Case kk -  Crohns Disease Causing Obstruction - Introduction to Small Bowel Thickening

There are dilated loops of proximal small bowel with air fluid levels.  Distally, there is decompressed but thickened and abnormal terminal ileum.  Just proximal to the transition point the small bowel is more markedly dilated with fecalization of its contents.  (We earlier discussed fecalization of contents as an appearance of the small bowel lumen just proximal to an abnormality).  These findings indicate a small bowel obstruction.  The cause of the obstruction is small bowel thickening most consistent with Crohns disease.  Endoscopy with terminal ileum biopsies confirmed the diagnosis.

This case is a good introduction to small bowel thickening because the varied appearances of the small bowel wall are present.  Proximally, the small bowel appears normal (1, 2).  The duodenum and jejunum contain muliple submucosal folds.  When distended, the folds can be individually seen and are thin.  When partially collapsed, the small bowel has a feathery appearance (1, 2) because although dense oral contrast interdigitates between the folds, there is not enough distension to see the folds individually.  The appearance is secondary to volume averaging of oral contrast and mucosal folds.  It is important to become familiar with this appearance.  The folds in the ileum are less prominent.  The wall of the ileum usually appears smooth and thin, though not as paper thin as the colonic wall.  Even when markedly dilated, the wall remains thin (1,2).  See the normal small bowel schematic for a summary.

Now look at the thickened segment of bowel.  The mucosa and serosa clearly enhance.  The wall is thick and has low attenuation.  Seen in cross section, this loop would have the classic target sign, which will be demonstrated in another case. 

In this case the thickening is obvious.  However, sometimes collapsed bowel appears thickened.  One of the most reliable methods to convince yourself that bowel is either thickened or normal is to look for air against the bowel wall.  If the air is separated from the extraluminal contents (i.e. fat, muscle, other loops of bowel) the wall is thickened.  If the air is adjacent to extraluminal contents (1,2), the wall remains thin.  Thickened bowel may or may not distend with contrast or fluid. 

Proximal thickened duodenum and ileum may have a different appearance.  The appearance of the mucosal folds are accentuated as the wall thickens.  They become thick and prominent as in this example, not taken from this case.  The appearance is similar to thumbprinting in a thickened colon and is also referred to as "Stack of Coins."  See the abnormal small bowel schematic for a summary.

Small bowel wall thickening is a non-specific finding. Submucosal  thickening represents one of only a limited number of processes.  These are edema, hemorrhage, pus, and tumor infiltration.  I tailor my differential diagnosis based on location and extent of disease. 

Focal thickening is usually due to focal hemorrhage from trauma, anticoagulation, or vasculitis; edema from infection or ischemia; or cellular infiltrate from processes like Crihns disease or lymphoma.  Distribution then plays a very important role.  Involvement of the terminal ileum strongly suggests Crohns disease, especially if there are multiple other focal areas of involvement or signs of obstruction.  Other causes of terminal ileitis include infection with Campylobacter, Shigella, Yersinia, and TB.  Involvement of the duodenum or co-existing solid organ injury suggests hemorrhage from trauma. Localized duodenal and jejunal involvement is usually seen with gastro-enteritis (and also infections such as Giardia and Cryptosporidium).  Lastly, history plays a large role.  A patient with INR of 7 and a focal area of thickening likely has hemorrhage into the bowel.

The differential diagnosis for diffuse thickening is almost as broad:  hemorrhage from vasculitis or anticoagulation; edema from ischema (shock bowel), radiation, graft versus host disease, or angioedema; and diffuse infiltration from lymphoma.

In this case the abnormal bowel is limited to the distal and terminal ileum.  The terminal ileum is also thickened.  Notice the thickened submucoa and the distance between intraluminal air and the mesenteric fat.  Thickened terminal ileum is the hallmark of Crohns disease.  Crohns disease often causes focal areas of obstruction.  Other common appearances of Corhns disease include multiple non continuous areas of thickening throughout the large and small bowel, fistula formation, and abscess collections. 

This case would be an unusual appearance for infection which does not often cause obstruction.  It is an unusual location for ischemia and hemorrhage.

Key points