Characterization and Etiopathogenic Approach of Pediatric Renal Biopsy Patients in a Colombian Medical Center from 2007-2017

Introduction Renal biopsy is the principal instrument to evaluate the diagnosis and prognosis of children with kidney disease. There are relatively few studies establishing epidemiology of its findings in the pediatric population. Methods A descriptive study was conducted to describe characteristics of pediatric patients who had undergone a renal biopsy over the last 10 years in a national reference center, trying to accomplish an etiopathogenic approach of biopsy findings. Results 241 patients were included. Most frequent indications were nephrotic syndrome (34.1%) and systemic disease with renal involvement (30.2%). The most prevalent biopsy diagnosis was glomerulonephritis (44%) and among these patients, glomerulonephritis mediated by immune complexes was the most frequent pathogenic type (90.5%). When the biopsy was indicated for proteinuria plus hematuria and systemic disease with renal involvement, the most frequent biopsy diagnosis was glomerulonephritis (60 and 85%, respectively). For isolated hematuria, the predominant biopsy diagnosis was inherited diseases of the glomerular basement membrane (70%) and for nephrotic syndrome, podocytopathy (82%). Glomerulonephritis was more frequent in patients older than 10 yrs (65%) and the rate of postbiopsy major complications was low (1.2%). Conclusion Immune complex glomerulonephritis was the most frequent histological finding, differing from previous reports. To our knowledge this is the first description that classifies biopsy findings according to the probable pathogenic mechanism.


Introduction
Percutaneous renal biopsy allows direct study of renal pathology [1]. Although many pediatric patients with kidney diseases can be properly diagnosed without it, in some situations biopsies are not only a diagnostic resource. Findings also determine prognosis and guide treatments [2,3]. In order to establish an adequate diagnostic and therapeutic approach to kidney disease based on biopsy histological findings, it is essential to recognize the pathogenic mechanisms involved, improving the understanding of kidney disease [2]. Mayo Clinic 2015 Nephrologists and Renal Pathologists Consensus is an example of a diagnostic and classification system of glomerulonephritis based on etiology and pathogenesis [4].
Histological findings were classified as detailed in Table 1. Biopsy complications were classified as follows: (1) major: if they involved additional interventions or outcomes (resuscitation, transfusion of blood components, surgery, prolonged hospital stay, readmission, death, infection, or macroscopic hematuria); (2) minors: if they did not involve additional interventions or outcomes.
Patients were admitted on the previous biopsy day and underwent a clinical evaluation including: clinical history, physical examination, and complementary bloodwork (complete blood count, partial thromboplastin time, and prothrombin time). Postbiopsy evaluation was performed as follows: (1) Complete blood count 6 hours after the intervention or earlier if required (hemodynamic instability, macroscopic hematuria, etc.). (2) Hourly vital signs and macroscopic hematuria surveillance over the first 6 to 12 hours. (3) Kidney ultrasound taken after 24 hours.
All the biopsies were ultrasound guided and performed by interventional radiologists under general anesthesia. An automatic percutaneous biopsy instrument and 18G needles were used for all patients regardless of their stature.
Samples were studied under light microscopy, immunofluorescence, and electron microscopy.

Statistical Analysis.
Description of demographics, clinical findings, and relevant variables related to the histopathological findings, through relative frequency measurements. Comparative tables and graphs with frequency distribution.
All analyses were performed using STATA version 9 software (StataCorp LP, College Station, TX).    was the most frequent indication in children older than 10 yrs while nephrotic syndrome was the most frequent indication in children younger than 10 yrs. Other indications showed a variable distribution between the groups.

Renal Biopsy Indications and Histopathological Findings
Correlation. In the subgroup of patients undergoing renal biopsy due to proteinuria and hematuria, systemic disease with renal involvement, and nephritic syndrome, the most frequent biopsy diagnosis was glomerulonephritis that was present in more than 50% of the cases. Additionally, when the indication was isolated hematuria and nephrotic syndrome, the most common biopsy diagnosis was basement membrane disease and podocytopathy, respectively. On the contrary, for glomerular filtration rate impairment without a known cause and isolated subnephrotic proteinuria patients, the distribution of biopsy diagnoses was variable ( Figure 2).
In 7/10 isolated hematuria cases, the specific entity was thin basement membrane disease, in 2/10 IgA Nephropathy, and in one case the findings were nonspecific.
As far as biopsy diagnosis according to age group, glomerulonephritis was more frequent in children older than 10 yrs than in children under 10 yrs (Figure 4).

Complications.
The only postbiopsy complication was subcapsular hematoma, present in 56 out of 241 cases (23.2%). In 54/56 cases (98.8%) hematomas were < 2 cm and there was no need of additional interventions. Due to their size, hemodynamic instability, anemia, and transfusion need, 2/56 cases (1.2%) were considered as a major complication. There were no cases of perforation, infection, fistula, or death.

