Ultrasound Zoom: A New Mode to Diagnose Pneumo(Peritoneum): Using POCUS to Identify Intraabdominal Free Air

A New Mode to Diagnose Pneumo(Peritoneum): Using POCUS to Identify Intra-abdominal Free Air

By Brittney Giuffre, MD, PGY-2 Jackson Memorial Hospital

Edited by Leila Posaw, MD, MPH Emergency Ultrasound Director, Jackson Memorial

For me, training in a busy public hospital has not only been an incredible opportunity to see a broad range of pathology, but also to experience many of the challenges faced by busy emergency departments. One of our biggest challenges at Jackson Memorial Hospital is the high volume of critically-ill patients, which may result in them waiting several hours for evaluation and imaging.

Almost reflexively, after determining a patient is “sick,” I find myself running to grab our ultrasound machine. The ability to get quick answers, with the bonus of low cost and no radiation risk to the patient, has spiked my interest to learn and use point-of-care ultrasound (POCUS), especially on our sickest patients with life-threatening diagnoses. For example, even before the portable x-ray machine rolls into the patient’s room, I can confidently determine whether a patient with shortness of breath has acute decompensated heart failure or a COPD exacerbation. However, I first had to learn to identify B-lines.

Recently, I came across an intriguing blog post in which POCUS was used to diagnose intra-abdominal free air (IFA). Now that I’ve learned the technique and signs, I am undoubtedly going to apply this to my next patient with acute abdominal pain, and hopefully, I will diagnose pneumoperitoneum.

PNEUMOPERITONEUM Pneumoperitoneum is an all-encompassing term for IFA inside the peritoneal cavity but outside the bowel lumen. Most often, patients will present with severe, sudden onset abdominal pain, associated with nausea and vomiting. The physical exam is variable, and ranges from mild to severe abdominal tenderness with distension, rebound and guarding.

There is a long list of causes for IFA (Table 1), and often the history is enough to narrow the differential diagnosis. Up to 85-95% of the time, IFA is caused by a perforated viscus or a defect in the gastrointestinal tract. These patients require emergent surgery, and a missed diagnosis is associated with high mortality. Non-operative etiologies include increased abdominal pressure (severe coughing, CPR, COPD) and iatrogenic causes (post-laparoscopic or open abdominal surgery). 1

IMAGING

Plain film radiography Radiography alone is not sensitive enough to rule out IFA, and published sensitivities range between 55-85%. 2 As patients need to be positioned upright or on their side for at least 10 minutes, plain films may prove difficult in our critically ill patients.

Of upright and supine views, the former is more sensitive. Classically, you would look for air under the diaphragm; however, this is only seen in 60-80% of cases. 3 Some studies describe the ability of upright films to detect as little as 1 mL of free air, 4 though this involves a great deal of technical precision and patience. If unable to obtain upright views, the supine left lateral decubitus view is the next best choice. Free air can be

Table 1: Common causes of IFA requiring surgical intervention, based on location

Esophageal perforation Risk factors: recent instrumentation, esophageal cancer, history of forceful vomiting ROS: +chest pain, +dysphagia, +epigastric pain Other clues: subcutaneous emphysema

Small bowel Acute mesenteric ischemia

Risk factors: atrial fibrillation, CAD History: postprandial pain, weight loss

Labs: elevated lactate Other causes

Malignancy Meckel’s diverticulum

Perforated peptic ulcer Gastric or duodenal ulcer

Risk factors: NSAID use, H. pylori

Colorectal Toxic megacolon

Risk factors: C. difficile colitis, IBD Other causes

Diverticulitis Appendicitis Malignancy Ischemic colitis Iatrogenic (post colonoscopy) Bowel obstruction

Subtle signs of IFA include the Falciform Ligament sign (when air collects in bilateral subphrenic spaces to reveal a linear density on the ventral surface of liver) and the Rigler sign (when air is visualized on both sides of the bowel wall). 5

CT CT is the imaging modality of choice as it has the highest sensitivity and specificity. It is excellent at detecting air in the retroperitoneal spaces, such as with perforated sigmoid diverticulitis. IV contrast is useful to detect fat stranding, bowel wall defects, or bowel wall thickening. In 90% of positive scans, air will be most pronounced at the site of the perforation. 6

Unfortunately, CT is not always the wisest first choice in EDs due to considerations of vague presenting symptoms, risks of radiation, cost, and potential delays in overcrowded EDs.

