| Abstract|| |
The objective of this study was to determine the incidence and risk factors of nephrocalcinosis (NC) in pre-term neonates in the neonatal intensive care unit (NICU) at the Al-Minya University, Egypt. The study included 97 pre-term neonates with a gestational age 34 weeks. Data on duration of hospitalization, sex, gestation, birth weight, family history of renal stone, need for respiratory support, intake of calcium and use of total parenteral nutrition (TPN) and nephrotoxic drugs were collected. Blood urea nitrogen, serum creatinine, sodium, potassium, calcium and phosphate were measured within the first week of life and again at term. Blood gases, urinary pH, urinary calcium/creatinine (U Ca/Cr) ratio and urinary oxalate/creatinine (U Ox/Cr) ratio were measured once at term. Three renal ultrasound (US) scans were performed; one before the first week of life, the second at term and the third at a corrected age of one year. Of the 97 infants studied, 14 (14.4%) developed NC diagnosed by renal US at term. NC was bilateral in 11 infants. Factors significantly associated with NC were gestational age, need for respiratory support, high calcium intake, TPN, use of post-natal dexamethasone, furosemide, theophylline, and/or aminoglycosides and U Ca/Cr ratio and U Ox/Cr ratio (all P < 0.05). Low gestational age (P = 0.004), use of respiratory support (P = 0.005), furosemide therapy (P = 0.002) and increased U Ca/Cr ratio (P = 0.001) were the strongest independent risk factors after logistic regression analysis. Eight of the 14 infants (57.1%) with NC had spontaneous resolution of calcification at a corrected age of one year. Screening at term with a renal US scan and long-term follow-up of renal function is needed for early diagnosis and better management of NC. Future research pertaining to prevention of NC in pre-term neonates is required.
|How to cite this article:|
Mohamed GB, Ibrahiem MA, Abdel Hameed WM. Nephrocalcinosis in pre-term neonates: A study of incidence and risk factors. Saudi J Kidney Dis Transpl 2014;25:326-32
|How to cite this URL:|
Mohamed GB, Ibrahiem MA, Abdel Hameed WM. Nephrocalcinosis in pre-term neonates: A study of incidence and risk factors. Saudi J Kidney Dis Transpl [serial online] 2014 [cited 2019 Nov 15];25:326-32. Available from: http://www.sjkdt.org/text.asp?2014/25/2/326/128524
| Introduction|| |
Nenhrocalcinosis (NC) refers to deposits of calcium crystals diffusely located in the parenchyma of the kidney. In children, NC can be caused by a wide variety of renal, urological, endocrinal and metabolic disorders. In pre-term neonates, the etiologic factors of NC have not yet been fully clarified. NC in pre-term neonates has been reported frequently and studies on small numbers of patients suggest an unfavorable effect on renal function.  The incidence of NC has been found to be 16-64% in very low-birth-weight babies. The reported risk factors include low gestational age, low birth weight, furosemide use, aminoglycoside use, male sex, duration of assisted ventilation, high serum calcium, low serum phosphate, high urinary calcium, oxalate, uric acid, low urinary citrate and length of hospital stay. ,
NC is a result of spontaneous and therapy-induced imbalance between promoters (e.g., calcium, oxalate, uric acid) and inhibitors (e.g., citrate and magnesium) of crystallization in urine.  Urine from pre-term neonates in the first weeks of life is highly supersaturated and has a defective ability of inhibiting calcium oxalate crystal agglomeration. This ability improves with age and is citrate mediated. Small studies have shown that urinary citrate excretion is reduced in pre-term babies with respiratory disease, and this may predispose them to NC.  Until recently, NC has been studied very little in pre-term infants.
Although NC is thought to predispose to urinary tract infection, renal colic and hematuria, the long-term consequences in pre-term babies are unclear. Several studies have suggested a spontaneous resolution of NC in 20-60% of cases. Some other reports have suggested that NC may predispose to glomerular and tubular dysfunction, while others have proposed that prematurity alone leads to renal dysfunction, independent of NC. 
