so, distension is easily appreciated but is usually not gross. Plain radiology usually shows distended loops of small bowel in the centre of the abdomen. Vomiting occurs several times a day and may appear as a brown liquid.
Distal obstruction often presents on a background of being unwell for many months. Distension is usually gross. Plain radiology often reveals a distended colon. Vomiting is infrequent and faeculant.