'Congenital solitary functioning kidneys: which ones warrant follow-up into adult life?'

Abstract

Congenital anomalies of the kidney and urinary tract (CAKUT) are the leading cause of paediatric end-stage renal failure (ESRF) [1, 3]. The clinical spectrum of CAKUT includes a variety of malformations of the urinary tract. Understanding the rate of progression for the different CAKUT categories and predicting long-term outcome is critical for a correct clinical management of these patients and for the transition of care from paediatric to adult nephrology. Among CAKUT categories, congenital solitary functioning kidney (SFK) has been the object of debate whether it constitutes a benign condition or presents a significant risk of progression to ESRF [2–14]. In the present issue of Nephrology Dialysis and Transplantation, Westland et al. report on the results of the KIMONO-study (REF ID pending). The authors analysed a large cohort of 206 children with SFK of congenital origin to identify indicators of renal injury. Among them, 116 had a primary congenital solitary functioning kidney (pSFK) and 90 had a secondary solitary functioning kidney (sSFK) after unilateral nephrectomy due to congenital anomalies of the kidney or urinary tract. Ipsilateral CAKUT was present in ~30% of children from both groups. Among the patients with pSFK (which encompasses unilateral renal agenesis and multicystic dysplastic kidney), ipsilateral anomalies were significantly more frequent in cases with renal agenesis. Similarly, high blood pressure was present in comparable proportions between pSFK and sSFK, and it was significantly higher in the subgroup with renal agenesis. About 20% of children were using renoprotective agents and ~12% had albuminuria. Overall, >30% of patients with SFK showed evidence of renal injury. These data indicate that a significant fraction of children with congenital solitary kidney show evidence of renal parenchyma damage early in life and can potentially progress to ESRF in adulthood. These numbers are consistent with our previous report on long-term outcome in CAKUT patients, in which 50% of children with solitary kidney reached ESRF by the age of 30 [11]. Westland et al. explored the rate of progression by generalized estimated equation (GEE) analysis (REF ID pending). Microalbuminuric-range proteinuria appeared at the age of 18 years in both pSFK and sSFK, and children with CAKUT showed a more pronounced increment. GEE analyses (to be taken cautiously) point to a slow progression of renal injury during adolescence, which suggests that a significant fraction of patients will progress to renal failure during adulthood. Again, these data seem to be supported by our outcome data, in which the majority of patients with SFK reached ESRF after the age of 18 [11]. Therefore, a large number of children who are diagnosed with congenital solitary kidney in early life will progress to chronic renal failure requiring replacement therapy in adulthood, without a smooth transition from paediatric care. The difficulties of understanding the natural history of congenital solitary kidney stand on several observations that have not been elucidated so far and remain unresolved by the present report. First, a clear definition of the phenotype of the patients investigated is not available. Westland et al. cluster unilateral renal agenesis and the involution of multicystic dysplastic kidney together into one category and all solitary functioning kidneys that result from other congenital anomalies of the urinary tract that require nephrectomy or lead to a complete loss of function of one kidney in another category. This stratification, although potentially useful, is arbitrary in consideration of the possibly different molecular origin and clinical course of the different conditions. Therefore, until molecular genetics can improve the diagnosis, any classification and grouping for statistical purposes is arbitrary and invariably results in