HUS is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and ARF. It is secondary to a specific strain of E. coli producing a Shiga-toxin. It is the most common cause of ARF in pre-school children. Five to 7 days after the ingestion of the toxin, gastro-intestinal symptoms occur. Among them, 5–15% of patients will develop symptoms of HUS. The key pathophysiological mechanism involves dissemination of the bacterial toxin to the renal microvasculature leading to swelling and apoptosis of endothelial cells. Occlusive thrombi develop in the renal arterioles and glomerular capillaries. Between 40–70% of children with HUS will become oligo-anuric necessitating renal replacement therapy. Thrombotic microangiopathy may also occur outside the kidneys and will affect the brain (10% of patients), the heart, or the pancreas. The digestive tract is almost always involved (especially the colon). Approximately 5–10% of HUS are not related to the shiga-toxin and appear as atypical HUS (aHUS). aHUS seems to be related to genetic mutations; it is characterized by recurrent bouts and more permanent kidney failure [1, 2].

On US, in both HUS and aHUS, during the acute oligo-anuric phase, the kidneys are slightly enlarged or remain within normal limits; the cortex becomes hyperechoic with increased cortico-medullary differentiation (CMD)(Fig. 21.2a). On duplex Doppler, the diastolic flow disappears or even a reverse diastolic flow may appear (Fig. 21.2b). Noteworthy, the return of a diastolic flow is predictive of the resuming spontaneous diuresis (Fig. 21.2c).

US of the abdomen is able to demonstrate evidence for microangiopathic colitis through the thickening of the bowel wall and disappearance of the normal bowel wall stratification (Fig. 21.2d). In the appropriate clinical setting, swelling of the pancreas indicates pancreatitis and echogenic sludge in the gallbladder may indicate hemobilia (Fig. 21.2e). Cerebral MR imaging will be mandatory to evaluate the brain lesions in case of seizures [8–13].

Most cases of HUS (less for aHUS) return to normal. In some instances, renal growth will be impaired and hypertension will eventually develop [12].

Acute glomerulonephritis (AGN) refers to a disease affecting the glomerular functional unit. Inflammatory processes result in swelling of the glomerular epithelial cells. Immune deposits along the basement membrane are associated findings. AGN is classified by the type and site of deposits and according to associated underlying diseases. On US, the most usual finding is normal size kidney with increased echogenicity and “thickened” cortex. These features are non-specific. Noteworthy, on color Doppler, the renal parenchyma may appear hypervascularized.

Acute tubular necrosis (ATN) may result from tubular epithelial injury resulting from vasomotor autoregulatory or drug induced mechanisms. The findings on US will be aspecific as well: mainly hyperechoic parenchyma (Fig. 21.1); still, due to the focused tubular insults, the pyramids may become small and irregular [1, 2, 8].

Renal disease associated with vasculitis is variable and dependent on the size of the vessel involved in the inflammatory process. Large vessel involvement like in Takayasu arteritis would lead to renal artery stenosis and hypertension resulting in abnormal duplex Doppler profile. With smaller vessels involvement, the clinical presentation would more likely correspond to AGN [14].

Kawasaki disease is an acute, febrile vasculitis that predominantly develops in children less than 5 years. The disease has a striking predilection for the coronary arteries. In Kawasaki disease (KD), sterile pyuria is a common feature, occurring in 30–80% of patients and therefore a complete UTI workup including renal US, VCUG, and/or DMSA scan should be performed in such patients [14, 15].

Henoch-Schonlein purpura (HSP) is an immune mediated systemic vasculitis generally found in children. The clinical manifestations include purpura arthritis and abdominal pain. In about 50% of patients with HSP, the renal function is compromised due to nephritis resembling IgA nephritis with few US changes beside potential hyperechogenicity. Occasionally, the ureter may be affected resulting in ureteritis and ureteral obstruction [14, 16].

Polyarteritis nodosa (PN) is a necrotizing vasculitis associated with aneurysmal nodules along the walls of medium-sized muscular arteries; renal manifestations include hematuria, proteinuria, and hypertension. The kidneys appear patchy on DMSA scanning of the kidneys as well as on US. MR angiography may demonstrate renal and extrarenal aneurysms or stenosis on the medium size arteries. Angiography represents the gold standard examination for ascertaining the diagnosis [14].

Solid tumors like Wilms’ tumors very rarely induce ARF unless the tumor is bilateral and involves the vessels. Renal involvement in malignant hemopathies, especially acute lymphoid leukemia, is more likely to compromise renal function. Leukemic infiltration of the kidneys occurs in 3–5% of patients at the time of diagnosis and can represent the initial finding of the disease. It occurs in 7–42% in the later stages of ALL. ARF in patients with hematological malignancies can present a major clinical problem; it develops as a result of a direct invasion by malignant cells determining obstruction of the ureters, renal artery or renal vein thrombosis. Other more indirect causes include AGN due to immunologic reactions, sepsis, hemolysis, and antileukemic therapy.

In cases with renal infiltration, on ultrasound, the kidneys could appear massively enlarged with heterogeneous patchy appearance of the renal parenchyma (Fig. 21.3a). Sometimes real nodules will be visualized that can be misinterpreted as primary tumors or renal lymphoma [17, 18]. Whenever necessary, MR imaging should ideally be performed to evaluate the renal involvement. Still, in an emergency setting a CT (Fig. 21.3b) can be performed with hyperhydration in order to prevent complications.

Once the chemotherapy has been started, ARF may develop due to the so-called “tumor lysis syndrome” or to urosepsis in immune-compromised patients related in particular to candidiasis. The latter may determine diffuse renal involvement with urinary sludge or fungus balls [19].

Various clinical and/or biological symptoms may lead to the discovery of metabolic diseases: failure to thrive, dehydration, signs of hepatic failure, or HT. In terms of renal failure several congenital metabolic entities will feature (A)RF (progressing into CRF) in the course of the disease. Tyrosinemia, primary oxalosis, and Bartter syndrome should be highlighted.

Hepato-renal tyrosinemia is a rare autosomal recessive disorder of tyrosine metabolism. Tyrosinemia affects liver and kidney function. Hepatocellular carcinoma is a known long-term complication. On US, the association of hepatomegaly with steatosis and medullary nephrocalcinosis is suggestive of the disease (Fig. 21.4a). Hepatic nodules may develop corresponding to benign adenomas (Fig. 21.4b) [20].

Bartter syndrome is an inherited renal tubular disorder associated with hypokalemic alkalosis. In the course of the disease, ARF followed by CRF supervenes. On US, medullary nephrocalcinosis develops very rapidly during the first weeks or months after birth (Fig. 21.5) [21].