(Brief translation from Pubmed: Detection of intraperitoneal free gas by ultrasound, Australas J Ultrasound Med, 2013 May)
Acute abdominal pain is a common symptom in patients going to the emergency department. The differential diagnoses in these patients are varied, and often difficult to be sure based solely on history and examination. Therefore, imaging tests are often ordered to find additional causes of the pain. Ultrasound is commonly used, although it is generally not considered the test of choice for perforation diagnosis. The common explanation for this is that the gas, as a strong antireflux, prevents the propagation of sound waves and produces a reflected polyphony. In addition, the physiological gas in the gut will also prevent an accurate interpretation.
However, when looking at the criteria used to diagnose pneumoperitoneum on X-rays and CT scans, it can be seen that the same standards can be used on ultrasound. This article will discuss the markers and criteria on ultrasound that can be used to diagnose peritoneal gas, describing the accuracy and limitations of this technique.
Standard on ultrasound for the diagnosis of gas in the peritoneal cavity
Diagnosis of intra-peritoneal gas, X-ray, CT or ultrasound is based on detecting gas in the abdominal cavity outside of its physiological site, i.e. outside the lumen of the intestine. Due to its light density, the gas will normally move in a counter-gravity direction to the highest regions. In the case of peritoneal gas, the patient is lying on his back, the gas is seen next to the peritoneal line or above the surface of the liver. Gas can also be seen, especially small gas molecules, outside the intestine at the site of the puncture.
The gas appears on ultrasound with reverberation artifacts that obscure the anatomical structures behind it (for high qi) or the hyperplasia of the marginal vein. air bubbles (EPSS- Enhancement of the peritoneal stripe sign) (for low air volume). Based on this, there are many signs on ultrasound described in the past 30 years.
Gas builds up on the surface of the liver
Usually the liver is below the diaphragm, with no air in between. In the case of peritoneal gas, the gas may be located on the surface of the liver, obscuring the underlying liver parenchyma. Unlike air bubbles from the lungs, the gas does not move with the rhythm of the breath. Furthermore, the liver parenchyma can be seen above for free gas in the peritoneal cavity and below for the lungs (Figure 1). If the gas is concentrated in the lower and mid-abdomen, it can be seen next to the peritoneal fold (eg the sickle ligament groove). Patient should be in either supine or left-side position. The hepatomegaly may increase in sound similar to that of free gas, but will not lose the underlying liver structure. Chilaiditi syndrome is a variation in which the intestine interjects between the liver and the diaphragm, which can have a similar picture. In this case, high frequency straight probe examination will find that if the intestinal lumen is discontinuous, it is discontinuous with the peritoneum and visible folds in the colon (Haustral folds).
Figure 1. Qi is seen on the surface of the liver, obscuring the underlying liver parenchyma (down arrow). The normal lung is seen separately above (arrowhead).
Gas can move
The free air can either move in the patient position or squeeze the transducer. The patient posture movement is described when intraperitoneal gas or intraperitoneal gas is suspected. Usually the patient is asked to lie on his left side and check again. With a large amount of free gas, a pressure probe can be used to rapidly move the gas (pain and resistance do not prevent this test). Pressing will cause the air to separate, creating an effect similar to the opening of a curtain (curtain sign)(Figure 2.3. Video 1).
Figure 2. A bright echo line with a back shadow of a posterior gas across the abdomen behind the abdominal wall (arrow).
Figure 3. Pressing the probe, the air line will split to reveal the gas in the lower intestine (arrowhead)
Video 1.https: //www.youtube.com/watch?v=-nk8jNpHbVA&feature=youtu.be
Abdominal pain of unknown cause. Bright reverberation lines with a posterior polyphonic pseudo-image are seen when cut across the abdomen. Pressing the probe causes the gas to separate to expose the lower intestine, confirming it is free gas in the abdomen.
Figure 4. Step-ladder sign: When the gas is high, the step sign can be seen. The ladder mark is formed by 2 air curtains: (1) gas in the lungs and (2) free gas. The gas in the lung is higher than the gas in the peritoneal cavity, creating a ladder image. Short black arrow: gas in the lungs, long black arrow: free gas.
