Results

There was no significant difference between the median age, gestational age, birthweight and sex between the study group and the control group (table 1). Five out of 11 children, aged 4 to 5 years, in the study group had ultrasonic evidence of renal calcification. This was present in both kidneys in three of them. Renal calcification was not identified in any of the control group. An additional child in the study group had renal colic at the age of 3, secondary to a right sided ureteric calculus. An ultrasound scan at that time also showed calcification. A calcium load test indicated that this patient had absorptive hypercalciuria and she was then started on a low calcium diet. At the age of 5 her ultrasound scan was normal.

One child in the study group had only one kidney and another had a renal cyst in the upper pole of the left kidney. No structural renal abnormalities were found in the control group. Renal length was measured ultrasonically in each kidney and plotted against height, using a published reference range.21 Out of 55 kidneys, the lengths of eight were below the 95% confidence limits. In two children out of the study group the lengths of both kidneys were below the 95% confidence limits. Four children out of the control group had a small kidney on one side.

Urinalysis showed that there was neither blood nor protein in all the early morning urine samples. Two children had an albumin:creatinine ratio >2.5 mg/mmol (3.9 and 4.5 mg/mmol), indicating microalbuminuria.22

The results of the calcium load test and renal concentrating capacity after the administration of intranasal desmopressin are shown in table2. There was no significant difference between the study group and the control group regarding urinary calcium:creatinine ratio before and after calcium load or maximum urine osmolality. Three children in the study group and five in the control group had an abnormal calcium load test. Four of these (two in the control and two in the study group) had a fasting urinary calcium:creatinine ratio >0.7 mmol/mmol, suggesting renal hypercalciuria,15 23 and in three of these children (one in the control and two in the study group) the post load value was >1 mmol/mmol. Four children (one in the study group and three controls) had a fasting urinary calcium:creatinine ratio <0.7 mmol/mmol and a post load value >1 mmol/mmol, suggesting absorptive hypercalciuria.15

The median value of the maximum urine osmolality in the study group was 1022 mosmol/kg (range 646–1086) and that in the control group was 928 mosmol/kg (range 727–1247). The mean value for normal children of this age has been reported to be 1062 mosmol/kg, with a range of 795 (−2SD) to 1330 (+2SD) mosmol/kg.17 The median maximum urine osmolality of both the study and the control groups was 973 mosmol/kg (95% CI 888 to 1033 mosmol/kg) with a corresponding median SD score of −0.7 (95%Cl −1.21 to −0.23) (fig1).

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Figure 1

Renal concentrating ability after administration of DDAVP.

In the study group the median estimated glomerular filtration rate was 61 ml/min/1.73m² (range 46-79 ml/min/1.73m²), (published range of 89 to 165 ml/min/1.73m²).24 The median Tp/GFR was 3.8; the published mean value for age is 4.4 (SD 0.6)18 and the median calculated SD score for Tp/GFR was −0.94. Three children in the study group had a PTH value below the normal range.23The serum values of calcium and phosphate were all within the normal range. These results are summarised in table 3.

There were no significant associations between decreased renal concentrating capacity or hypercalciuria and gestational age, birthweight, days of supplemental oxygen, retinopathy of prematurity or intraventricular haemorrhage. There was a trend towards those children who had idiopathic hypercalciuria to have received an aminoglycoside following a high trough concentration in the neonatal period, but this was not significant (P=0.07, Fisher’s exact test).

In three children (two in the study group, one control) urinary tract infection had been confirmed on a midstream urine sample since discharge from the neonatal unit. In the two children from the study group this was associated with systemic upset but did not result in renal scarring on a subsequent dimercaptosuccinic acid technetium isotope (DMSA) scan.

Discussion

Few studies have looked at the short to medium term effects of renal calcification in children who were born preterm. Ezzedeenet al looked at nine children aged between 9 and 56 months, who had been identified as having renal calcification in the neonatal period.2 Sonography revealed complete resolution of renal calcification in four children and improved, but persisting, calcification in five children. One child had a small kidney on one side, and the serum creatinine and calculated GFR were abnormal in four. There were no controls in this study.

In the study by Downing et al those children who had received frusemide and had developed renal calcifications had a significantly higher urinary calcium:creatinine ratio and fractional excretion of sodium compared with children who had not developed calcifications but had received frusemide.12 The first group also had a lower tubular reabsorption of phosphate and lower tubular acidification capacity. The average chronological age in this study was 14.1 months and some of the children would have been less than 12 months corrected age, when renal tubular and glomerular function are not yet mature. Renal calcification seems to diminish with increasing age. Downing et al suggested that as calcification improves renal function may also improve. We therefore felt that it was important to study a group of children aged 4 to 5 years old, who had been identified as having renal calcifications in the neonatal period.

In our study five out of 11 children, aged 4 to 5 years, had ultrasonic evidence of renal calcification. Ultrasound scanning is the most sensitive imaging technique for the detection of renal calcification13 and there is good correlation between ultrasound findings and histological evidence of nephrocalcinosis and nephrolithiasis.25 There was no clinical evidence of other pathological processes that might give rise to similar ultrasonic appearances.26 The patients classified as having renal calcification in the original study had areas of hyperechogenicity on at least two consecutive scans, performed at weekly intervals.11 The age at diagnosis of renal calcification ranged from 27 days to 66 days. Hyperechogenicity associated with renal insufficiency in the preterm infant usually occurs earlier than this and tends to be transient.27

Predicted GFR and Tp/GFR were reduced in some children from the study group. As blood was not taken from the control group it is uncertain whether prematurity or renal calcification was the predisposing factor for this.

