Radiological Findings of Pneumoperitoneum - An Overview

Introduction:

Pneumoperitoneum is a medical condition in which free gas or air is found inside the abdominal or peritoneal cavity. The peritoneum is a covering membrane divided into the parietal layer (covering the abdomen) and the visceral layer (covering the abdominal organs). The space between the parietal and visceral layer of the peritoneum is called the peritoneal cavity. The peritoneal cavity contains organs such as the stomach, liver, spleen, intestine, and peritoneal fluid (a fluid that lubricates the layers of the peritoneum).

What Are the Causes of Pneumoperitoneum?

Perforation of hollow viscus (hole or lack of continuity in the gastrointestinal tract wall causes leakage of the gastric content). The causes of bowel perforation are as follows:

In Adults:

• Peptic ulcer (ulcer or sores in the lining of the small intestine or stomach).

• Ischemic bowel (lack of blood supply to intestine due to narrowing or blockage of the blood vessels).

• Bowel (intestine) obstruction.

• Appendicitis (swelling of the appendix- a finger-shaped projection that arises from the beginning of the large intestine).

• Typhoid fever.

• Tuberculosis.

• Diverticulitis (swelling of the diverticulum - small pouch arising from the digestive tract lining).

• Malignancy (cancer).

• Inflammatory bowel disease (a disease that causes painful swelling of the intestine).

In Children:

• Necrotizing enterocolitis (a disease that causes swelling of the intestine in premature babies).

• Hirschsprung's disease (congenital disease affecting the large intestine that causes difficulty passing stool).

• Meconium ileus (obstruction of the small intestine by meconium [thick and sticky] like stool).

• Mechanical perforation - Trauma, colonoscopy (an imaging technique of the colon of the large intestine), foreign bodies.

• Postoperative free intraperitoneal gas (collection of free air in the peritoneum after surgical procedures) - This usually resolves in three to six days after surgery; sometimes, it takes up to 24 days.

• Pneumothorax (collection of air in the space between the lungs and chest wall).

• Peritoneal dialysis (a medical procedure used to remove waste products from the blood).

• Pneumomediastinum (collection of air in the mediastinal space).

What Are the Imaging Techniques Used in the Diagnosis of Pneumoperitoneum?

Pneumoperitoneum can be diagnosed with the help of the following imaging techniques. They are:

1. Chest Radiograph - The chest radiograph shows the following signs in pneumoperitoneum patients.

• Leaping Dolphin Sign: In the chest radiograph, the collection of intraperitoneal free in the bow shape causes slipping of the long coastal muscles of the diaphragm resembling a leaping (jumping up) dolphin. Hence, this sign is called leaping dolphin or diaphragm muscle slip. It is commonly seen in the right upper quadrant and rarely in the left upper quadrant. This is a sign of moderate to large volume pneumoperitoneum.

• Cupola Sign: In pneumoperitoneum, the free air may be present below the central tendon of the diaphragm, which appears as a cupola (small dome). It is also called a saddlebag or mustache sign.

• Continuous Diaphragm Sign: In a chest radiograph, the diaphragm's central portion is usually not visible as it merges with the base of the heart. If the diaphragm is seen continuous in the midline, it indicates pneumoperitoneum.

• Subdiaphragmatic Free Gas Sign: In a chest radiograph of pneumoperitoneum patients, if the intraperitoneal free gas is found in the anterior (front) subhepatic space, it is known as the subdiaphragmatic free gas sign.

2. Supine Abdominal Radiograph - An abdominal radiograph can be taken in the following ways:

• Erect.
• Supine.
• Lateral decubitus.

In pneumoperitoneum patients, a supine abdominal radiograph shows the following signs.

• Football Sign: In a supine abdominal radiograph, the free air is moved to the anterior abdominal wall as it is a less dependent area. In the massive (large) pneumoperitoneum case, the free air outlines the entire abdominal cavity and appears as an American football-shaped (oval-shaped) hyperlucent area.

