Answers

Short answers

1 This patient has mesenteric ischaemia.

2 The computed tomogram shows portal venous gas and pneumatosis intestinalis—that is, air within the liver and bowel wall, respectively—and some intra-abdominal fluid. These signs are consistent with bowel ischaemia.

3 The most common cause of acute mesenteric ischaemia is thrombosis or thromboembolism within the superior mesenteric artery. Atherosclerosis and cardiac arrhythmias are the greatest risk factors.

4 Surgical resection is the main treatment for bowel infarction. In this case, however, the patient was managed conservatively.

Long answers
1 Diagnosis
The diagnosis is acute mesenteric ischaemia. Patients with acute mesenteric ischaemia tend to have acute onset of symptoms—including abdominal pain, nausea, and vomiting—followed by a rapid deterioration in their clinical condition. The pain is classically described as out of proportion to the clinical signs and the patient is often dehydrated. Patients with underlying superior mesenteric artery thrombosis may complain of a prodromal symptom complex of postprandial pain, nausea, and weight loss owing to chronic intestinal insufficiency.

Blood tests might show haemoconcentration, deranged renal function secondary to hypovolaemia, metabolic acidosis with a raised lactate concentration, and base deficit. If bowel infarction has occurred, the patient may present with hypovolaemic shock, peritonism, and signs of sepsis with multiorgan failure. It is mandatory to aggressively resuscitate these patients and prepare for urgent transfer of the patient to theatre, where appropriate.

The differential diagnoses include bowel obstruction, bowel perforation, intra-abdominal sepsis, and rupture of an abdominal aortic aneurysm. A history of peripheral vascular disease would raise suspicion for mesenteric atherosclerosis. The diagnosis in this patient was only reached on the basis of the computed tomography findings.

2 Imaging
The computed tomogram shows gas within the liver along the portomesenteric vessels and pneumatosis intestinalis (figs 3 and 4 ). These radiological findings can occur either in isolation or together, although the presence of both signs is highly specific for a transmural bowel infarction—that is, acute mesenteric ischaemia—and is associated with high mortality.^{1 2} Very rarely these signs can be caused by non-ischaemic conditions such as infection, inflammation, trauma, neoplasm, and obstruction.³

View larger version (78K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 3 Axial unenhanced computed tomogram of the upper abdomen showing portal venous gas (arrow)

 

View larger version (68K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 4 Axial unenhanced computed tomogram of the mid-abdomen showing air in the bowel wall (arrow)

 
If the patient is sufficiently stable, computed tomography angiography is the optimal investigative modality for confirming or refuting a diagnosis of mesenteric ischaemia, and has now largely superseded direct catheter angiography. Some studies have suggested that the sensitivity of computed tomography for the diagnosis of acute bowel ischaemia is comparable to that of catheter angiography (82% v 87.5%),⁴ although computed tomography angiography is still considered to be the "gold standard" for diagnosis. Computed tomography has additional advantages in that it can demonstrate bowel wall changes and the extent of these changes, as well as help to either confirm or exclude differential diagnoses. Contrast enhanced magnetic resonance angiography is a second line non-invasive technique for patients suspected of having mesenteric ischaemia.

Magnetic resonance angiography is the technique of choice for children and patients with mild renal impairment because the technique does not involve radiation or iodinated contrast agents.⁵ Gadolinium contrast would be relatively contraindicated in patients with a glomerular filtration rate of less than 30 ml/min owing to the possibility of nephrogenic systemic fibrosis. Compared with magnetic resonance angiography, computed tomography angiography has higher spatial resolution and faster acquisition times, permitting more accurate assessment of the peripheral visceral branches and the inferior mesenteric artery. In addition, computed tomography angiography facilitates the identification of calcified plaques.

Computed tomography angiography was not performed in this patient because she had renal failure; however, an example from a different patient is shown in fig 5. Magnetic resonance angiography was not performed in this patient as the computed tomography findings were sufficient for the diagnosis to be reached.

