Chapter 15 Recognizing Extraluminal Air in the Abdomen
Recognition of extraluminal air is an important finding that can have an immediate effect on the course of treatment.
Air is normally not present in the peritoneal or extraperitoneal spaces, bowel wall, or biliary system.
Signs Of Free Intraperitoneal Air
There are three major signs of free intraperitoneal air arranged below in the order in which they are most commonly seen:
Air Beneath the Diaphragm
Air will rise to the highest part of the abdomen. Therefore, in the upright position, free air will usually reveal itself under the diaphragm as a crescentic lucency that parallels the undersurface of the diaphragm (Fig. 15-1).
While 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.
Figure 15-1 Free air beneath the diaphragm.
There are thin crescents of air beneath both the right (solid white arrow) and left (dotted white arrow) hemidiaphragms representing free intraperitoneal air. The patient had undergone abdominal surgery 3 days earlier. Free air can remain for up to 7 days after surgery in an adult, but serial studies should demonstrate a progressively decreasing amount of air.
Figure 15-2 Large amount of free air.
Upright view of the chest demonstrates a large amount of free air (A) beneath each hemidiaphragm (solid white arrows). The top of the liver (solid black arrow) is made visible by the air above it. The patient had a perforated gastric ulcer.
Figure 15-3 Free air seen on CT scan of the abdomen.
Axial CT scan of the upper abdomen performed with the patient supine shows free air anteriorly (solid white arrows). The air is not contained within any bowel. Free intraperitoneal air will normally rise to the highest point of the abdomen which, in the supine position, is beneath the anterior abdominal wall.
On conventional radiographs, free air is best demonstrated with the x-ray beam directed parallel to the floor (i.e., a horizontal beam) (see Figs. 13-14 and 13-15). Small amounts of free air will not be visible on supine radiographs.
Free air is easier to recognize under the right hemidiaphragm because only the soft tissue density of the liver is usually located there. 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, already reside in that location and may mask the presence of 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 his or her left side taken with a horizontal x-ray beam may show free air rising above the right edge of the liver. This is the left lateral decubitus view of the abdomen (Fig. 15-5).
Figure 15-4 Normal left hemidiaphragm (A) and free air under hemidiaphragm (B).
Close-up views of the left upper quadrant demonstrate the difficulty in recognizing free air beneath the left hemidiaphragm because of the normal location of gas-containing structures such as the stomach (S) and splenic flexure (SF). There is no free air seen in (A) but the other patient (B) does have a crescent of free air (solid white arrows). It is easier to recognize free air beneath the right hemidiaphragm because there is usually no air present above the liver on the right side.
Figure 15-5 Left lateral decubitus view showing free air.
Close-up of the right upper quadrant in a patient lying on his or her left side in the left lateral decubitus position shows a crescent of air (dotted white arrows) above the outer edge of the liver (solid black arrow), beneath the right hemidiaphragm (solid white arrow). The head/foot orientation of the patient is indicated. If the patient is unable to stand or sit up for an upright view of the abdomen, a left lateral decubitus view with a horizontal beam can substitute.
Pitfall: Chilaiditi syndrome
Figure 15-6 Chilaiditi syndrome.
Close-up of the right hemidiaphragm on a conventional chest radiograph (A) and an axial CT scan at the level of the diaphragm (B) both demonstrate air beneath the diaphragm that could be confused for free air (open black arrows in both photos). Careful evaluation of this air demonstrates several haustral folds (solid black arrows in [A] and solid white arrows in [B]) which indicate this is a loop of colon interposed between the liver and the diaphragm (Chilaiditi syndrome) rather than free air.
Visualization of Both Sides of the Bowel Wall
In the normal abdomen, 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 surrounded by tissue of the same density.
Introduction of air into the peritoneal cavity enables us to visualize the wall of the bowel itself since the wall is now surrounded on both inside and outside by air.
When air fills the peritoneal cavity, both sides of the bowel wall will be outlined by air (solid white arrows) making the wall of the bowel visible as a discrete line. This is known as Rigler sign and indicates the presence of a pneumoperitoneum.
Pitfall: When dilated loops of small bowel overlap each other, they may occasionally produce the mistaken impression that you are seeing both sides of the bowel wall (Fig. 15-8).
Figure 15-8 Overlapping loops mimicking free air.
Don’t let overlapping loops of dilated small bowel (solid white arrows) fool you into thinking you are seeing both sides of the bowel wall due to free air. If doubt exists about the presence of free air, confirmation may be obtained through an upright or left lateral decubitus view of the abdomen or a CT scan of the abdomen.
Visualization of the Falciform Ligament
The falciform ligament courses over the free edge of the liver anteriorly just to the right of the upper lumbar spine. It contains a remnant of the obliterated umbilical artery.
When a relatively large amount of free air is present and the patient is in the supine position, free air may rise over the anterior surface of the liver, surround the falciform ligament, and render it visible.Related posts:
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