Renal abscess is a rare infectious disease, and the majority of existing reports are either case reports or single-center cohort studies; as such, the overall prevalence of renal abscesses remains clear. In one study in Chang Gung Hospital in Taiwan, 17 children over 8 years of age were diagnosed with renal abscesses, with an average age of 6.1 years and a male-to-female ratio of 7:10 (3). Scholars in the United States retrospectively analyzed 36 children diagnosed with renal abscess over the course of 10 years, finding a median age of 9.3 years and a male-to-female ratio of 13:23 (4). The present study also found that the incidence of renal abscesses was slightly higher in female children than in male children. The male-to-female ratio of children with renal abscesses in our center was 8:12, which was similar to the above study; however, the median age of the children at diagnosis was 3.5 years, which was relatively young compared these prior studies.
Owing to the atypical clinical symptoms, it is difficult to diagnose renal abscesses during the early stages of the disease. One 10-year retrospective study in China which included 98 adult patients with renal abscesses found that lower back pain (76.5%) and fever (53.1%) were the most common clinical manifestations (5). However, in contrast to adults, children with renal abscesses more commonly present with fever, lower back pain, and urinary tract irritation symptoms (1, 3, 4). Fever is the most common clinical manifestation in children with renal abscesses. Some children may further present with abdominal pain, vomiting, loss of appetite, and other symptoms. Only 5 children had obvious urinary tract irritation symptoms, while 3 experienced renal percussion pain on physical examination. It has been suggested that the symptoms and physical examination of children with renal abscesses lack specificity, particularly in infants with fever of unknown origin, to avoid missed diagnoses and misdiagnoses.
The data from this group showed that all children with renal abscesses had elevated white blood cell count, C-reactive protein, and erythrocyte sedimentation rate, while only 10 children (50%) had normal procalcitonin levels, which was consistent with most previous reports (1, 2, 4, 6). Accordingly, it is our viewpoint that regular blood examinations, levels of C-reactive protein, and the erythrocyte sedimentation rate are viable as markers of inflammation for the identification and assessment of kidney abscesses. On the other hand, we do not advocate the utilization of procalcitonin in the diagnosis and treatment of these conditions.
Several previous studies at home and abroad have shown that children with renal abscesses have a very low positivity rate on blood culture, suggesting that hematogenous dissemination may not be a primary pathology of renal abscesses in children (4, 7–9). Among the children with renal abscesses in our center, 18 (90%) had different degrees of pyuria, 8 (40%) had a positive urine culture, 12 (60%) had a positive urine mNGS, and only one had a positive blood culture, which was similar to the above study. At the same time, all male children in this study had phimosis, including 2 (25%) with redness and swelling of the urinary tract. Further, six (50%) of the female patients had increased secretions from the vulva, suggesting that an ascending infection is the main cause of renal abscesses in children.
The pathogenic bacteria of renal abscesses also differ in various studies, and the most common pathogens isolated from children were Escherichia coli and Staphylococcus aureus (10, 11). In adults, the most common organisms identified in culture are Escherichia coli and Klebsiella pneumoniae (12). Anaerobic microorganisms are significantly implicated in the development of renal abscesses among pediatric patients (13). Other studies have found that urinary tract fungal infections are common in premature infants, particulalry in low-birthweight infants, leading to renal abscesses and recurrent fungal infections (14). The pathogenic bacteria identified in children with renal abscesses in our center were different from those in the above studies, with Enterococcus faecium ranking first, followed by Pseudomonas aeruginosa, and Klebsiella pneumoniae. Combined with the data of this group, six children had a history of recurrent urinary tract infection, nine had CAKUT, and nine had a history of urinary tract infection. There were 6 cases of primary vesicoureteral reflux (2 cases of grade 3, 3 cases of grade 4, and 1 case of grade 5), 1 case of hydronephrosis, 1 case of neurogenic bladder, and 1 case of posterior urethral valve reflux. Children with CAKUT are prone to recurrent urinary tract infections due to congenital urinary tract structural abnormalities; as such, antibiotics are more widely used, and the duration of anti-infection treatment is longer. However, the unreasonable application of antibiotics in clinical practice can easily lead to changes in the composition of pathogenic bacteria (15), and the proportion of other non-Escherichia coli pathogens causing urinary tract infection has been shown to increase in such situation (16, 17). One Japanese study showed that children with vesicoureteral reflux were more likely to develop renal abscess (18). Therefore, among children with CAKUT, particulalry those with vesicoureteral reflux, who present with unexplained fever, clinicians should be alert to the possibility of renal abscesses.
