Ascites in relation with urinary tract leakage (usually post-obstruction) is a classical yet unusual finding as the urinary tract is retroperitoneal. Still, its presence within the peritoneum can be understood by leakage and rupture of the bladder (as in posterior urethral valves) or by a retrograde filling of the peritoneum through the uterus and fallopian tube when the urethra and vagina have a common opening. Furthermore, abdominal trauma may induce a communication between the retroperitoneum and the peritoneal cavity and explain the intraperitoneal presence of urinary ascites in case of trauma to the urinary tract.

Peritoneal inclusion pseudocysts (PIP) correspond to fluid located in (abdomino-) pelvic compartments delineated by intestinal adhesions that can be related to previous (sometimes neonatal) surgery, endometriosis, or pelvic inflammatory diseases in adolescents (see also Chap. 24). They may contain accumulation of the fluid resulting from ruptured follicles (Fig. 14.2). PIP do not have a proper wall. Their size may be quite large (over 8–10 cm) and it may not always be possible to differentiate PIP from real ovarian cysts. Treatment includes puncture, sclerotherapy, or surgery [8].

Intraabdominal cerebrospinal fluid (CSF) is an expected consequence of ventriculoperitoneal shunting. The fluid is usually distributed in the entire abdominal cavity in small volumes. Multiloculated, sometimes large, collections of CSF can develop when intestinal adhesions develop around the tip of the shunt catheter or around the shunt in its course within the abdominal wall (Fig. 14.3). The adhesions interfere with the normal drainage and may therefore induce uphill cerebral hypertension. These pseudocysts do not have a wall, still they may become infected and limited by a pseudo-wall representing inflammatory peripheral tissue from the omentum and bowel walls. Drainage under US control or surgery with replacement of the shunt may be necessary [9].

Peritonitis corresponds to a diffuse inflammatory process involving parts or all of the peritoneum associated with complicated infected effusion. The most classical cause is appendicular peritonitis and abscess formation following a rupture of appendicitis (Fig. 14.4) (see Chap. 11). Infectious collections or abscesses may develop after rupture of Meckel’s diverticulum or perforation of a bowel segment (Fig. 14.5). It can be a complication of any abdominal surgery. Collections/abscesses may also be associated with inflammatory bowel diseases. These infectious effusions may collect in single or multiple locations (communicating or not). Fistulae may develop between these abscesses and intestinal loops.

A meconium peritonitis is a particular peritonitis encountered in the neonatal period. It results from an in utero perforation of the bowel and spilling of the meconium in the abdominal cavity. It can organize as a meconium pseudocyst (see Chap. 5).

(Sclerosing) peritonitis is a complication of peritoneal dialysis; in such case, the peritoneum thickens and may calcify. The dialysis catheter will be visualized within it.

US is also able to detect abscesses; they appear as a relatively hypoechoic (sometimes septated) collection without evidence for peristalsis, without own wall but with peri-abscess inflammatory hypervascularized tissues (Figs. 14.4a and 14.5b). Some hyperechoic foci within the collection may represent air and anaerobic superinfection. In case of infection, the peritoneum, the mesentery, and the omentum may appear diffusely hyperechoic (especially the mesentery). Some enlarged lymph nodes can be observed as well. This infectious hyperechogenicity must be differentiated from thickened and hyperechoic mesenteric fat as encountered in inflammatory bowel disease.

The potential extent and the number of collections is difficult to define by US alone and therefore, in acute clinical presentations as well as in cases with unfavorable evolution, a CE-CT may help to evaluate the entire peritoneum (Fig. 14.4b). MR imaging is surely accurate but is less easy to perform in an emergency setting mainly due to its lack of availability.

On CT, in case of peritonitis, the peritoneal sheets would enhance after contrast enhancement. CT is more accurate than US to establish the map and extent of the collections as well as the relation with the adjacent organs. It will also help to differentiate intraperitoneal from retro- or extraperitoneal collections. In case of posterior abdominal collection, a psoas abscess is a potential differential diagnosis to consider and will be easier to demonstrate on CT (and on MR imaging) [10–13].