View larger version (68K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 5 Axial contrast enhanced computed tomography angiogram through the upper abdomen showing thrombus in the superior mesenteric artery (arrow)

 
3 Causes
There are two theories postulated to account for the pathogenesis of portomesenteric gas—mechanical and bacterial. The mechanical theory proposes that damage to or distension of the intestinal wall causes intraluminal gas to dissect into the intramural tissue, damaging the mucosa and giving rise to intramural gas (or pneumatosis intestinalis). The bacterial theory suggests that gas forming bacteria such as Escherichia coli and Clostridium species enter the submucosa through mucosal defects and produce gas within the intestinal wall and the portal vein.⁶ This gas then tracks into the vein of the bowel wall and along the portal venous system.

The most common causes of intestinal wall alteration or mucosal damage are intestinal ischaemia with bowel necrosis and inflammatory bowel disease. Arterial embolism, arterial thrombosis, non-occlusive causes, and venous thrombosis are also possible causes (table 1). Age, atrial fibrillation, peripheral vascular disease, and malignancy are all risk factors for acute mesenteric ischaemia. The cause in this patient is thought to have been thromboembolism secondary to atrial fibrillation.

View this table: [in this window] [in a new window]   Table 1 Causes of and risk factors for mesenteric ischaemia

 
4 Management
Acute bowel ischaemia is one of the most challenging emergencies, with an overall mortality of 60-80%, because of its wide range of manifestations.⁷ Management of this disorder requires a high index of suspicion, early confirmation of diagnosis, and aggressive treatment, with prompt active resuscitation, resection of non-viable bowel, and treatment of secondary peritonitis and multi-organ failure.

There are no universally accepted guidelines for the management of acute bowel ischaemia, but various schema have been proposed.^{7 8} Generally in the UK, patients diagnosed with acute bowel ischaemia who are fit for surgery undergo emergency laparotomy plus bowel resection. No absolute contraindications exist for this surgery because the mortality rate in patients who do not have surgery is 100%, but palliative measures may be more appropriate for some patients with extensive infarcts.

Early angiography can offer the opportunity for therapeutic intervention, including the administration of intra-arterial vasodilators or thrombolytic agents and angioplasty with or without stent in patients with mesenteric arterial occlusion. Other techniques such as superior mesenteric artery embolectomy and visceral artery bypass have also been described in cases where infarction has been detected early and rapidly, but these procedures are performed rarely and only in some tertiary centres.^{7 9} Unlike chronic bowel ischaemia, there is no widely accepted endovascular treatment option for acute bowel ischaemia.

Much of the success of bowel resection depends on the length of bowel removed, as a malabsorption syndrome (short bowel syndrome) can develop if more than 60% of the small bowel is lost.¹⁰ This proportion represents approximately 4 metres out of the 6 metres of small bowel in the adult, but loss of proximal or distal sections is particularly significant. Symptoms of short bowel syndrome largely derive from nutritional deficiencies, such as deficiency of vitamins and minerals, and from fluid and electrolyte imbalance. Patients typically report abdominal and bone pain, fatigue, prolonged blood clotting, and anaemia. Fluid retention and inadequate absorption of fat can occur, which lead to diarrhoea and steatorrhoea. Nutrient deficiencies are normally correctable by dietary supplements, but some patients might require more complex feeding approaches such as parenteral nutrition. Over time, the remaining section of small bowel can adapt to improve its absorptive capacity by, for instance, reducing the peristalsis rate, increasing in diameter, and increasing the length of villi. Surgical procedures to lengthen dilated bowel are controversial. Such procedures include Bianchi’s procedure and serial transverse enteroplasty. Small intestine transplantation is limited to the most specialised centres and has a high postoperative mortality: 90% and 60% survival at one and four years, respectively.¹¹

Patient outcome
The patient was managed conservatively and the decision was made with her family not to perform cardiopulmonary resuscitation in view of the patient’s frailty and poor prognosis. She died peacefully on the same day. No postmortem examination was performed.

Cite this as: BMJ 2009;339:b3192