What are the key clinical features that differentiate ileus from mechanical bowel obstruction in adults?

The key clinical features differentiating ileus from mechanical bowel obstruction in adults include the nature of symptom onset, abdominal examination findings, and radiological characteristics.

In ileus, symptoms typically develop more gradually and are associated with diffuse abdominal distension and generalized, often mild, abdominal tenderness without localized peritonism. Bowel sounds are usually reduced or absent due to the functional paralysis of the bowel. In contrast, mechanical bowel obstruction often presents with more acute onset of colicky abdominal pain, localized tenderness, and high-pitched, tinkling bowel sounds early on, reflecting hyperactive peristalsis proximal to the obstruction NICE NG123.

Vomiting is common in both conditions but tends to occur earlier and be more profuse in mechanical obstruction. Additionally, patients with mechanical obstruction may pass flatus and stool initially but develop obstipation as the obstruction persists, whereas ileus may present with more generalized constipation without a clear transition point NICE NG123.

Radiologically, ileus shows diffuse gaseous distension of both small and large bowel without a clear transition point, reflecting a functional impairment of motility. Mechanical obstruction, however, typically demonstrates a distinct transition point with proximal bowel dilatation and distal bowel collapse on imaging such as abdominal X-ray or CT scan Merlin et al. 2008. Multidetector row helical CT can further delineate the site and cause of obstruction, which is absent in ileus Merlin et al. 2008.

In summary, the differentiation hinges on the clinical presentation of pain character and bowel sounds, the pattern of vomiting and bowel movements, and imaging findings showing either a functional motility disorder (ileus) or a mechanical blockage with a transition point (mechanical obstruction) NICE NG123; Merlin et al. 2008.