Renal calcification in preterm neonates does not seem to affect subsequent renal calcium excretion or tubular concentrating ability, as there were no significant differences between the study and control groups. Twenty eight per cent of all the children (both study and control groups) had an abnormal calcium load test (four children had a fasting urinary calcium: creatinine ratio >0.7 mmol/mmol and four children had a normal fasting urinary calcium: creatinine ratio but a post load value of >1 mmol/mmol). Five children from the control group had an abnormal calcium load test, which suggests that preterm infants may have an increased risk of developing idiopathic hypercalciuria that is not associated with renal calcification. Hypercalciuria is defined in children as a urinary calcium excretion greater than 4 mg/kg/day.28 Other authors have demonstrated a correlation between 24 hour urinary calcium and a urinary calcium:creatinine ratio on an overnight urine sample,29 or a second fasting urine after liquid intake.30 We did not measure calcium excretion on a 24 hour urine collection because of its unreliability in this age group. We chose the calcium load test. This involves a fasting urine sample and a urine collection following a calculated dose of calcium, because urinary calcium excretion is affected by dietary calcium.31 The upper reference limit for the second fasting urine sample was taken from a study measuring urinary calcium on a population of children from the North of England, who were eating a normal diet.23 The calcium load test may appear normal in children with hypercalciuria, defined by a 24 hour urinary calcium >4 mg/kg/day, if they have low salt or restricted calcium intake before the test.32 33 An abnormal calcium load test in a child with a normal 24 hour urinary calcium has been described in a child with a history of renal colic and renal stones.34 We are not aware of other studies that have shown an abnormal calcium load test in children with normal urinary calcium on a 24 hour collection. It is therefore unlikely that we overestimated the number of patients with raised urinary calcium excretion. One child who was felt to have absorptive hypercalciuria developed a ureteric calculus and was placed on a low calcium diet.

Urinary calcium excretion parallels sodium excretion31 35and is affected by both dietary sodium and calcium. We did not find any difference in dietary sodium or calcium in the three days before the calcium load test between those children who had hypercalciuria and those who did not. The pathogenesis underlying idiopathic hypercalciuria is not yet fully understood. Several hypotheses have been suggested, including increased bone resorption36 and a disordered control of renal phosphate handling and 1,25(OH)2 vitamin D production.37 38 Two children who had a Tp/GFR <2SD also had raised urinary calcium excretion and in one of these children PTH was undetected. A further child in this group, who also had idiopathic hypercalciuria, had a SD Tp/GFR of −1.29 and a PTH value of 1. Blood was not taken from the control population, so we cannot comment on renal phosphate handling in the five controls with raised calcium excretion.

One explanation of decreased phosphate reabsorption may be aminoglycoside induced nephrotoxicity which causes tubular damage of the convoluted and straight portions of the proximal tubule. Reabsorption of phosphate predominantly occurs in the proximal convoluted tubule. Although the number of patients with idiopathic hypercalciuria was only eight, there were proportionally more patients in this group who received aminoglycosides after a high trough concentration. We are not aware of any long term studies which suggest that renal tubular function may remain altered four to five years after the administration of aminoglycosides. Our findings may be an epiphenomenon and relate to the severity of illness in the neonatal period. However, there was no difference in other indicators of illness severity, including birthweight, gestational age, days in oxygen and days requiring total parenteral nutrition.

Concentrating capacity in both the study and control group combined was below normal values.17 The reduction in concentrating capacity is unlikely to cause any clinical problems. There are limitations to the assessment of renal length by ultrasound scanning. We attempted to minimise these by correlating renal length to height, using one observer who was blind to which group the child belonged. We compared renal length to standards published by Han and Babcock for American children, and used the same scanning methods.21Out of 55 kidneys, eight were on and eight were below the lower limit of the 95% confidence interval when comparing renal length to the child’s height. There are many predisposing factors early in the neonatal period which may cause renal cortical or medullary necrosis. These include renal vein thrombosis, hypoxia, hypotension and nephrotoxic drugs. They are more likely explanations than renal calcification for the reduction in renal size and GFR present in some of our children. Lerner et al reported that renal cortical and medullary necrosis at post mortem examination was associated with congenital heart disease, prematurity, respiratory distress syndrome and bleeding diathesis.39

Our results indicated that renal calcification in these children decreased with increasing age. This has also been reported in the study by Downing.12 This may explain why the differences in renal function seen in the neonatal period and early childhood, observed in other studies,11 12 and considered to be associated with renal calcification, are not present at the age of 4 years.

In summary, the results of this study indicate that children born preterm may have small kidneys and a degree of renal dysfunction. Renal calcification detected in the neonatal period does not seem to be a major predisposing factor to the renal excretion of calcium, concentrating ability, and renal size in these children. The clinical importance and aetiology of these abnormalities of renal function in our patients are unclear.