Ileus

01Pathophysiology

Normal intestinal motility depends on a complex interplay between the enteric nervous system (the intrinsic neural plexuses of Auerbach and Meissner), the autonomic nervous system (sympathetic inhibition vs. parasympathetic stimulation), and local hormonal and inflammatory mediators. Ileus occurs when this coordinated system is disrupted, resulting in a functional paralysis of the gut wall without any physical blockage.

The most heavily tested cause is postoperative ileus. Surgical manipulation of the bowel triggers a local inflammatory cascade. Handling of intestinal loops activates resident macrophages within the muscularis externa, releasing cytokines (IL-6, TNF-alpha) and prostaglandins that directly suppress smooth muscle contractility. Simultaneously, the surgical stress response drives a sympathetic surge that inhibits parasympathetic tone throughout the gut. The combination of local inflammation and systemic sympathetic overdrive produces a diffuse, non-propulsive state across the entire intestinal tract.

Electrolyte imbalances are the second most common tested trigger. Hypokalemia is the highest-yield association: potassium is essential for smooth muscle depolarization and contraction. When serum $K^{+}K^+$ drops below 3.0 mEq/L, the resting membrane potential of smooth muscle cells becomes hyperpolarized, making it harder for the cell to reach the threshold for contraction. The same principle applies to hypomagnesemia (magnesium is a cofactor for $K^{+}K^+$ channels and $N{a}^{+}\mathrm{/}{K}^{+}Na^+/K^+$-ATPase) and hypocalcemia ($C{a}^{2+}Ca^\{2+\}$ is the final common pathway for smooth muscle contraction).

Opioids act on mu-receptors in the myenteric plexus, directly inhibiting acetylcholine release from excitatory motor neurons. This decreases propulsive contractions and increases non-propulsive segmental tone, effectively bringing transit to a halt. This explains why the exam frequently places the patient on a morphine PCA pump and then presents worsening distension on postoperative day 3.

The result of all these mechanisms is the same: intestinal contents (gas, fluid, swallowed air) accumulate within a hypotonic, dilated bowel. Because the process is diffuse, both small and large bowel dilate uniformly. This is the critical imaging distinction from mechanical obstruction, where dilation occurs proximal to a single point and decompressed bowel is seen distally.

02Classification and Clinical Manifestation

Ileus is not formally staged, but it is classified by etiology because the cause dictates management. The table below organizes the categories most frequently tested.

Clinical manifestation across all categories follows a predictable pattern: progressive abdominal distension is the earliest and most consistent sign. Patients then develop nausea and vomiting (initially gastric contents, later bilious if the small bowel is significantly dilated). Absent bowel sounds on auscultation are the hallmark physical exam finding; this contrasts with the high-pitched, tinkling bowel sounds of mechanical obstruction. Patients report inability to pass flatus or stool (obstipation). Importantly, the abdomen is usually non-tender and soft unless an underlying inflammatory process is present. Pain, when present, is mild and diffuse rather than colicky.

03Diagnostic Workup

The diagnostic approach begins with a plain abdominal X-ray, which is the best initial test. The hallmark finding is gaseous distension involving both the small intestine and the colon, distributed diffusely throughout the abdomen. You will see air-fluid levels on the upright film, but unlike mechanical obstruction, these are scattered and do not form a step-ladder pattern confined to one region. The critical negative finding is the absence of a transition point: there is no abrupt cutoff between dilated and decompressed bowel.

If the clinical picture is straightforward (a postoperative patient on opioids with classic findings on X-ray), no further imaging is necessary. However, when the presentation is ambiguous or when you cannot confidently exclude a mechanical obstruction, the next step is a CT abdomen and pelvis with IV contrast. CT is the most accurate test because it can identify a transition point (confirming mechanical obstruction), detect closed-loop obstruction or strangulation, and reveal underlying causes of ileus such as an abscess, pancreatitis, or retroperitoneal pathology.

