Although acute pyelonephritis is a common infection in children, the primary development of a renal abscess or progression of pyelonephritis to a renal or perinephric abscess is an uncommon occurrence. In one study conducted over a 10-year period, eight children with a renal abscess were found among 43,224 discharge diagnoses—approximately one case per 5400 pediatric admissions. Six of the eight children were 11 years or older. In a study from a children’s hospital in Taiwan, 45 children were found to have a renal abscess over a 10-year period. In a third study from the Children’s Medical Center of Dallas, 36 children were identified with a renal or perinephric abscess between 2000 and 2010. The median age of the patients in this report was 9.3 years (range, 0.3–17 years). Although rare, renal abscesses have been reported in neonates. Renal abscess may be a primary problem (i.e., one that develops in a kidney without an antecedent infection or underlying anatomic abnormality), or it may occur secondarily in a patient with previously recognized acute pyelonephritis or in a child with congenital urologic abnormalities known to predispose to the development of pyelonephritis.
A primary renal abscess is thought to develop most often after an episode of bacteremia and frequently occurs in younger children. Hematogenous spread of bacteria to the kidney usually results in a cortical abscess. The most common organisms involved in these abscesses are gram-positive cocci, primarily Staphylococcus aureus and less often Streptococcus spp. In some cases, a cutaneous infection might have been present before development of the renal abscess, and this infection is thought to be the primary source for the bacteremia. Most children in whom a renal abscess develops hematogenously are normal hosts. Bartonella henselae has been reported to cause microabscesses in the kidney, presumably by the hematogenous route.
In older children and teenagers, tuberculous abscesses and caseous necrosis of the renal parenchyma also should be included in this primary classification. These infections tend to be indolent, although renal tuberculosis may be complicated by bacterial infection because of associated ureteral strictures and severe tuberculous cystitis.
When a renal abscess occurs in association with a recognized urologic disorder, the organism responsible most often is a gram-negative bacillus or an enterococcus, bacteria usually seen in simple urinary tract infections (UTIs) and pyelonephritis. In the Dallas study, E. coli alone was isolated in 44% and in combination with other organisms in another 6% of the 36 patients. Enterobacter aerogenes , Proteus mirabilis , Citrobacter spp. and Klebsiella spp. together accounted for about 10% of the isolates in this study. No organism was identified in 22% of patients. Examples of urologic disorders that one might encounter with these infections include congenital and acquired obstructions (e.g., ureteropelvic and ureterovesical obstruction, retrocaval ureter, ureteral stricture after surgical intervention), calculous disease (obstructing and nonobstructing), infundibular stenosis, and renal dysplasia with cystic changes. Abscesses that occur as a result of infection of the urinary tract generally are found in a corticomedullary location.
Anaerobic organisms also have been implicated as a cause of renal abscess. They often are present simultaneously with the more usual aerobic bacteria, but they can cause infections and abscesses alone. These anaerobic renal infections develop most commonly in association with infections complicating bowel injury or surgery, renal transplantation, malignancy, and orodental infections. Bacteroides fragilis is likely to arise from an intraabdominal source, whereas an oral site is more common for Prevotella oralis .
Children with human immunodeficiency virus infection seem to have an increased risk for development of renal abscesses from the more common traditional organisms and from unusual opportunistic fungal organisms, especially Aspergillus. As expected, these children also tend to have a more fulminant course that often requires extensive surgical intervention and drainage.
The presence of a renal abscess implies the destruction and liquefaction of tissue in a confined space. Two other infectious disorders of the kidney, xanthogranulomatous pyelonephritis and acute lobar nephropathy (acute focal bacterial nephritis), frequently are included in this general category, although technically true abscesses do not always develop in these disorders. Acute lobar nephropathy may progress to renal abscess, however, if not treated appropriately.
