Lawrence, Tara L. MS, PA-C*; Franklin, Jeremy A. MD†
Aperinephric abscess is an uncommon occurrence in the pediatric age range.1,2 The widespread use of antibiotics has changed the predominant mode of abscess formation and the most common etiologic organisms. Diagnosis of perinephric abscesses can be problematic and is often delayed because of the generally vague clinical features and the relatively limited specificity of routine laboratory testing. Clinicians need to recognize the underlying conditions that place a patient at increased risk for developing a perinephric abscess and maintain a high index of suspicion with any urinary tract infection (UTI) that fails to respond quickly to appropriate antibiotic therapy. Imaging studies need to be obtained promptly to expedite diagnosis and treatment in an effort to decrease the high rate of morbidity and mortality associated with this infection. Percutaneous abscess drainage with adjuvant intravenous antibiotics is the recommended treatment. Pediatric patients are especially at risk for significant renal damage as a result of a UTI complicated by a perinephric abscess.
A 2-year-old previously healthy girl presented to the university clinic with persistent fever, increasing fatigue, and swelling of the left flank. Approximately 3 weeks prior, she had been seen by an emergency department physician for fever, malaise, cough, and rhinorrhea. She was treated for an upper respiratory tract infection with a 10-day course of amoxicillin. Despite resolution of her upper respiratory tract infection symptoms, she remained febrile and fatigued; therefore, her primary care physician performed a urinalysis (UA). Based on the results of the UA, she was diagnosed with a UTI and started on nitrofurantoin.
Three days into treatment with nitrofurantoin, she continued to have fever and malaise. She also began to develop redness and swelling over her left flank. She was brought to the university clinic for further evaluation. In addition to the aforementioned symptoms, she also had a 3-day history of decreased appetite, foul-smelling urine, and left leg weakness. Further review of systems was negative. She had no history of surgery, hospitalization, or prior UTI. Family history was negative for recurrent UTIs, nephrolithiasis, and renal masses or tumors.
Physical examination revealed an alert female child in no acute distress with a temperature of 98°F, respiratory rate of 28, and pulse rate of 120. Physical findings were remarkable only for a 3-cm diameter mass that was tender to palpation over the left flank. No ecchymosis, erythema, or calor of the overlying skin was noted. The mass was not detectable by palpation of the abdomen.
A computed tomography (CT) scan of the abdomen demonstrated a left-sided enhancing perinephric mass containing air-fluid levels consistent with an abscess. It measured 2.5 × 4.6 cm in diameter and was located posterior to the left kidney, displacing it anteriorly (Fig. 1). The patient was admitted to the hospital for further evaluation and initiation of treatment.
Laboratory results were significant for a white blood cell count of 15,900/μL with 22% bands, hemoglobin level of 8.6 g/dL, hematocrit of 25.3%, and platelet count of 884 × 103/μL. Urinalysis was remarkable for 2+ leukocyte esterase, greater than 100 white blood cells per high-power field, and 3+ bacteria. Therapy was initiated with intravenous ceftriaxone at a dosage of 80 mg/kg per day in combination with intravenous vancomycin at a dosage of 40 mg/kg per day. Urine culture later grew greater than 100,000 colony-forming units of Escherichia coli that was resistant to ampicillin but sensitive to cephalosporins and trimethoprim-sulfamethoxazole. Blood culture was negative.
The day after admission, the child underwent CT-guided percutaneous drainage of the abscess. Approximately 10 mL of purulent material was aspirated from the perinephric abscess and sent for culture. An automated peritoneal dialysis (APD) drain was advanced into the abscess and left in place for further drainage. The specimen from the abscess grew E. coli with an antibiogram identical to the isolate from the urine culture. Vancomycin was discontinued. Five days after placement, the APD drain was removed. A renal ultrasound (US) performed after removal of the APD drain demonstrated normal kidneys with no residual abscess. A repeat catheterized urine specimen was obtained to document sterilization of the urine. Ceftriaxone was discontinued after 7 days of treatment, and the patient was started on a 14-day course of oral cefprozil. She continued to improve clinically after the change to oral antibiotics and was discharged home 2 days later. At her follow-up appointment, the patient was afebrile and doing well. Repeat UA and urine culture were negative. Follow-up renal US was normal. A voiding cystourethrogram showed no evidence of vesicoureteral reflux.
