The most common renal causes of an apparent acute abdomen are upper urinary tract infection, especially pyelonephritis, renal colic due to a stone in the urinary tract, and acute presentation of a pelvi-ureteric junction obstruction. Children may localize the pain to the abdomen, not the loin. Urinary tract infections, usually related to vesicoureteral reflux, may cause similar symptoms to those of intussusception, mostly in young children. In most of these cases US is normal and only in high-grade reflux
(grades III-IV) pyelocaliceal dilatation may point to the right urologic diagnosis. When evident, foci of infection are often hypoechoic with focal loss of the corticomedullary differentiation (Riccabona 2002a,b). A renal abscess appears as a heterogeneous mass lesion with central necrosis. Structural abnormalities of the urinary tract that may be found in children presenting with infection include duplex systems, renal ectopia, horseshoe kidney, and renal malrotation. A VCUG is indicated in children presenting with an acute urinary tract infection, but should not be performed during acute infection. The child should remain on antibiotic prophylaxis and the VCUG should be done as a planned investigation (Pennington and Zerin 1999).
Renal lithiasis is more common in infants than in older children, and 20% of cases manifest as renal colic. Underlying causes are multiple, proteus being the most common pathogen. The role of imaging is to diagnose lithiasis, to detect any underlying anatomical abnormality, and to demonstrate the effect on the urinary tract so that treatment can be appropriate. Renal lithiasis can be detected radiographically or sonographically. The latter method may also substantiate the presence of obstructive uropathy and in many cases it may demonstrate its level and etiology. A calculus manifests as a hyperechoic area with acoustic shadowing located within the pelvicaliceal system or ureter (Fig. 1.82). In suspected obstruction by a calculus, US examination should include the kidneys, ureters, and bladder to determine the level of obstruction. On plain abdominal radiographs, calculi are seen as radiopacities in the renal areas, the line of the ureters or the bladder region. Differential diagnosis includes appendicolith, intracolic foreign body, calcified ovarian mass, or adenopa-thy. CT should not be used as a routine investigation tool due to its associated radiation dose (Eshed and Witzling 2002); however, it is useful in patients with renal colic and a negative US study, and in patients with US findings suggestive of ureteric obstruction in which US failed to demonstrate the calculus. In such cases, a non-contrast, low-dose CT scan usually allows demonstration of the stone, even a non- or poorly radiopaque stone (Meaghar et al. 2001).
Pelviureteric junction obstruction may be detected on prenatal US or on US examination of infants who have a urinary tract infection; however, a significant proportion of cases present later in childhood with abdominal pain. US demonstrates a dilated renal collecting system with no associated dilated ureter. There are multiple hypoechoic cystic spaces, the largest being medial and representing the dilated pelvis. The cysts intercommunicate and infundib-ula and calyces, as well as surrounding renal parenchyma, can usually be identified (Ward et al. 1998). In severe cases the distended renal pelvis has a classically convex contour (Fig. 1.83). The patient should be well hydrated at the time of examination. Follow-
ing a US diagnosis of pelviureteric junction obstruction, the patient should have a Mag 3 renogram after IV furosemide, to assess the degree of obstruction and the relative filtration of the obstructed kidney. The excretion curves that are generated during this examination provide useful information regarding the degree of obstruction. They also provide a useful baseline for future follow-up in patients in whom immediate surgical treatment of their obstruction is not warranted. Intravenous urograms may be useful when further anatomical details are required prior to surgery; however, this imaging modality is being substituted by MR urography if facilities are available (RiccAboNA 2002). Most patients with severe pelviureteric junction obstruction are surgically treated with a dismembered pyeloplasty, while those with mild or moderate pelviureteric junction obstruction are followed-up with periodic US.
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