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Clinical Epidemiology |
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* Renal Research Group, Institute of Medicine,
Locus for Registry Based Epidemiology, