Aorta

In contrast to adults, pathology of the abdominal aorta in children is rare. There are only a handful of reported cases of congenital anomalies of the abdominal aorta, consisting mainly of atresia and duplication. Other reported pathologies of the abdominal aorta include middle aortic syndrome ( Fig. 8.3 and Fig. 8.4 ), Takayasu arteritis, thrombosis ( Fig. 8.5 ), and aneurysms. Of special interest are the long-term survivors of abdominal neuroblastoma. There have been reports of midaortic syndrome in these children, both in those who have and in those who have not had radiation therapy.

In all of these rare cases, additional imaging with computed tomography (CT) and/or magnetic resonance (MR) imaging is mandatory.

Inferior Vena Cava

Congenital anomalies of the inferior vena cava (IVC) are in general an incidental finding and do not warrant specific follow-up. Because of the complex embryology of the IVC (which comprises four different segments: hepatic, suprarenal, renal, and infrarenal), multiple variants can be encountered. The most common variant is absence of the hepatic segment of the IVC, leading to the azygos continuation ( Fig. 8.6 ; Video 8.6). Other variants include left-sided IVC, double IVC, and anomalous/multiple origins of the veins draining into the IVC ( Fig. 8.7 and Fig. 8.8 ).

Knowledge of the normal variants of the IVC is important in differentiating between left-sided para-aortic masses and an aberrant IVC and, in cases of percutaneous venous intervention, in IVC stent placement.

Thrombosis/occlusion of the IVC can be encountered in children with central lines and in oncology patients (especially children with Wilms tumor; Fig. 8.9 ; Video 8.9 and Fig. 8.10 ). Ultrasound (US) is in these cases the imaging method of choice. The optimal way to visualize the IVC is from the front. However, in some children this will yield a suboptimal view because of such factors as body shape and gas-distended bowels. In such cases, a lateral approach to the IVC can be used, in which the retroperitoneum is used as a window.

Mesenteric Lymphadenopathy

The mesenteric lymph nodes in children are easily visualized, and to the inexperienced radiologist they can look like a pathologic finding. The normal short-axis diameter of the mesenteric lymph nodes is up to 8 mm ( Table 8.2 ). Mesenteric lymphadenopathy is commonly seen in mesenteric adenitis, which is most often caused by a viral infection. However, enlarged, ovoid lymph nodes can also be seen in a significant proportion of otherwise healthy children ( Fig. 8.11 and Fig. 8.12 ). The sole finding of such lymph nodes should not lead to a further work-up. If, however, the lymph nodes show a change in texture, with loss of the hilum and a round, hypoechoic appearance, further analysis is mandatory, either by additional work-up, including cross-sectional imaging, or by biopsy ( Fig. 8.13 ). In patients with large and bulky lymph nodes, further work-up is mandatory. In many cases, a percutaneous or a surgical biopsy must be performed.

Retroperitoneal Lymphadenopathy

Unlike mesenteric lymphadenopathy, enlargement of the retroperitoneal lymph nodes should always be regarded as abnormal ( Fig. 8.14 ). Therefore, this finding warrants further follow-up. In children, the most common causes of retroperitoneal lymphadenopathy are malignancies, such as Wilms tumor, neuroblastoma, rhabdomyosarcoma, and malignant lymphoma; however, infectious diseases can also cause lymph node enlargement.

On US, lymphadenopathy can be obscured by gastrointestinal gas or, in an increasingly large group of children, obesity. In children with large abdominal tumors, the retroperitoneum can be difficult to visualize. In these children, disease staging with additional imaging (e.g., MR imaging) is mandatory.

Ascites

Ascites is the accumulation of free fluid in the peritoneal cavity. In healthy girls, a small amount of fluid in the Douglas pouch is a normal physiologic finding ( Fig. 8.15 ). In all other cases, ascites should be regarded as a pathologic finding.

The causes of ascites can be differentiated according to whether a transudate, with a low protein content, or an exudate is present. The most common cause of a transudate is portal hypertension, and therefore a transudate is seen in children with cirrhosis and heart failure ( Fig. 8.16 ; Video 8.16). A transudate can also be seen in severe malnutrition (kwashiorkor) and nephrotic syndrome. An exudate is seen primarily in peritoneal metastatic disease and infection ( Fig. 8.17 , Fig. 8.18 , Fig. 8.19 , Fig. 8.20 ; Videos 8.19 and 8.20). There is, however, a large differential diagnosis for the presence of ascites, which is different for neonates ( Table 8.3 ) and children ( Table 8.4 ; Fig. 8.21 and Fig. 8.22 ).