Small-Bowel Obstruction Workup

If the diagnosis is unclear, admission and observation are warranted to detect early obstructions. Essential laboratory tests are needed, including the following:

• Serum chemistries: Results are usually normal or mildly elevated; abnomal results early in the disease are generally due to vomiting or dehydration

• Blood urea nitrogen (BUN)/creatinine levels: May be increased due to a decreased volume state (eg, dehydration)

• Complete blood cell (CBC) count: The white blood cell (WBC) count may be elevated with a left shift in simple or strangulated obstructions; increased hematocrit is an indicator of volume state (ie, dehydration)

• Serum lactate levels: Increased levels are suggestive of dehydration or tissue underperfusion

• Lactate dehydrogenase studies

• Urinalysis

• Type and crossmatch as well as prothrombin time (PT), international normalized ratio (INR), and partial thromboplastin time (PTT): These are adjunctive laboratory tests used in the evaluation of SBO; the patient may require surgical intervention

Laboratory studies to exclude biliary or hepatic disease are also needed and include the following:

• Phosphate level

• Creatine kinase level

• Liver panel: aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), and bilirubin levels

Obtain plain radiographs first for patients in whom small-bowel obstruction (SBO) is suspected. At least 2 views, supine or flat and upright, are required. Plain radiographs are diagnostically more accurate in cases of simple obstruction. However, diagnostic failure rates of as much as 30% have been reported. ^[4]

In a study of 103 patients with suspected SBO, the sensitivity of plain radiography was reported to be 75%, with a specificity of 53%, whereas computed tomography (CT) scanning had a 92% sensitivity and 71% specificity. ^[11]

The experience of a radiologist also appears to play a role in the evaluation of SBO. Among 6 reviewers in one study, the sensitivity of SBO was between 59% and 93%, with senior staff members having significantly higher sensitivity. ^[4] The radiographic signs that were most significant included, two or more air-fluid levels, air-fluid levels wider than 2.5 cm, or air fluid levels differing more than 5 mm in same bowel loop. ^[4]

X-ray imaging is further limited by the decreased ability to visualize the transition point or grade the degree of bowel obstruction.

Plain radiography is of little assistance in differentiating strangulation from simple obstruction. Some have used abdominal radiography to distinguish between complete obstruction and partial or no SBO.

A study by Lappas et al proposed that 2 findings were more predictive of a higher grade or complete SBO: (1) the presence of an air-fluid differential height in the same small-bowel loop and (2) the presence of a mean level width greater than 25 mm. ^[12]  When the 2 findings were present, the obstruction was most likely high grade or complete. When both were absent, the authors proposed, a low-grade (partial) SBO was likely or nonexistent.

Dilated small-bowel loops with air-fluid levels indicate SBO, as does absent or minimal colonic gas. SBO is demonstrated in the radiographs below.

Enteroclysis

Enteroclysis is the use of a contrast agent normally administered through a nasogastric tube. Enteroclysis is valuable in detecting the presence of obstruction and in differentiating partial from complete blockages. This study is useful when plain radiographic findings are normal in the presence of clinical signs of small-bowel obstruction (SBO) or when plain radiographic findings are nonspecific. Enteroclysis is often less desirable when compared to computed tomography (CT) scanning due to the risk of perforation or aspiration with the administration of contrast medium. 

Enteroclysis distinguishes adhesions from metastases, tumor recurrence, and radiation damage. It offers a high negative predictive value and can be performed with 2 types of contrast agents. Barium is the classic contrast agent used in this study; it is safe and useful when diagnosing obstructions, provided that no evidence of bowel ischemia or perforation exists. Barium has been associated with peritonitis and should be avoided if perforation is suspected.

CT enterography/CT enteroclysis

This modality is replacing enteroclysis in clinical practice. ^{[13, 14, 15]}  In addition, it is the examination of choice for intermittent SBO and in patients with a complicated surgical history (eg, prior surgery, tumors). ^{[16, 17, 18]}

CT enterography displays the entire thickness of the bowel wall and allows evaluation of surrounding mesentery and perinephric fat. ^[13]  It uses CT-scanning technology to scan thin slices of bowel while simultaneously using large-volume enteric contrast material for imagery. ^[13]

CT enterography is more accurate than conventional CT scanning at finding the cause of SBO (89% vs 50%, respectively), as well as at locating the site of the obstruction (100% vs 94%, respectively). ^[19]  It is useful in patients being managed conservatively (ie, nonoperatively). ^[19]

Computed tomography (CT) scanning

CT scanning is the study of choice if the patient has fever, tachycardia, localized abdominal pain, and/or leukocytosis.

CT scanning is useful in making an early diagnosis of strangulated obstruction and in delineating the myriad other causes of acute abdominal pain, particularly when clinical and radiographic findings are inconclusive. It also has proved useful in distinguishing the etiologies of small-bowel obstruction (SBO), that is, in distinguishing extrinsic causes (such as adhesions and hernia) from intrinsic causes (such as neoplasms and Crohn disease). In addition, CT scanning differentiates the above from intraluminal causes, such as bezoars. The modality may be less useful in the evaluation of small bowel ischemia associated with obstruction.

CT scanning is capable of revealing abscess, inflammatory process, extraluminal pathology resulting in obstruction, and mesenteric ischemia and enables the clinician to distinguish between ileus and mechanical small bowel obstruction in postoperative patients. ^[20]

The modality does not require oral contrast for the diagnosis of SBO, because the retained intraluminal fluid serves as a natural contrast agent.

Obstruction is present if the small-bowel loop is greater than 2.5 cm in diameter dilated proximal to a distinct transition zone of collapsed bowel less than 1 cm in diameter. A smooth beak indicates simple obstruction without vascular compromise; a serrated beak may indicate strangulation. Bowel wall thickening, portal venous gas, or pneumatosis indicates early strangulation.

One series of 32 patients reported a sensitivity of 93%, a specificity of 100%, and an accuracy of 94% for CT scanning in the detection of obstructions. ^[21] Another series reported a sensitivity of 92% and specificity of 71% in the correct identification of partial or complete SBO. ^[11] Additional studies have shown sensitivities above 95% for CT scanning in identifying obstructions and its complications. ^[5]

Magnetic resonance imaging (MRI)

The accuracy of MRI almost approaches that of CT scanning for the detection of obstructions. ^[13] MRI is also effective in defining the location and etiology of obstruction. ^[22] MRI has several limitations, however, including lack of availability (transporting sicker patients is difficult) and poor visualization of masses and inflammation. ^{[23, 24]}

In a retrospective study (2005-2015) of 12 prenatally diagnosed cases of SBO evaluated by both ultrasonography and MRI, Rubio et al noted that MRI was useful in providing the following information ^[25] :

• The distribution of meconium in the small bowel, clarifying the level of obstruction
• Abnormally diminished meconium in the rectum, suggestive of cystic fibrosis or combined SBO and colonic obstruction (which may be useful in family counseling and postnal care preparation)
• The evaluation of colon and rectal contents, serving as a fetal enema

Ultrasonography is less costly and invasive than CT scanning and may reliably exclude SBO in as many as 89% of patients; specificity is reportedly 100%.

In a small study by Jang et al in which the use of bedside ultrasonography by emergency physicians was compared with radiography for the detection of small-bowel obstruction (SBO), emergency physician ̶ performed ultrasonography compared favorably with radiography. Dilated bowel on ultrasonography had a sensitivity of 91% and a specificity of 84% for SBO, while radiography had a sensitivity of 46% and a specificity of 66%. ^[26]