Xanthogranulomatous pyelonephritis describes a more chronic form of severe renal parenchymal destruction that often is associated with chronic stone disease. The process may involve the whole kidney or may be focal. In children, the focal form occurs more commonly. The pathognomonic histologic finding is an accumulation of lipid-laden macrophages that coalesce into discrete yellow nodules. Small abscess cavities often are studded throughout the kidney. The organism most frequently recovered from the kidney is Proteus, and urinary calculi are common findings. Although these patients often have acute symptoms, the symptoms frequently are superimposed on more chronic manifestations, such as weight loss, failure to thrive, and anemia. Treatment is complete or partial nephrectomy because the renal destruction generally is severe.
Acute lobar nephropathy describes a focal area of intense edema at the site of infection in acute pyelonephritis. It usually is recognized as a mass effect on an initial screening renal ultrasonogram. Severe nephromegaly (renal length >3 standard deviations above the mean for age) is another finding on renal ultrasound suggestive of acute lobar nephropathy. Computed tomography (CT) of the kidney shows poor uptake in the involved segment but no well-defined liquefaction ( Fig. 40.1 ). Whether this edema is just an exaggerated response to infection or represents a change before an abscess is unknown. Klar and colleagues described 13 children, 4 months to 8 years of age, with acute lobar nephropathy in a prospective study during a 4-year period. Bacteremia was documented in only one child. Evolution to abscess formation occurred in four (31%). In another prospective study, Cheng and associates found bacteremia occurred in four of 80 children (5%) with acute lobar nephropathy. In further studies these investigators noted that patients with acute lobar nephronia who had CT findings with heterogeneously decreased nephrographic density after contrast enhancement were more likely to develop a renal abscess than those with a homogeneous decrease in density. These lesions generally heal satisfactorily with administration of antibiotics alone.
Clinical Findings
Children with acute renal or perirenal abscess typically are febrile and have localized pain in the costovertebral location. Prolonged fever is a common finding, especially in older children. Most patients are febrile for longer than 7 days before the diagnosis is established. A clue to the diagnosis is persistence of symptoms and fever in a child being treated appropriately for an acute UTI. The initial findings generally do not differentiate, however, between acute pyelonephritis with and without an abscess. If the abscess has spread into the perinephric region, it may be possible to recognize psoas muscle irritation in the patient—that is, the patient is more comfortable with the ipsilateral leg in a position of flexion and experiences pain with full extension. A large abscess may be palpable as a flank mass.
Findings on urinalysis can be confusing. With a primary (hematogenous) abscess, the urine may be deceptively benign, and culture generally is negative. Blood cultures may be positive, depending on the duration of the illness when blood is obtained for culture. Abscesses associated with underlying urologic disorders can be expected to contain organisms and pyuria. Generally, severe leukocytosis is present. The erythrocyte sedimentation rate and C-reactive protein typically are elevated. All of these findings are nonspecific, however, and often further studies are needed to establish the diagnosis. If a more indolent process is encountered, appropriate testing and cultures for tuberculosis should be considered.
Diagnostic Evaluation
All children admitted to the hospital with a febrile UTI should undergo renal ultrasonography as soon as is reasonable after admission. The findings on this initial study can have a significant effect on the choice of therapeutic options. If both kidneys are normal and unobstructed, organism-specific therapy is satisfactory in most patients. If significant obstruction or stones are present, antibiotic therapy may not be as effective, and interventional drainage may become necessary if the response to antibiotics is inadequate. At the time of this initial screening study, findings may suggest the presence of a renal abscess. Such findings include a mass effect within the margins of the kidney, along with a thickened wall and material of varying sonographic density within the mass ( Fig. 40.2 ). Similar findings can be seen in infections accompanied by severe ureteropelvic junction obstruction or infundibular stenosis with isolated calyceal dilation, in which case purulent material within the dilated collecting system layers out and can mimic a renal abscess ( Figs. 40.3 and 40.4 ). Nephromegaly on ultrasonography also has been found to be a clue to recognizing a renal abscess with subsequent CT imaging studies.
