Recognizing Extraluminal Air in the Abdomen

Air Beneath the Diaphragm

• Free air will generally rise to the highest part of the abdomen. In the upright position, free air will usually reveal itself under the diaphragm as a crescentic lucency

  Case Quiz 15 Question

  

  A 48-year-old male presented to the emergency department with severe abdominal pain and a tympanitic abdomen. A supine radiograph of the abdomen was obtained and is shown. Some time later, the patient was discharged after undergoing which one of these most likely events? (a) neostigmine administration, (b) insertion of a rectal tube, (c) abdominal surgery, or (d) polypectomy. See the answer at the end of the chapter.

  that parallels the undersurface of the diaphragm ( Fig. 15.1 ) .

  • The size of the crescent will be roughly proportional to the amount of free air. The smaller the amount of free air, the thinner the crescent; the larger the amount of free air, the larger the crescent ( Fig. 15.2 ) .

    

  

• Although free air is best demonstrated on CT scans of the abdomen because of its greater sensitivity in detecting very small amounts of free air ( Fig. 15.3 ), most surveys of the abdomen begin with conventional radiographs. Conventional radiographs serve as an important screening tool on which many previously unsuspected cases of free air are discovered.

  

Important Points

• On conventional radiographs, free air is best demonstrated with the x-ray beam directed parallel to the floor (i.e., a horizontal beam) (see Fig. 12.14 ). Small amounts of free air will not be visible on radiographs in which the x-ray beam is directed vertically downward, such as supine or prone views.

• Free air is easier to recognize under the right hemidiaphragm because that is normally the location of the liver, which has soft-tissue density. Free air is more difficult to recognize under the left hemidiaphragm because air-containing structures, such as the fundus of the stomach and the splenic flexure of the colon, already reside in that location, and they may mimic or hide free air ( Fig. 15.4 ) .

  

• If the patient is unable to stand or sit upright, then a view of the abdomen with the patient lying on their left side (meaning the right side is pointing up) taken with a horizontal x-ray beam may show free air rising above the right edge of the liver (see Fig. 12.15 ). This is called a left lateral decubitus view of the abdomen ( Fig. 15.5 ) .

  

Diagnostic Pitfalls

• Chilaiditi sign

  • Occasionally, normal colon may be interposed between the dome of the liver and the right hemidiaphragm and, unless a careful search is made for the presence of haustral folds characteristic of the colon, it may be mistaken for free air ( Fig. 15.6 ) . This normal interposition of the colon between the liver and right hemidiaphragm is called the Chilaiditi sign (pronounced kyla-ditty). If in doubt, obtain a left lateral decubitus view of the abdomen or, if necessary, a CT scan of the abdomen.

    

Visualization of Both Sides of the Bowel Wall

• On the normal abdominal radiograph, we visualize only the air inside the lumen of the bowel, not the wall of the bowel itself. This is because the wall is soft-tissue density and is normally surrounded by intraperitoneal tissues of the same density.

• Introduction of air into the peritoneal cavity enables us to visualize the wall of the bowel itself because the wall is now surrounded on both inside and outside by air.

• The ability to see both sides of the bowel wall is a sign of free intraperitoneal air called the Rigler sign ( Fig. 15.7 ).

  

• The Rigler sign can be seen on supine, upright, or prone films of the abdomen so long as there is a relatively large amount of free air present.

Diagnostic Pitfalls

• When dilated loops of small bowel overlap , they may occasionally produce the mistaken impression that you are seeing both sides of the bowel wall ( Fig. 15.8 ) . If in doubt, confirm the presence of free air with an upright or left lateral decubitus view of the abdomen, or a CT scan of the abdomen.