Owing to the lack of specificity of clinical symptoms, signs, and laboratory tests, it diagnosis of renal abscess in the early clinical stages is difficult, and a delay in diagnosis may lead to increased morbidity and potential mortality. As such, it is essential to perform imaging examinations in order to diagnose renal abscesses in children. In terms of imaging examination, renal ultrasound has always been the preferred examination method for children with renal abscesses due to its non-invasiveness, low cost, and low cooperation requirements (10, 19). Some scholars have found that CT is not superior to ultrasound for the diagnosis of renal abscesses (1). However, before the formation of abscess cavities in microabscesses, or abscesses in the early stages of the disease, diagnosis may be missed because of the low resolution of ultrasound examination, whereas CT has a higher sensitivity and accuracy for such lesions (10, 20). In addition, some scholars have suggested that even if a renal abscess is found on ultrasound, CT should be performed to further confirm the diagnosis (21). In our study, all patients underwent both ultrasound and CT examinations, but only nine patients (45%) with renal abscesses were diagnosed using ultrasound. Renal abscesses were identified on MRI in all 14 cases. Based on our results, we recommend performing ultrasonography as a screening and follow-up examination, while CT or MRI can be used to confirm the diagnosis of renal abscesses in children.
The treatment of renal abscesses includes broad-spectrum intravenous antibiotics and percutaneous or open surgical drainage. Most prior studies have classified renal abscesses based on size, with a cutoff diameter of 3 cm, suggesting that renal abscesses larger than 3 cm can be treated with percutaneous abscess puncture or open surgery combined with anti-infective treatment, whereas renal abscesses smaller than 3 cm can be treated with conservative anti-infective treatment (3). However, some researchers have alternatiely proposed that puncture drainage or surgical treatment may not be superior to conservative anti-infective treatment for renal abscesses with a diameter greater than 3 cm (4). The average diameter of renal abscesses imaged in this study was 24.8 ± 7.3 mm. Five patients with renal abscesses larger than 3 cm were treated with conservative treatment, comprising intravenous infusion of antibiotics for at least 2 weeks, followed by sequential oral anti-infection treatment for at least 2 weeks, with a mean total treatment course of 47.5 days. Most renal abscesses disappeared within 2 months, and the prognosis was good. The initial anti-infective therapy at our center was mainly based on third-generation cephalosporins. However, we observed that the pathogens were mostly multidrug-resistant, and often had poor initial treatment effects. In our study, eleven patients (55%) changed their medication to carbapenem antibiotics, such as meropenem or imipenem. Considering the severity of renal injury, de-escalation treatment is the first choice for children with renal abscesses. Treatment can be initiated with broad-spectrum antibiotics, with subsequent reductions in antibiotic levels based on the therapeutic response and susceptibility testing. As most children in this group had been treated with broad-spectrum antibiotics before hospitalization, the positive rate of urine culture was only 40%, whereas the positive rate of urine mNGS was as high as 92.3%. Among the eight patients with positive urine culture results, the results of urine mNGS were consistent with those of the urine culture. Therefore, we believe that for children with renal abscesses who have poor treatment outcomes and negative blood and urine culture results, urine mNGS can be applied to help adjust the anti-infection treatment plan.
We recognize that there are some limitations to this study. Including it is a single center retrospective study, although reported the largest number of cases of pediatric renal abscess in China, the sample size is still limited. In addition, we only described the duration of anti-infective therapy in this study, and there is a lack of prospective, multi-center, large-sample studies to verify the standard duration of antibacterial therapy for renal abscess in children.
Renal abscess is a rare infectious disease in children, but should be considered in pediatric patients who present with unexplained fever, vomiting, abdominal pain, elevated white blood cell count, C-reactive protein level, erythrocyte sedimentation rate, pyuria, and renal abscesses. Ascending infection may be the primary cause of renal abscesses in children; however, the pathogenic bacteria causing renal abscesses in children differ, particularly in children with CAKUT, and attention should be paid to the possibility of atypical pathogenic bacteria. Ultrasonography can be used for screening and follow-up examinations, and CT or MRI can be used to confirm the diagnosis of renal abscesses in children. Conservative broad-spectrum antibiotics are effective in the treatment of renal abscesses in children, and could be applied as the first choice of treatment.