Discussion
Over the years there is growing interest in recognizing the significance of histomorphological biopsy findings and pathogenic mechanisms involved in diseases affecting glomerular, vascular, or tubulointerstitial kidney structures. Thus, terms such as podocytopathy arise to group those entities in which podocyte biology is affected, as FSGS or MCD [24]. Likewise, membranoproliferative glomerulonephritis classification has been reconsidered, taking into account not only histomorphological findings, but also the presence of immune complex or complement as a pathogenic mechanism [25,26]. These new approaches help to understand histopathological lesions, identify new or under diagnosed entities, and establish therapeutic guidelines and further investigations needed [26].
Based on this need, the Mayo Clinic/Renal Pathology Society Consensus Report on Pathologic Classification, Diagnosis, and Reporting of Glomerulonephritis emerged in 2015. This approach includes a primary diagnosis constituted by the pathogenic type, specific entity, injury pattern, and the lesion classification according to the case, followed by a secondary diagnosis, reporting coexisting injuries not necessarily associated with the primary diagnosis [4].
To our knowledge, there are relatively few studies describing renal biopsy indications, findings, and complications epidemiology in pediatrics ( Table 5). None of them have a pathogenic approach of biopsy histological findings being this, the first pediatric description trying to achieve it.
Past descriptions showed "primary glomerular diseases" as the most frequent biopsy finding, including entities like MCD, FSGS, IgA Nephropathy, among others [2,6,[8][9][10][11][12][13][14][15][16][17]. Opposite to this, in our study, glomerulonephritis was the most common finding. Considering immune complex, complement, and other immunological mechanisms involved in this entity, the presence of systemic diseases in a significant proportion of the children included seems to be like a plausible explanation (Table 2). However, it is also possible that a pathophysiological approach of biopsy findings made us categorize immune complex glomerulonephritis entities that are traditionally considered as primary.
The finding of lupus nephritis as the most common specific entity among immune complexes mediated glomerulonephritis is striking. Considering that in previous series the most frequent specific entity was IgA Nephropathy [13,21,27]. This may suggest a different epidemiological behavior in our population.
Due to insufficient data in medical records, classification of tubulointerstitial diseases according to pathogenic mechanisms was not possible. This aspect should be taken into account in future descriptions.
This study ratifies variability in glomerulonephritis clinical manifestations [28,29] but shows that glomerulonephritis mediated by immune mechanisms as a finding could be more likely in children with proteinuria and hematuria, systemic disease with renal involvement, nephritic syndrome, and SRNS or being older than 10 yrs, unlike podocytopathies, which were more prevalent in children with SDNS/FRNS and younger than 10 yrs.
In recent years emphasis has been placed on nephrotic syndrome epidemiology changes, due to an increase in FSGS lesions [30,31]. But, in our series, MCD was the most International Journal of Nephrology    International Journal of Nephrology Table 5: Selected studies on the renal biopsy in children.
Autor, year   common histopathological lesion. Nonetheless, when analyzing by subgroups according to corticosteroids response, FSGS was more frequent in SRNS while MCD was more frequent in SDNS/FRNS. The results endorse clinical-pathological correlation relevance, considering agreement in some prebiopsy indications and postbiopsy findings.
Postrenal biopsy complications frequency is variable according to the series previously published (Table 5). In our study, the only complication was subcapsular hematoma in 23.2% of the cases. Nevertheless, only 1.2% (2 cases) was considered as a major complication. These two cases corresponded to patients admitted to the pediatric intensive care unit due to systemic lupus erythematous debut associated with acute kidney injury and dialysis requirement. Factors as uremia, arterial hypertension, and autoimmune disease may have contributed to this complication development. In the present study, major complications prevalence is much lower compared to other reports 12 to 30.8% of the cases [18,32,33]. This allows us to conclude that, in our center, percutaneous renal biopsy is a safe procedure with a major complications prevalence of less than 5%, concordant to previously established standards [32].

Conclusion
In children, classification of histopathological findings in renal biopsy based on the probable etiopathogenic mechanisms constitutes a key instrument not only for an adequate diagnostic and therapeutic approach, but also for the understanding of renal disease epidemiological behavior, according to the population.

Data Availability
The data used to support the findings of this study are property of Fundación Cardioinfantil. Access to these data will be considered by the author upon request, with permission of Research Department and Institutional Ethics Board Committee.

Ethical Approval
The study protocol was reviewed and approved by the Fundación Cardioinfantil Ethics Board Committee.

Conflicts of Interest
The authors declare that they have no conflicts of interest.