Due to the strong negative feedback, the gas produces a bright echogen which causes thickening of the peritoneum. It is essential to ensure that the gas is visible side by side or continuously with the peritoneum. The physiological gas in the intestine emerges near the peritoneum, but it separates from the peritoneum because of the thickness of the intestinal wall, which appears as a thin, poorly echogenic line between the light line of the peritoneum and the gas surface shining. The use of a high frequency straight probe can prove this. In addition, if monitored, gas in the intestine will be seen either moving away from the peritoneum as the lumbar folds fold deeper into the abdomen or show the normal mucosal folds of the intestine. Peristalsis or moving in rhythm can also be observed, confirming that gas is in the loops of the intestine. By comparison, the peritoneum is seen as continuous, no folds located throughout the abdomen. The difference can be seen in Figure 5. If there is an ascites attached, then the presence of gas to the peritoneum is more conspicuous (Figure 6).
Figure 5. The downward arrow shows the negative gas feedback is continuous with the peritoneum, representing free gas. The upward arrow shows the negative light response of the gas that bends away from the peritoneum and is separated by a thin, poor echogenicity of the intestinal wall, representing gas in the intestine.
Figure 6. Patient with alcoholic liver disease presents with abdominal pain and swelling. The back shadow of the visible gas emanates from the peritoneum (arrowhead), easily visible due to ascites.
Gas outside the intestine
The gas can be found in anatomical sites, not its physiological sites. The most common site in patients with acute abdominal pain is around the umbilical cord (porta hepatis) as a result of peptic ulcer perforation. Small gas molecules can be seen around the gallbladder and under the liver (Figure 8). Perforation of the colon usually leads to large amounts of free gas, easily visible on ultrasound. An intra-abdominal or retroperitoneal abscess (for example, appendix, diverticulum, or postoperative) can create small air bubbles outside the intestine. Uncomplicated diverticulosis can lead to gas molecules appearing on the outside of the lumen, although the mucosa can be seen around the gas. Gas in superficial structures (gas in the skin and in muscles) can also have similar characteristics, but it is above the peritoneal tract and is not as mobile as peritoneal gas (Figure 7).
Figure 7. Subcutaneous air spillage. The gas in the subcutaneous layer (long arrow) and the gas in the lungs (short arrow) form the step mark. The ladder mark due to subcutaneous air spill, however, is not separated by the diaphragm (open arrow).
Similar to the prominent ‘A lines’ in pleural pneumothorax, reflections of light and the distances of the reflected polymorphic image generated from a large amount of gas can be expected in the case of peritoneal gas. large amounts. However, this can also occur when the stomach or intestines are dilated, and should therefore be interpreted with caution (Figures 8,9,10).
Figure 8. Small gaseous molecules found outside the intestine in the gallbladder fossa and the liver umbilical cord (arrow).
Figure 9. Pictured polyphonic response at evenly spaced, bright images (similar to the ‘A lines’ in the lungs) is shown. It is not clear whether they originate from the peritoneal tract in this image (next image with a high frequency straight probe shows the intestinal wall thickness separate from the anechoic peritoneum).
Figure 10. X-rays show that the intestines are filled with gas and dilate, creating a false image.
Previous animal models and many patients after laparoscopy showed that ultrasound was very sensitive to detect even small amounts of free gas in the peritoneal cavity. A series of patients with abdominal trauma or abdominal pain have shown that ultrasound is highly sensitive (85-100%). Most studies also show high specificity (84-100%), although this specificity is only 53% for radiographs in that study. Overall, these studies demonstrate that ultrasound is as accurate as X-rays, although CT remains the gold standard in imaging.
Performing an ultrasound in a patient with abdominal pain may be limited due to the severity of pain and resistance. This may prevent performing the solutions described above. A small amount of gas, especially one that has not yet reached the peritoneum, may be difficult or impossible to see with ultrasound due to obstruction of gas in the intestine.
Although not commonly seen as the primary indication for perforated viscera and is not as sensitive by CT, ultrasound can often detect signs of peritoneal gas if present. When a patient with unexplained abdominal pain is often prescribed an ultrasound as part of the diagnostic process, all ultrasound physicians should be on the lookout for these signs.
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- Ultrasound diagnosis of slightly free peritoneal cavity, techniques and applications. Nguyen Hoang Thuan.
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BS. Le Thi Ny Ny
Department of Diagnostic Imaging, Nguyen Tri Phuong Hospital