US and CT may be performed to guide drainage of the collections if required.

Primitive peritonitis (PP) is a rare disease usually related to pneumococcal infection. It may complicate cirrhosis, nephrotic syndrome or occur as an isolated disease. On US, the effusion appears echogenic, the bowel loops may be dilated but no obstruction will be demonstrated neither on US nor on CT or even during surgery (if performed). The diagnosis is confirmed by the culture of the effusion [14, 15].

Primary omental torsion is an extremely rare cause of abdominal pain in children. It occurs when the omentum twists around its long axis causing edema and vascular compromise. Omental torsion can be primitive or secondary. Secondary omental torsion can be associated with tumors (Fig. 14.7), adhesions or hernia (see below). Segmental infarct may be an associated undistinguishable finding.

The underlying pathogenesis is unknown. Obesity is a favoring factor. The clinical symptoms commonly mimic those of appendicitis. It is mainly diagnosed during laparotomy.

On US, a painful superficial echogenic abdominal mass is visualized, anywhere but most commonly in the right flank or RLQ (Fig. 14.8a). On CT, without contrast injection, the omental fat may appear somewhat infiltrated with areas of mixed low attenuation areas and hyperattenuating streaks. The same pattern is observed after contrast injection (Fig. 14.8b). Similar findings can be visualized on MR imaging. The latter should be preferred when a tumor is suspected (Fig. 14.7b, c).

Surgical excision is the treatment of choice, especially when an underlying tumor is suspected [20].

Epiploic appendagitis is a rarely reported cause of acute abdominal pain in children arising from inflammation of epiploic appendages that are distributed along the entire colon. Epiploic appendages may undergo torsion or thrombosis leading to ischemia and inflammation of the peripheral tissue, including the bowel wall, mesentery, and peritoneum. The most common location is the cecum, therefore, this rare diagnosis is usually misled as appendicitis. CT appears characteristic: oval to round fat attenuation lesion less than 5 cm diameter located on the antimesenteric side of the colon with surrounding inflammatory changes anterior to the colon (Fig. 14.9).

The evolution is towards spontaneous resolution [21].

At 10 weeks gestation, the bowel re-enters the abdomen. The cephalad midgut (proximal small bowel) enters first and undergoes a third 90° counterclockwise rotation leading to the normal C loop duodenal pattern. The caudal midgut (distal ileum cecum and proximal colon) enter later and undergo additional 180° counterclockwise rotation. At this stage, the position of the cecum varies. Further elongation of the colon develops throughout the remainder of the gestation and postnatally. The second, third, and fourth portions of the duodenum are fixed in the retroperitoneum. The ligament of Treitz fixes the duodeno-jejunal junction. Descending and ascending colon mesenteries fuse with the retroperitoneum. The transverse mesocolon fuses partially with the greater omentum, the sigmoid mesocolon fuses with the retroperitoneum. The small bowel is fixed by a broad mesentery extending from the DJJ in the left upper abdomen to the ileocecal valve in the RLQ. The broad base of the mesentery stabilizes its position and prevents volvulus.

Arrest of the embryologic development may occur at any phase with variable consequences, leading to the different malrotation patterns. Early failure of rotation corresponds to the pattern of non-rotation (no further than the 90° first rotation). In such cases, the small bowel is located at the right and the colon at the left. Incomplete rotation represents a failure of the 180° counterclockwise rotation of the small bowel and the 180° rotation of the colon. The result abnormality is a spectrum from complete non-rotation to almost normal rotation. The risk for volvulus will vary with the degree of mesenteric attachment which might be difficult to directly assess with imaging. In reversed rotation, the duodenum rotates clockwise instead of counterclockwise. The duodenum becomes anterior to the SMA and the colon posterior. This may result in an internal hernia (and obstruction) (see below).

Normal rotation leads to a broad mesenteric root and no risk for volvulus. In case of non-rotation, the mesenteric root is wide and the risk of volvulus is low; still Ladd’s band may cause obstruction. Incomplete rotation with the DJJ to the right of midline and a high positioned cecum are associated with a short mesenteric root predisposing to volvulus and Ladd’s bands.