Simultaneously, you must order a complete metabolic panel. This is not optional. Correctable electrolyte abnormalities (particularly $K^{+}K^+$, $M{g}^{2+}Mg^\{2+\}$, and $C{a}^{2+}Ca^\{2+\}$) are both a cause and a perpetuating factor of ileus. Finding and correcting these is considered a core management step and frequently appears as the "next best step" answer choice.

One important distinction for the exam: if the vignette describes a patient with only colonic dilation (cecum markedly dilated, small bowel normal or minimally dilated), consider Ogilvie syndrome (acute colonic pseudo-obstruction) rather than generalized ileus. This distinction matters because the management pathways diverge.

04Management and Treatment

Acute management follows a supportive algorithm. The initial steps are simultaneous: place the patient NPO, insert an NG tube if there is significant vomiting or distension, begin IV fluids, and draw a metabolic panel. Electrolyte repletion is the first targeted therapy and should be aggressive. If the patient is on opioids, transitioning to multimodal non-opioid analgesia (IV ketorolac, IV or PO acetaminophen, regional nerve blocks) is the single highest-yield intervention and the most commonly tested "next best step."

Early ambulation is consistently supported by evidence and is a favorite answer choice on exams. Mobilizing the patient out of bed promotes vagal tone and helps restore peristalsis.

For postoperative ileus prevention, alvimopan is the only FDA-approved pharmacologic agent. It selectively blocks mu-opioid receptors in the GI tract without reversing central analgesic effects. Its use is restricted to the inpatient setting (a maximum of 15 doses over 7 days) due to a historical concern for cardiovascular events with long-term use. It is most commonly tested in the context of a patient undergoing elective bowel resection.

When ileus fails to resolve after 5 to 7 days of conservative management, you must reassess the diagnosis. Order a CT scan to rule out mechanical obstruction, abscess, or anastomotic leak. Prolonged ileus is not a benign entity: it increases the risk of aspiration, bacterial translocation, and prolonged hospitalization.

A key contraindication to remember: do not use promotility agents like metoclopramide or erythromycin as a routine treatment for ileus. Their efficacy is not established in this context, and erythromycin's prokinetic effect is primarily on the upper GI tract. Neostigmine is reserved strictly for Ogilvie syndrome and must be administered with cardiac monitoring due to the risk of symptomatic bradycardia.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on ileus questions is by confusing it with mechanical small bowel obstruction. Both present with distension, vomiting, and air-fluid levels on X-ray. The trap is that students see "dilated bowel" and immediately jump to obstruction without checking whether the dilation involves both the small and large bowel (ileus) or is limited to the small bowel with a visible transition point (SBO). The vignette will always contain a distinguishing clue: in ileus, bowel sounds are absent and the abdomen is diffusely tympanic without tenderness; in SBO, bowel sounds are initially hyperactive and pain is colicky.

A second common trap involves the "next best step" question in a postoperative patient with ileus who is receiving IV morphine. Students often choose NG tube placement or a CT scan. The tested answer is to discontinue or minimize opioids and switch to non-opioid analgesia. This is the most impactful intervention and should be selected before additional imaging or invasive procedures.

Another high-yield pearl: do not confuse ileus with Ogilvie syndrome. If the vignette describes an elderly, hospitalized patient with isolated colonic dilation (cecum $>>$ 12 cm) and no small bowel involvement, the answer is Ogilvie syndrome and the treatment is IV neostigmine, not conservative management alone. Students who lump these two together will select the wrong treatment.

Finally, remember that ileus is almost never the final diagnosis in a patient with peritoneal signs (rebound, guarding, rigidity). If the vignette includes these findings, the patient has a surgical abdomen (perforation, strangulated obstruction, mesenteric ischemia) and the answer is emergent surgical consultation, not conservative management for ileus. The core competency being tested is the ability to distinguish functional bowel hypomotility from conditions requiring urgent intervention.