A perinephric abscess is a collection of suppurative material within the perinephric space, bound by Gerota fascia and the renal capsule.3 In 60% to 90% of cases, this occurs via extension of a renal abscess, previously confined to the renal parenchyma, that has perforated the renal capsule and entered the perinephric space4 (Fig. 2). Other mechanisms of perinephric abscess formation include hematogenous dissemination from distant foci of infection and direct extension of intra-abdominal infections.4,5
Before widespread use of antibiotic therapy, perinephric abscesses were predominantly caused by hematogenous dissemination of Staphylococcus aureus. Currently, perinephric abscesses are most frequently seen as a result of ascending UTI, and the most common pathogens identified are gram-negative bacteria including E. coli, Proteus, Pseudomonas, Klebsiella, and Aerobacter aerogenes. Less common etiologic organisms reported in the literature include Enterococcus, Enterobacter, Serratia, Citrobacter, Streptococcus pneumoniae, Mycobacterium tuberculosis, various anaerobes, and Candida.4,6,7 Perinephric abscesses may be polymicrobial in up to 25% of cases.7
The incidence of renal and perinephric abscesses is reported to be between 1 and 10 cases per 10,000 hospital admissions.4,8 Several predisposing factors have been implicated in the pathogenesis of perinephric abscesses (Table 1). Only 6% to 16% of cases of perinephric abscess have no predisposing factors identified.2,9 Complications of perinephric abscesses can include bacteremia, sepsis, and contiguous spread of the infection. Perinephric abscesses have a mortality rate of 20% to 50%.3,4,8
Early identification of perinephric abscesses is often difficult because of the insidious onset of nonspecific symptoms. Patients may be symptomatic for several weeks before presentation. Common presenting complaints include fever (55%-89%), chills (42%-47%), flank pain (40%-80%), and nausea and vomiting (23%-42%).4,10,11 In some series, dysuria is a relatively uncommon presenting complaint (23%-39%).4,11 Other presenting symptoms and physical examination findings are included in Table 2.
Diagnosis of a perinephric abscess requires a high index of suspicion so that radiographic evaluation can be performed expeditiously. Patients with a UTI who are clinically failing therapy and those patients with a known history of urinary tract abnormalities, diabetes mellitus, or immunosuppression should be considered for prompt diagnostic imaging studies. This becomes especially important in pediatric patients because young children with UTIs often present with few recognizable signs, other than fever, but are at higher risk for developing renal damage subsequent to kidney infection.12 A recent study of perinephric abscesses found that only 35% of patients were correctly identified upon presentation, and the mean time to diagnosis after admission was 3.4 days.13 Prompt and accurate diagnosis is necessary to improve outcome.
Routine laboratory tests are nonspecific. Peripheral leukocytosis with a neutrophilic predominance is often present; however, this tends to be mild, and leukocyte counts of greater than 15,000/μL are infrequent.4 A recent study demonstrated that 8% of patients with a diagnosis of upper UTI and thrombocytosis (platelet count, >500 × 103/μL) upon admission had a perinephric abscess.14 Anemia, azotemia, and increased levels of nonspecific markers of inflammation such as the erythrocyte sedimentation rate are common findings.3,4,8,15
Urinalysis may demonstrate pyuria, proteinuria, and hematuria;8,13 however, it may be normal in up to one third of patients.3,4,8 A sterile urine culture does not exclude the diagnosis of a perinephric abscess; indeed, urine cultures may be positive in as few as 38% to 50% of cases.2,8,11 Blood cultures may be positive in 15% to 40% of cases.2,8,11
Both renal US and CT can be used to diagnosis and monitor perinephric abscesses. On US, an abscess is usually portrayed as a hypoechoic mass with thickened irregular walls.3,16 Computed tomography is the preferred diagnostic test because it is more sensitive and more accurate in defining the location and extent of involvement in relation to surrounding retroperitoneal structures.2,3,7,13
Radiographically guided percutaneous abscess drainage with adjuvant intravenous antibiotic therapy is the recommended treatment for perinephric abscesses.1,9 Initial intravenous antibiotic selection should include broad-spectrum coverage of the common gram-negative uropathogens and S. aureus. Once adequate drainage and clinical improvement have been demonstrated, the patient may be switched to oral antibiotics with appropriate coverage for the isolated pathogen. Patients should receive a minimum of 14 days of antibiotics. Clinical failure despite percutaneous drainage and intravenous antibiotic therapy may require open drainage. In severe infections with large abscesses of thick fluid, renal parenchymal damage, or loss of kidney function, nephrectomy may be indicated.3,7,15
The case presented in this article demonstrates several key issues related to perinephric abscesses: (1) the insidious onset of nonspecific protean symptoms, (2) the typical association with a UTI, (3) gram-negative uropathogens such as E. coli are frequently the etiology, (4) the characteristic findings on CT scan, (5) the use of percutaneous abscess drainage and adjuvant antibiotics as the preferred treatment regimen, and (6) the good clinical outcome that can be achieved with proper diagnosis and therapy. Of interest, this patient had an anatomically normal urinary tract, no previous history of UTI, and no other established risk factor for perinephric abscess. Very few cases of perinephric abscesses occur in pediatric patients with no risk factor. The strong association of urinary tract abnormalities with perinephric abscesses and the higher risk for renal damage from UTIs in pediatric patients necessitate close follow-up and evaluation with renal US and VCUG.
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