The Rigler sign, or double-wall sign, is an indication of free air enclosed within the peritoneal cavity (pneumoperitoneum), imprinting a visible pattern on a plain radiographic image of the abdomen, in supine technique. This sign presents because of the separation between free air and intraluminal by the intestinal wall, marking the air radiolucency and radiopacity of the wall, then both serosal and luminal surfaces of bowel are visible.
American radiologist Leo G. Rigler (1896-1979) in 1942 described this sign of pneumoperitoneum. He described the pattern in four cases report in 1941. He observed this sign was present when large quantities of free air are in the peritoneal cavity.
In upright chest radiography or abdominal radiography, pneumoperitoneum is detectable under special conditions in small quantities as little as 1mL. CT scan has greater sensitivity and specificity than conventional radiographic technique is more available and cheaper. Because a large percentage of patients who require abdominal radiography for suspected pneumoperitoneum are ill and unable to stand or sit, the plain supine projection becomes an option. This modality has lower diagnostic accuracy for pneumoperitoneum (56%) compared with other projections (left lateral decubitus 96%, chest 85%, upright 60%). Other studies sustain that radiography can detect pneumoperitoneum only in 69% to 89% of cases with visceral perforation. Sensitivity and specificity for detection of pneumoperitoneum by abdominal radiography are low compared with CT scan. Dissemination of modern CT scan technology has made abdominal radiography less common in most developed health systems, and conventional abdominal radiology is omitted in the workup of most adult patients with acute abdominal pain.
Rigler sign is the second most common sign of pneumoperitoneum, the first being the right upper quadrant subdiaphragmatic free air. The incidence of Rigler sign has been reported in study cases with a lesser prevalence then right upper quadrant subdiaphragmatic free air (46% vs. 32%). For its presence, a large quantity of free air must be around 1000mL for clear recognition. Pseudo-Rigler Sign occurs when two loops of gas-distended bowel make contact, enhancing the intestinal wall radiopacity image against the air-filled loops, misleading the diagnosis of pneumoperitoneum. If Rigler sign is believed to be present, then abdomen/pelvis CT scanning and surgical consultation are warranted in the proper clinical context.
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