Ultrasound Comfort with the POCUS technique is an invaluable skill for emergency physicians for several reasons. A busy ED might have imaging delays with CT scanners occupied with trauma patients or no available patient transporters, and a bedside exam might save the day. Another advantage of POCUS is that it is a quick, low radiation option in patients who are “too unstable” to leave the department. It is also a more appropriate first choice in pregnant women and infants, in whom we Ultrasound is more sensitive for free air than plain radiography. One study found ultrasound to have a 95% sensitivity in detecting pneumoperitoneum compared to 78% with plain films. 7 Additionally, ultrasound has improved ability to detect “indirect” findings of bowel perforation, such as decreased peristalsis and free fluid between intestinal loops. 8

That being said, it is important to remember that POCUS is operator-dependent, and an average user might forget the subtleties of how to perform the scan or might not be familiar with the ultrasound findings altogether. POCUS is a “rule-in” test and is extremely useful as a screening test to expedite next steps. Given the significant morbidity and mortality associated with this diagnosis, a negative POCUS exam should not be interpreted as an absence of pathology.

TECHNIQUE This exam seeks to answer one simple question: Is there free air in the abdomen?

Probe selection This depends on the patient’s body habitus. The higher frequency linear probe (10-12 MHz) is preferred in thin patients due to superior resolution. Unfortunately, this probe is limited by shorter depth penetration. Thus, in patients with more adipose tissue, the lower frequency curvilinear

probe (3-5 MHz) will allow greater depth visualization. While Nazerian et. al. found improved specificity using the linear probe compared to the curvilinear probe (95.5% vs 81.8%), there was no significant difference in sensitivity. 2 An additional benefit is that the same probe can be used to perform several other scans simultaneously, such as the gallbladder, aorta or FAST.

Several scanning techniques have been published. Hefney et.al. describe scanning the patient in the epigastrium and in the RUQ, in both supine and left lateral decubitus positions. Deep inspiration was shown to enhance image acquisition. 9 Karahan et. al. also describe scanning the patient in the supine position, with the sagittal probe in the right paramedian epigastric area, and in the left lateral position with the transverse probe placed in the right mid axillary line, parallel to parallel to 8th and 10th intercostal spaces. Strong reverberations may shift with position change, and this is known as the shifting phenomenon. 10

Scissor Maneuver Karahan et. al. have described a novel and useful technique called the “scissor maneuver,” which can assist in identifying pneumoperitoneum. When pressure is applied to the caudal aspect of the probe, the reverberation artifacts disappear as the free air is pushed away. When pressure is released, the reverberation artifacts

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Pneumoperitoneum.To.Go.

Stick in your wallet. Reference on-the-go. Courtesy of authors Leila Posaw, MD, MPH and Brittney Giuffre, MD

Normal

Pneumoperitoneum Point-of-Care Ultrasound: Signs

EPSS: Enhanced Peritoneal Stripe Sign

Reverberation artifacts: A-lines Gas Bubbles in Ascites Pitfall

� Look for peristalsis when using the scissor technique to differentiate between normal air in the lumen and free air.

For a better visual idea of what the Scissor Maneuver looks like, scan to read this blog post from EM Docs:

2 Scan FAST exam Nazerian et. al. describe a quick 2-location exam, which has been shown to be equally efficacious as scanning five abdominal locations (epigastrium, right hypochondrium, left hypochondrium, umbilical, and right hypochondrium with patient in left lateral decubitus position). 2 The best two locations were found to be the epigastric region with the patient in supine position, and the right hypochondrium with the patient in lateral decubitus position for at least 2 minutes. 2 The goal of the 2-minute wait is to bring the free air anterior and close to the probe.

Signs Four of the most well described POCUS signs in patients with IFA are: (a) the enhanced peritoneal stripe sign, or EPSS, (b) the presence of

(a) EPSS: Normally, the peritoneal stripe will appear as a thin, echogenic line between the anterior abdominal wall and underlying organs or peritoneal fluid. With disruption by air, the interface between the gas and underlying soft tissue scatters sound waves to result in an “enhanced” peritoneal stripe. 11,12 (Fig. 1)

(b) Reverberation artifacts: Large amounts of free air can produce echogenic reflections known as reverberation artifacts. These appear similar to the “A-lines” seen by ultrasound in normal lungs. (Fig. 2)

(c) Comet tail artifacts: Comet tails are essentially several echogenic reverberations stacked together to resemble a streak similar in appearance to a comet’s tail.

(d) Air bubbles in ascites: Often, free air is found with free fluid in the abdomen. Air bubbles can be seen clearly as hyperechoic, mobile “bubbles” floating in dark pockets of ascites. (Fig. 3)

2 SCAN FAST EXAM

1. Epigastric region with the patient in supine position (or with head of bed slightly elevated) 2. Right hypochondrium with the patient in lateral decubitus position for at least 2 minutes

Fig. 2: Reverberation Artifact

Pitfalls There are several pitfalls to remember. The most frequently encountered one is confusing air in the bowel (Fig. 4) with free air in the abdomen. The presence of peristalsis can help differentiate the two. Another way to differentiate both is by scanning in the right hypochondrium, where the solid liver will prevent bowel loops from causing confusion. While scanning in the right upper quadrant, take heed to not confuse normal reverberation artifacts in the lung with that caused by IFA. Normal lung artifacts will move with respiration and will be located above the peritoneal line. 9

POCUS is commonly used in patients presenting with acute abdomi

Pneumoperitoneum Point-ofCare Ultrasound: Techniques

Purpose: to detect free air in the abdomen

Probe Selection: Linear > curvilinear

Scissor Technique: Pressure on the caudal probe will dissipate the reverberation artifacts. Release pressure and the artifacts reappear.

