CT Findings in Small Bowel Obstruction Faisal Budhani Diagnostic Radiology PGY-3 Resident

What is Important On-Call ? Must answer the following questions: Bowel obstruction: Y or N Complete vs Partial Obstruction Location of Transition Point Closed Loop or Internal Hernia Complications: perforation, strangulation and ischemia

Outline Introduction Role of CT in SBO Diagnosis of SBO Level of Obstruction Degree of Obstruction Causes of SBO Intrinsic Extrinsic Intussusception Intraluminal Closed-loop Strangulation following SBO Management of SBO What is Important On-Call ?

Introduction Relatively common accounting for 20% of all acute surgical admissions Diagnosis based on history, physical signs and radiographic findings Site and cause of SBO and presence of strangulation must be determined to ensure appropriate treatment Conventional radiology is first imaging modality with an accuracy of diagnosing presence of SBO 46-80%

Role of CT in SBO CT able to determine presence, level, degree and cause of SBO and identify associated strangulation CT able to depict pathology in bowel wall, mesentery, mesenteric vessels and peritoneal cavity Sensitivity of CT in detecting high grade SBO is 78-100%

Diagnosis of SBO Dilated proximal bowel with collapsed distal bowel separated by a transition zone is diagnostic Small bowel caliber > 2.5 - 3 cm is considered dilated

Diagnosis of SBO Small bowel feces sign – gas bubbles mixed with particulate matter in small bowel loops proximal to site of obstruction

Level of Obstruction Cannot be determined by intra-abdominal location of transition zone Dilated bowel loops migrate from their expected anatomic positions Relative length of dilated versus collapsed bowel must be considered

Degree of Obstruction Complete vs. Partial SBO based on degree of collapse and amount of residual contents distal to obstruction Passage of oral contrast distal to the transition zone always indicates partial obstruction

Adhesions Responsible for more than half of all SBO Etiology: Surgery  80% Peritonitis  15% Other (congenital, idiopathic)  5% Adhesions may be single, multiple or extensive Not seen on CT other causes of bowel obstruction must be ruled out

Hernia 2nd most common cause of SBO (10%) External Hernia Prolapsed of viscera through defect in abdominal/pelvic wall CT useful in detecting hernias in unsuspecting sites and obese patients Internal Hernia herniation of bowel loops through developmentally or surgically created defect in peritoneum, omentum or mesentery Less common than external hernias

Other Extrinsic Causes Variety of neoplastic, inflammatory or vascular lesions can cause SBO through direct compression or desmoplastic reaction Most common extrinsic is peritoneal carcinomatosis Multiple transitions zones of nodular wall thickening Mycobacterial infections, carcinoid and desmoid tumors have similar imaging findings

Intrinsic Lesions Neoplasms, hematomas, inflammatory and vascular lesions may cause bowel wall thickening leading to SBO Intrinsic lesions are located at the transition zone Most common causes include adenocarcinoma, crohn’s disease and radiation enteropathy Rare causes include intramural hematoma and eosinophilic gastroenteritis

Intussusception Relatively rare cause of adult SBO (5%) Unlike infants, 80% of cases caused by underlying neoplasm, adhesion, inverted Meckel’s, foreign body or previous surgery  serves as lead point Collapsed proximal segment (intussusceptum) with its mesenteric fat and vessels within the wall of the distal bowel (intussuscipiens)  characteristic target sign on axial images

Intraluminal Lesions Gallstones, foreign bodies and bezoars may cause SBO Gallstone ileus Triad of ectopic stone, pneumobillia and SBO Seen in elderly patients, particularly in women If foreign body detected, underlying obstructive lesion must be excluded

Closed Loop Obstruction 2 points of bowel obstructed at a single site Most often caused by adhesive bands; external and internal hernias less common Tends to involve the mesentery  prone to volvulus C-shaped/U-shaped loop of bowel with vessels converging towards site of torsion 2 adjacent collapsed loops with interposed dilated fluid filled bowel

Strangulation Mechanical obstruction associated with bowel ischemia Majority of cases associated with closed-loop obstruction Wall thickening (“halo sign”), mesenteric hazziness, pneumatosis and portal venous gas With IV contrast: lack of enhancement, asymmetric enhancement or delayed enhancement of bowel wall CT detection rate of strangulation is 63-100%

Management Acute complete SBO  surgical Partial SBO  conservative Follow up imaging recommended (CT or small bowel enteroclysis) in indeterminate cases Closed loop obstruction in absence of ischemia is a surgical emergency as it can progress to strangulation Risk of strangulation in compete SBO increases with time Surgery within 36 hrs  mortality rate = 8% Surgery after 36 hrs  mortality rate = 25% Exploratory laparotomy recommended in all patients with closed loop or signs of ischemia If CT findings not in keep with clinical presentation, patients must undergo laparotomy

What is Important On-Call ? Must answer the following questions: Bowel obstruction: Y or N Complete vs Partial Obstruction Location of Transition Point Closed Loop or Internal Hernia Complications: perforation, strangulation and ischemia