Peritoneal stripe sign (pneumoperitoneum)
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The peritoneal stripe sign in abdominal ultrasonography is considered indicative of intraperitoneal free air, present in pneumoperitoneum. Free air in the abdomen will collect in an anti-dependent manner, typically the anterior prehepatic space in the supine patient, settling against the parietal peritoneum, thereby creating an unusually reflective interface. A high-amplitude linear echo results, forming the basis of the sign 1.
This sign is referred to as the "enhanced peritoneal stripe sign" by some authors. Beyond nomenclature, some heterogeneity exists regarding the necessity of the presence or absence of associated reverberation artifacts in order to invoke this sign. Of note, the long path reverberation artifacts which are concomitantly present have been noted to occur in isolation 4.
Optimal positioning of the patient relies on ensuring the anterior prehepatic space will preferentially accumulate any intraperitoneal air if present. The patient is, therefore, typically placed supine with the head of the bed elevated to 10-20 degrees (just shy of a semi-fowlers position) 1.
The probe is typically placed at the right midclavicular line at the level of the epigastrium. Probe positioning should be optimized to recognize the dynamic excursion of the lung into the anterior costophrenic recess, which is an important potential sonographic mimic, and if observed to be discrete is considered supportive of the presence of intraperitoneal free air.
A reasonable sonographic approach to evaluate for the presence of pneumoperitoneum, with a patient situated as dictated above and/or with partial right lateral decubitus positioning, will systematically interrogate the right hypochondrium and epigastrium for the characteristic appearance of intraperitoneal free air.
Intraperitoneal free air will collect beneath the parietal peritoneum and create an abnormally reflective air-soft tissue interface; the resultant incongruity in acoustic impedance results in scattering; free air tends to distribute in a smooth, linear fashion and may act as a specular reflector. The corresponding "stripe" of overlying parietal peritoneum will thus demonstrate increased echogenicity 3.
Near complete reflection of ultrasound waves at an appropriate angle of insonation may result in the artifactual recapitulation of this peritoneal stripe at staggered, equidistant intervals in the far field, akin to the "a-lines" one may observe during lung ultrasonography.
Helpful associated findings
- the location of the sign will move with the patient's position, conforming to peritoneal barriers
- the abnormally hyperechoic peritoneal stripe should be distinguished from the pleural interface, which demonstrates respirophasic movement
- pressure on the caudal aspect of the probe may abolish the collection of air in one's sonographic field 2
Linear, thin hyperechoic entities which may mimic the appearance of a peritoneal stripe sign include
- basal lung
- colonic interposition
- gas within small bowel