2 Scan FAST Exam: � (1) RUQ/epigastrium in supine position, HOB 10-20 degrees (2) Left lateral decubitus

2 Scan FAST Exam

Fig. 4: Pitfall

nal pain, especially if renal stones, gallstones or AAA is suspected. However, it is not utilized enough to diagnose IFA. Expanding our POCUS skills to include this diagnosis may prove invaluable in saving patients’ lives. Familiarity with the simple technique, signs and pitfalls is a bold move towards this. ■

1. Tanner, Tiffany Nicole, et al. “Pneumoperitoneum.” Surgical Clinics of North America, vol. 98, no. 5, 2018, pp. 915–932., doi:10.1016/j. suc. 2. Nazerian, P, et al. “Accuracy of

Ultrasonography for the Diagnosis of Pneumoperitoneum.” Critical Ultrasound Journal, vol. 7, no. S1, 2015, doi:10.1186/2036-7902-7- s1-a14. 3. Williams, N. M., and D. F. Watkin.

“Spontaneous Pneumoperitoneum and Other Nonsurgical Causes of Intraperitoneal Free Gas.” Postgraduate Medical Journal, vol. 73, no. 863, Jan. 1997, pp. 531–537., doi:10.1136/pgmj.73.863.531. 4. Braccini, G., et al. “Ultrasound versus Plain Film in the Detection of Pneumoperitoneum.” Abdominal Imaging, vol. 21, no. 5, 1996, pp. 404–412., doi:10.1007/s002619900092. 5. Hokama, Akira, et al. “The

Falciform Ligament Sign of Pneumoperitoneum.” Journal of Emergencies, Trauma, and Shock, Medknow Publications, July 2011, www.ncbi.nlm.nih.gov/pmc/ articles/PMC3162727 6. Borofsky, Samuel, et al. “The

Emergency Room Diagnosis of Gastrointestinal Tract Perforation: the Role of CT.” Emergency Radiology, vol. 22, no. 3, 2014, pp. 315–327., doi:10.1007/s10140-014- 1283-4. 7. Chen, S.-C., et al. “Ultrasonography Is Superior to Plain Radiography in the Diagnosis of Pneumoperitoneum.” British Journal of Surgery, vol. 89, no. 3, 2002, pp. 351–354., doi:10.1046/j.0007- 1323.2001.02013.x. 8. Grassi, Roberto, et al. “Gastro

Duodenal Perforations: Conventional Plain Film, US and CT Findings in 166 Consecutive Patients.” European Journal of Radiology, vol. 50, no. 1, 2004, pp. 30–36., doi:10.1016/j. ejrad.2003.11.012. 9. Hefny, Ashraf, and Fikri Abu

Zidan. “Sonographic diagnosis of intraperitoneal free air.” Journal of Emergencies, Trauma, and Shock, vol. 4, no. 4, 2011, p. 511. 10. Karahan, Okkes Ibrahim, et al.

“New Method for the Detection of Intraperitoneal Free Air by Sonography: Scissors Maneuver.” Journal of Clinical Ultrasound, vol. 32, no. 8, 2004, pp. 381–385., doi:10.1002/jcu.20055. 11. 1Asrani, Ashwin. “Sonographic

Diagnosis of Pneumoperitoneum Using the ‘Enhancement of the Peritoneal Stripe Sign.’ A Prospective Study.” Emergency Radiology, vol. 14, no. 1, 2007, pp. 29–39., doi:10.1007/s10140-007- 0583-3. 12. Indiran, Venkatraman, et al.

“Enhanced Peritoneal Stripe Sign.” Abdominal Radiology, vol. 43, no. 12, 2018, pp. 3518–3519., doi:10.1007/ s00261-018-1628-7.

Daunting Diagnosis: A

By Karen Estrine, DO, FACEP, FAAEM Editor-in-Chief

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This female was diagnosed with a malignant nerve sheath tumor encroaching the brachial plexus.

The CT scan demonstrates a large, firm mass, approximately 16cm x 10cm over the upper left aspect of the chest, located immediately below the clavicle.

The MRI is of the left brachial plexus. It shows an enhancing and centrally necrotic mass at the left upper chest/ axilla. There is surrounding edema.

The patient was seen by orthopedic surgery. After discussion at a sarcoma tumor board, she underwent surgical intervention with a forequarter amputation. It was recommended that the patient undergo adjuvant chemotherapy after surgery. The patient was amenable to the plan due to lack of arm function and the high risk of metastatic spread without having operative intervention. ■

MRI of left brachial plexus