• Rigler's Sign: In a supine abdominal radiograph, if the gas present in the bowel lumen and intraperitoneal gas outlines the bowel wall, it is called Rigler's Sign, gas relief sign or double-wall sign. If the pneumoperitoneum gas is present in a large volume of more than 1000 mL, this sign appears. Sometimes a false double-wall sign can occur when the two loops are in contact. Hence the thickness of the bowel wall should be considered before the diagnosis (doubled thickness).

• Urachus Sign: In the supine abdominal radiograph, the free air of pneumoperitoneum outlines the median umbilical ligament and appears as a urachus sign. The median umbilical ligament is formed by the obliteration of urachus (remnants of the canal drain urine in the fetus). It is extended between the apex of the bladder and the umbilicus.

• Telltale Triangle Sign: The free air in the pneumoperitoneum forms a triangle shape between the abdominal wall and two bowel loops or between three loops of the bowel. Therefore, the telltale triangle sign is also called the triangle sign.

• Inverted "V" Sign: In the supine abdominal radiograph, the free gas of pneumoperitoneum forms the outline of lateral umbilical ligaments. Therefore, it is also called a lateral umbilical ligament sign.

• Silver Sign or Falciform Ligament Sign: Suppose a large amount of free gas is found in the pneumoperitoneum. In that case, it forms the outline of the falciform ligament (a ligament that connects the liver to the anterior abdominal wall), which is visible in supine abdominal radiography.

• Fissure for Ligamentum Teres Sign: In a supine abdominal radiograph, the free gas of the pneumoperitoneum forms the outline of the ligamentum teres (a cord created by the obliteration of the fetal umbilical vein).

• Hepatic Edge Sign: The hepatic edge sign appears as a cigar-shaped pocket of free air in the subhepatic region.

• Lucent Liver Sign: In the supine abdominal radiograph, the intraperitoneal (gas inside the peritoneum) gas collected anterior to the liver reduces the radiodensity of the liver, which is called the lucent liver sign.

• Morison Pouch Sign/Doge's Cap Sign: The free air in the Morison pouch appears as a triangular or semi-circular-shaped lucent area. It is usually found near the eleventh rib (inferior medial aspect).

• Periportal Free Gas Sign: In the supine abdominal radiograph, if the free air is found in the periportal area, that is called a periportal free gas sign. Periportal free gas sign is usually found in upper GI tract perforation.

3. Ultrasonography:

Ultrasound or ultrasonography is a non-invasive imaging technique. For ultrasound, patients with pneumoperitoneum are asked to lie in the supine position and sometimes in a lateral decubitus position for better visualization. Ultrasonography with the linear array transducer at a frequency of about 10 to 12 MHz (megahertz) is used to diagnose pneumoperitoneum. The free air in the pneumoperitoneum appears as a hyperechoic area. Peritoneal stripe sign in ultrasound indicates the presence of free air in the anterior perihepatic space.

4. Computed Tomography (CT Scan):

CT scan helps detect the extraluminal (outside the lumen) gas irrespective of the technique and the patient's position. CT scan in some instances of bowel obstruction can help localize the zone of transition and thereby locate the perforation site. Computed tomography (CT scan) helps detect the presence of inflammatory liquid within the peritoneum in perforation. However, CT cannot localize the perforation site accurately. In addition, in the supine position, gas located in the anterior aspect is difficult for CT to differentiate from gas within the bowel.

Conclusion:

CT is considered a standard imaging technique for the diagnosis of pneumoperitoneum. However, a supine abdominal radiograph can produce accurate images closer to the pneumoperitoneum CT images. Abdominal radiography with proper technique can reduce the radiation exposure and expense of the patient. Ultrasonography is considered a first-line imaging technique in emergent patients. It has about 85 percent sensitivity and 100 percent specificity in diagnosing pneumoperitoneum.