Sonographic diagnosis of intraperitoneal free air.

Detection of intraperitoneal free air is important for the diagnosing of life-threatening conditions in patients with acute abdominal pain. Point-of-care ultrasound is an extension of the clinical examination in patients presenting with acute abdomen. Failure of sonographers to detect intraperitoneal free air was clinically considered as a limitation of abdominal ultrasound. It is now increasingly appreciated that ultrasound may detect intraperitoneal free air. Emergency physicians should be familiar with the sonographic features of intraperitoneal free air which may be essential to recognize bowel perforation. Herein we review the literature on the sonographic findings of intraperitoneal free air and the technical manoeuvres that can improve clinicians' detection of intraperitoneal free air using point-of-care ultrasound.

INTRODUCTION

Ultrasound is widely available in emergency departments and point-of-care ultrasound is increasingly used by emergency physicians.[1] Detection of intraperitoneal free air (IFA) is of great value in identifying life-threatening conditions in patients with acute abdominal pain. IFA usually occurs as a result of a perforated hollow viscous, intraperitoneal gas insufflations, penetrating abdominal injury, infection with gas forming organisms, or spontaneous pneumoperitoneum.[2] Upright chest X-ray is a common method for detecting IFA.[3] However, many patients with acute abdominal pain cannot stand to have a chest radiograph. Decubitus abdominal X-ray is usually used in this condition. If perforation is suspected, those patients are usually subjected to abdominal CT scan exposing them to high dose of radiation. We have to highlight that point-of-care ultrasound examination is of great value in these circumstances. Point-of-care ultrasound is an extension of the clinical examination in patients having an acute abdomen.[4] It is a goal-directed study that can be used for rapidly diagnosing intraperitoneal fluid, aortic aneurysms, gallstones, appendicitis, and hydronephrosis.[1]

Failure of ultrasound to reveal IFA is considered by many as a weakness of abdominal sonographgy. This is due to the difficulty of differentiating IFA and intraluminal bowel gas.[5] The sonographic features of IFA in the literature are limited.[6] Herein we review the sonographic findings of IFA and technical manoeuvres that can improve clinicians’ detection of IFA using point-of-care ultrasound.

SONOGRAPHIC FINDINGS OF IFA

It is claimed that, sonography is superior to erect chest X-ray in diagnosing IFA and as little as 2 ml of IFA can be detected by ultrasound.[37] Nevertheless, its detection is difficult even for an experienced sonographer.[7] The sonographic appearance of IFA results from scattering of the ultrasound waves at the interface of soft tissue and air which is accompanied by reverberation of the waves between the transducer and the air. This, typically, results in a high-amplitude linear echo (increased echogenicity of a peritoneal stripe) accompanied by posterior artifactual reverberation echoes with characteristic comet-tail appearance [Figure 1].[89] This image can be changed by changing the patient's position.

Figure 1

Sagital sonographic section of the right hypochondrium using a curvilinear probe showing enhanced peritoneal stripe (empty arrow) and reverberation artefacts (small arrows) which partially obscure the right lobe of the liver (L) and right kidney (K). Laparotomy confirmed that the patient had perforated diverticulitis

Reverberation may not be seen in small air collections. When extensive pneumoperitoneum is present, then reverberation may even obscure the underlying abdominal organs.[7] Trapped air bubbles in ascitic fluid or in a localized fluid collection will give rise to echogenic foci due to air from perforated viscus [Figure 2].[410]

Figure 2

Transverse sonographic section of the right hypochondrium using a linear probe showing a hyperdence echogenic small area (arrow head) moving within a fluid collection. Laparotomy confirmed that the patient had a perforated duodenal ulcer

Associated sonographic findings can help in the diagnosis of the etiology of the IFA [Figure 3]. Detection of interaperitoneal free or localized fluid collections can be seen in perforated peptic ulcer or perforated appendix.[17] Thickened bowel or gall bladder wall and decreased bowel motility or ileus may point toward a peritoneal inflammatory process that can be associated with perforation of diverticulitis or small bowel perforation.[68] Lymph node metastasis may be seen in perforated tumors of the gastrointestinal tract.[3]

Figure 3

Sagital sonographic section of the right hypochondrium using a curvilinear probe showing a hyperdence interrupted echogenic lines under the liver in Morrison's pouch (arrow head), fluid collection (white arrow), and a hyperdense echogenic line in the anterior wall of the duodenum representing the scar of a duodenal ulcer (black arrow). Laparotomy confirmed that the patient had a perforated duodenal ulcer

TECHNICAL MANOEUVRES TO IMPROVE DETECTION OF IFA

Sonographers are familiar with the images of air present in the lung and bowel. The presence of intraperitoneal air outside the intestinal lumen is unusual and can be mistaken for air within the bowel.[5]

The linear-array transducer (10-12 MHz) is more sensitive than standard curvilinear abdominal transducer (2-5 MHz) for detecting IFA because it has higher resolution in the near field where air accumulates.[7]

IFA is best detected when scanning the right upper quadrant between the anterior abdominal wall and liver where there is no intervening bowels except rarely with interposition of the bowel between the liver and the diaphragm. Furthermore, the intraluminal gas can be seen inside a bowel loop having visible peristalsis and a normal wall thickness.[7] The presence of air in abnormal sites such as along with the fissure of ligamentum teres and Morrison's pouch should raise the suspicion of IFA.[4]

Patients should be first scanned in the supine position concentrating on the midline and right upper quadrant [Figure 1]. The patient should be then scanned in the left lateral decubitus position. The presence of even small amount of IFA behind the anterior abdominal wall causes an enhancement of the peritoneal stripe that will move when changing the patients’ position.[5] In morbidly obese patients, the identification of enhanced peritoneal stripe can be facilitated by asking the patient to take a deep breath. This can help in the differentiation between the enhanced peritoneal stripe and the moving intraperitoneal fat and bowel.[5]

When scanning the right upper quadrant, care should be taken to avoid artifacts resulting from the adjacent lung. This can be achieved by scanning this area both during inspiration and expiration. The shadows produced by the intervening ribs originate above the peritoneal line and move with respiration.[6]

It is important to repeat the abdominal ultrasound in clinically suspected cases of perforated bowels with non conclusive evidence of IFA because more free air may accumulate and become more evident.[11]

ADVANTAGES AND LIMITATIONS OF SONOGRAPHY

Sonography is a valuable tool not only for the detection of IFA, but also for the diagnosis of the etiology of IFA.[7] In comparison with CT scan, ultrasound is easily available, safe, portable, less expensive, and does not have radiation. Nevertheless, it has its own pitfalls. It is operator dependant and some ultrasound machines have low-quality images that may not be able to detect IFA. Furthermore, very ill and irritable patients may be less cooperative to allow for scanning of different regions. Sonography is difficult in obese patients and with those having subcutaneous emphysema.[512]

CONCLUSIONS

Ultrasound of IFA typically appears as an enhanced peritoneal stripe accompanied by posterior artifactual reverberation echoes with characteristic comet-tail appearance. Right upper quadrant and midline are the best regions to detect IFA. Air in unusual sites should raise the suspicion of a perforated hollow viscus. Emergency physicians performing point-of-care ultrasound should be familiar with the sonographic findings of IFA which has a great value in diagnosing life-threatening conditions. Repeated abdominal scanning of clinically suspected cases of bowel perforation is essential to detect accumulating free air overtime.