Renal consequences of preterm birth

Background

The developmental origin of health and disease concept identifies the brain, cardiovascular, liver, and kidney systems as targets of fetal adverse programming with adult consequences. As the limits of viability in premature infants have been pushed to lower gestational ages, the long-term impact of prematurity on kidneys still remains a significant burden during hospital stay and beyond.

Objectives

The purpose of this study is to summarize available evidence, mechanisms, and short- and long-term renal consequences of prematurity and identify nephroprotective strategies and areas of uncertainty.

Results

Kidney size and nephron number are known to be reduced in surviving premature infants due to disruption of organogenesis at a crucial developmental time point. Inflammation, hyperoxia, and antiangiogenic factors play a role in epigenetic conditioning with potential life-long consequences. Additional kidney injury from hypoperfusion and nephrotoxicity results in structural and functional changes over time which are often unnoticed. Nephropathy of prematurity and acute kidney injury confound glomerular and tubular maturation of preterm kidneys. Kidney protective strategies may ameliorate growth failure and suboptimal neurodevelopmental outcomes in the short term. In later life, subclinical chronic renal disease may progress, even in asymptomatic survivors.

Conclusion

Awareness of renal implications of therapeutic interventions and renal conservation efforts may lead to a variety of short and long-term benefits. Adequate monitoring and supplementation of microelement losses, gathering improved data on renal handling, and exploration of new avenues such as reliable markers of injury and new therapeutic strategies in contemporary populations, as well as long-term follow-up of renal function, is warranted.

A step forward towards accurately assessing glomerular filtration rate in newborns

In this edition of Pediatric Nephrology, Milena Treiber and colleagues have published a study on cystatin C (CysC) concentrations in relation to renal volumetry in 50 small-for-gestational age (SGA) and 50 appropriate-for-gestational age (AGA) neonates, deriving a new formula for estimating neonatal glomerular filtration rate (GFR). The study builds on previous work which established that renal volumetry together with CysC blood levels is a superior method for establishing GFR in term and pre-term newborns [The Journal of Pediatrics (2014) 164:1026-1031.e2]. Treiber et al. use the expected difference between SGA and AGA renal volumes to document the superiority of their new formula, which is based on total renal volume, CysC and body surface area, but does not incorporate gold-standard inulin clearance. Treiber et al.'s study adds new knowledge to the field that will hopefully improve the safety of renally excreted critical dose drugs in the newborn period. This editorial discusses the strengths and limitations of the current study.

A new serum cystatin C formula for estimating glomerular filtration rate in newborns

BACKGROUND

The levels of serum cystatin C (CysC) and creatinine (Cr) were determined in small-for-gestational-age (SGA) babies and compared with those for normal term newborns appropriate for gestational age (AGA), at birth and 3 days later. We then compared a number of cysC-based, Cr-based and combined formulas for estimation of glomerular filtration rate (GFR) with the neonatal reference GFR.

METHODS

Fifty full-term SGA and 50 AGA newborns were enrolled in the study. Kidney volume measurements were performed by ultrasound for each newborn.

RESULTS

At birth, the mean level of CysC in SGA babies was 1.48 ± 0.30 mg/l in cord blood and 1.38 ± 0.18 mg/l in day 3 blood samples, and the mean Cr level, determined simultaneously, was 67.08 ± 17.62 and 55.62 ± 14.91 μmol/l, respectively. These levels did not differ significantly from those determined in AGA babies. A 10 % reduction in kidney volume was associated with an increase in CysC value of 9.3 % in cord blood. The Cr-based and Schwartz-combined equations underestimated GFR relative to CysC-based and Zappitelli-based equations at birth and 3 days later.

CONCLUSIONS

A newly constructed Cys-C based formula which includes kidney volume and body surface area in the calculations for GFR is a reliable marker of GFR compared with neonatal reference clearance values.

The Importance of Accurately Assessing Renal Function in the Neonate and Infant

Measuring renal function in neonates and small infants is important to ensure that drugs are safely dosed and to detect acute kidney injuries early on. Serum creatinine (Cr) remains the most widely used marker, but its shortcomings are particularly important in neonates. For example, neonatal Cr largely depends on maternal renal function for at least the first 72 h of life. Novel approaches for assessing neonatal renal function include cystatin C and beta-trace protein. Another way to assess renal function is to measure renal volume by ultrasound. Although this approach may assess neonatal nephron endowment, it is insensitive to the postnatal adaptation of renal function in term and preterm neonates. The purpose of this review is to summarize what is known about measuring renal function in term and preterm newborns, and to summarize existing knowledge gaps, including a description of steps to take to close these gaps.

Beta-trace protein as a marker of GFR--history, indications, and future research

OBJECTIVES

Recent findings suggest that beta-trace protein (BTP), a small molecular weight protein, is at least equal if not superior to serum creatinine as a marker of glomerular filtration rate (GFR), particularly since it is independent from height, gender, age, and muscle mass. The authors sought to summarize knowledge on BTP and its use as a marker of GFR using the most recent literature available.

DESIGN AND METHODS

The authors compiled key articles and all relevant recent literature on this topic. Physical and chemical features of the molecule are described, as well as factors that may affect its expression. The use of BTP in estimating GFR as a whole and in specific patient groups, including pregnant women, neonates and infants, children and adolescents, and patients who have undergone renal transplantation is discussed. The use of BTP as a marker for cardiovascular risk factors is also briefly addressed.

RESULTS

Although its performance in the general population is marginally inferior to cystatin C, studies have suggested that it may be superior in accurately estimating GFR in select patient groups such as pregnant women and neonates.

CONCLUSIONS

This novel marker shows promise, but further research is required to clarify findings from available data.

Methods of assessing renal function

Accurate assessment of renal function is critical for appropriate drug dosing of renally excreted compounds. Glomerular filtration rate (GFR) is considered the best marker of kidney function. Inulin clearance forms the gold standard for measuring GFR, both in adults and in children. The method is invasive, cumbersome, and smaller children require urinary catheterization for accurate timed urine collections. Nuclear medicine methods replaced inulin clearance in the 1970s after (51)Cr EDTA clearance was introduced. Inulin has no plasma protein binding, whereas all commonly used radioisotopes have a small amount of plasma protein binding that leads to lower values. Only iohexol does not have significant plasma protein binding. The underestimation due to plasma protein binding is partially offset by overestimation due to the use of non-compartmental pharmacokinetic modeling of the plasma disappearance of the radioisotope. The problem could be overcome with a urinary nuclear medicine clearance method, but these have not been validated in children. Endogenous markers of GFR include serum creatinine and low molecular weight proteins such as cystatin C and beta-trace protein. Of these, estimation of GFR using cystatin C appears to be the most promising, although its accuracy in pregnancy and in the neonatal period may be limited.

Cystatin C adaptation in the first month of life

Je-Hyan Lee et al. have published a study on cystatin C concentrations in the first 30 days of life in 127 pre-term and 119 term neonates in this edition of Pediatric Nephrology, thereby closing a knowledge gap of detailed cystatin C concentrations beyond 72 h of life by day of life and by post-conceptional age. While the study objective has merit and a large number of measurements were included, there are some methodological limitations that bring the validity of the data into question as pure reference intervals for children up to 1 month of age, mostly because of the inclusion of patients that potentially could have an impaired glomerular filtration rate (GFR), for instance due to exposure to nephrotoxic drugs. We discuss the strengths and weaknesses of the study and outline an approach to definitely close this knowledge gap. We call for a worldwide collaboration to use Box-Cox transformations similar to the methodology used with growth charts to calculate age-independent z-scores and percentiles of neonatal and infant markers of GFR. This could also lead to better definitions of acute kidney injury in infants if GFR markers cross the percentiles based on post-conceptional or chronological age.