The aim of this study was to determine the incidence of NC in pre-term neonates at the Neonatal Intensive Care Unit (NICU) of the Al-Minya University Hospital, showing its pattern as well as identifying factors that influence development of NC in such high-risk neonates.
| Patients and Methods|| |
This prospective observational follow-up study was conducted in the NICU of the Al-Minya University Hospital. The study population included 150 neonates, all pre-term infants, admitted to the NICU from March 2008 to February 2010, with a gestational age of 34 weeks or less at birth. Pre-term babies with renal anomalies, hypoxic ischemic encephalopathy or those with compromised renal functions were excluded from the study as these might affect glomerular filtration rate and excretion of urinary solutes (N = 27). Twenty-six neonates died during the study and were excluded, while 97 pre-term neonates continued the study. The study protocol was approved by the local ethics committee and informed written parental consent was obtained.
The following data were collected: Duration of each infant's admission, sex, gestational age, birth weight, family history of renal stone, urine output, respiratory support (oxygen therapy and mechanical ventilation), antenatal and post-natal use of steroids, treatment with furosemide, theophylline and aminoglycosides (gentamicin), intake of calcium and use of total parenteral nutrition (TPN).
In the first 28 days, the mean intake of calcium per kilogram per day was calculated from prescribed supplements if parenterally fed, and using published concentrations of calcium if milk fed. Human expressed breast milk was assumed to contain 25.4-30.6 mg/dL of calcium. 
The blood urea nitrogen (BUN) and serum creatinine, sodium, potassium, calcium and phosphate were measured within the first week of life and again at term (at term was defined as a post-conceptional age of 38-42 weeks). Blood gases, urinary pH, urinary calcium/ creatinine (U Ca/Cr) ratio and urinary oxalate/creatinine (U Ox/Cr) ratio were measured at term. The BUN, serum creatinine and electrolytes (including calcium and phosphate) and urinary calcium, oxalate and creatinine were assayed on an automated analyzer (Dimension RxL Max).
Three renal ultrasound (US) scans were performed, one within the first week of life, the second at term and the third at a corrected age of one year. The renal US was performed by an experienced radiologist each time. The US examination was performed using a GE logic 4 machine with a 5.5 MHz small-part transducer. NC was diagnosed according to the criteria of Myracle et al.  Criteria for diagnosis were based on the presence of at least one bright reflection in the medulla or cortex seen in both transverse and longitudinal directions. Bright reflections of 1-2 mm without acoustic shadowing were defined as white flecks, and were not adequately measurable. Bright reflections larger than 2 mm, with or without acoustic shadowing, were defined as white dots.
In the NC group, at a corrected age of one year, clinical evaluation, BUN, serum creatinine and urinalysis were assessed in addition to renal US scan.
| Statistical Methods|| |
Data were analyzed using SPSS (version 16) software. The incidence rate of NC was described in a simple proportion. Student's t-test was used to compare the two groups regarding parametric data and chi-square test was used for non-parametric data. Pearson correlation was used to correlate two quantitative variables. A logistic regression model was performed for the risk factors that had been shown to be significant on univariate analysis. A P-value of <0.05 was considered significant.
| Results|| |
The study population included 97 pre-term neonates, all with a gestational age of 34 weeks or less at birth. [Table 1] shows the baseline characteristics of the studied patients.
The mean serum creatinine decreased and the mean serum calcium increased significantly when measured in the first week (0.71 ± 0.2 mg/dL and 7.8 ± 0.3 mg/dL, respectively) and at term (0.54 ± 0.1 mg/dL and 9.2 ± 0.66 mg/dL, respectively) (P = 0.009 and P = 0.000).
The cases with NC at term had a high U Ca/Cr ratio despite having normal serum calcium levels, and also had a high U Ox/Cr ratio. None of the studied NC cases had abnormalities in BUN, serum creatinine, serum sodium, potassium and phosphorus, blood gases or urinary pH.
[Table 2] shows the relationship between NC and risk factors. There was a significant relationship between the occurrence of NC and gestational age, respiratory support, calcium intake, TPN, use of post-natal dexamethasone, furosemide, theophylline and/or aminoglycosides, U Ca/Cr ratio and U Ox/Cr ratio (all P < 0.05). On the other hand, there was no significant relationship between the occurrence of NC and sex, birth weight, family history of renal stone, use of antenatal steroids and length of hospital stay (all P > 0.05).
|Table 2: Relationship between nephrocalcinosis in pre-term neonates and risk factors.|
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Those variables that were statistically significant on univariate analysis were further analyzed using logistic regression analysis. Low gestational age (P = 0.004), respiratory support (P = 0.005), furosemide therapy (P = 0.002) and increased U Ca/Cr ratio (P = 0.001) were the strongest independent risk factors.
There was a significant positive correlation between U Ca/Cr ratio and other variables such as respiratory support, intake of calcium, TPN and use of post-natal dexamethasone, furosemide, theophylline and/or aminoglycosides (all P <0.05) [Table 3].
|Table 3: Correlation of urinary calcium/creatinine ratio and urinary oxalate/creatinine ratio with different variables.|
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Also, there was a significant positive correlation between U Ox/Cr ratio and other variables such as TPN (P = 0.001). No significant correlation was found between U Ox/Cr ratio and any of the following variables: Respiratory support, intake of calcium and use of postnatal dexamethasone, furosemide, theophylline and/ or aminoglycosides (all P > 0.05) [Table 3].
Seventeen babies had abnormal ultrasonographic findings; three had unilateral hydronephrosis (antero-posterior diameter <20 mm without calyceal dilatation, which is an acceptable sonographic finding in neonates).  The dilatation decreased on follow-up over the period of the study. Fourteen babies (14.4%) had NC, which was diagnosed on US at term. NC was bilateral in 11 infants and unilateral in three infants. NC was medullary in 13 infants and cortical and medullary in one infant. White flecks appeared in five infants, white dots without shadowing in four infants and white dots with shadowing in five infants.
In the NC group, at a corrected age of 1 year, all infants were asymptomatic. Eight of the 14 infants (57.1%) with NC had spontaneous resolution of calcification. The BUN, serum creatinine and urinalysis were all within normal limits.
| Discussion|| |
NC in pre-term neonates has been reported earlier and this study demonstrates the various factors that contribute to its development in pre-term neonates. The incidence of NC in this study was 14.4%, which was lower than that previously reported. , This may be explained by the age of the studied group, as most of the other studies were conducted on more premature infants with an expected amelioration of conditions predisposing to NC with increased gestational age. Also, the incidence of NC varies in different studies, perhaps reflecting variations in ultrasonic identification of NC, improvements in neonatal intensive care, particularly the antenatal use of steroids and surfactant, and improved approaches to nutrition.
In this study, we reported a significant correlation between NC and gestational age but not with birth weight, which was comparable to other studies. ,, The high incidence of NC in pre-term neonates suggests that crystallization is common in immature kidneys.
Short and Cooke  found that the duration of oxygen administration was the strongest clinical indicator of occurrence of renal calcification. They confirmed that severe respiratory disease, as indicated by duration of ventilation, duration of oxygen administration and oxygen dependence at 36 weeks after conception, was significantly associated with NC. In this study, we found that use of respiratory support was significantly associated with the development of NC. Urinary citrate, known as a potent inhibitor of renal calcification in adults and children,  is decreased in pre-term infants with lung disease,  which may predispose them to NC.
In this study, high calcium intake was significantly associated with NC. Routine calcium intake without measurement of serum calcium can lead to high urinary calcium excretion in pre-term newborns;  however, this was not confirmed by other authors. 
In this study, TPN was significantly associated with NC. Glycine and ascorbic acid are precursors of oxalate, and both are present in TPN. Narendra et al  found that TPN was significantly associated with NC and that the association is through phosphate deficiency, which is typically associated with hypercalciuria, while phosphate supplementation decreases urinary calcium excretion. 
We found an association between NC and use of post-natal dexamethasone (through hypercalciuria), which was in agreement with the findings of Saarela et al.  However, Sonntag and Gaude  found no increase in urinary calcium excretion in a group of pre-term infants who received dexamethasone compared with a control group.
The well-known effects of the administration of furosemide are hypercalciuria and NC. , Some authors have already suggested that in the first four weeks of life, furosemide therapy is not a major cause of the development of NC in pre-term neonates.  In our study, furosemide (through hypercalciuria) contributed as a strong factor in the etiology of NC in pre-term infants.
NICU protocols recommend the use of theophylline till maturity is reached. The hypercalciuric effect of xanthines (including theophylline) has been well described; different suggested mechanisms are increased diuresis and natriuresis, increased prostaglandin synthesis and antagonism of adenosine-mediated effects with a change in renal blood flow and glomerular filtration rate.  We found a significant correlation between the use of theophylline and the occurrence of NC.
Use of aminoglycoside antibiotic is well recognized as a cause of tubular dysfunction in the newborn period, but whether this is long lasting is not known. Nephrotoxic antibiotic levels were strongly associated with NC, independent of gestation.  Hypercalciuria with therapeutic doses of gentamicin has been found in full-term infants.  The renal toxicity of aminoglycoside is related to the frequency and duration of treatment rather than the actual serum toxic level.  In our study, we found that use of aminoglycosides was significantly associated with the development of NC.
We found a significant association between U Ca/Cr ratio measured at term and NC. The results obtained were in agreement with some authors ,, and in disagreement with some others.  We found a significant association between U Ox/Cr ratio measured at term and NC. Results obtained were in concordance with the results of Narendra et al. 
In only one study, male sex was reported to be significantly associated with NC.  In agree-ment with Nasseri et al,  we did not find a similar association. Family history of renal stone was reported to be significantly associated with NC.  We did not find such an association, which is in agreement with other studies. ,
In our study, the duration of hospital stay had no effect on the occurrence of NC. This was similar to the results reported by Nasseri et al. 
US scan is a non-invasive and sensitive diagnostic tool that does not involve radiation in identifying NC. It has been confirmed that US is more sensitive than plain X-rays and computer tomography for detecting calcifications in the kidney. 
In our study, which was in concordance with other authors,  the bright foci of calcification were almost always located in the medulla and hardly located in the cortex. It is not yet known what mechanisms predispose to different locations of NC and whether the location of NC has different consequences on the course of NC and, eventually, renal function.
In this study, we found that NC resolved spontaneously in 57.1% (eight of 14) of the patients scanned at a corrected age of one year. Downing et al  found resolution in 60% (six of ten) pre-term babies followed-up at 1-2 years of age. Chang et al  found resolution in 50% (three of six) followed-up at one year of age. Long-term follow-up of affected patients is needed to assess the significance of this calcification. There is understandable concern about long-term renal functions in the presence ofNC. 
The incidence of NC in this study was 14.4%. Low gestational age, respiratory support, furosemide therapy and increased U Ca/Cr ratio are the major risk factors for NC. NC was transient in 57.1% of the affected infants. Screening at term with a renal US scan and long-term follow-up of renal function is needed for such high-risk neonates. Further studies are required on the preventive role of inhibitors of urinary calcification and supplementation with citrate and magnesium in these high-risk neonates.
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Gamal B Mohamed
Department of Pediatrics, Faculty of Medicine, Al-Minya University
[Table 1], [Table 2], [Table 3]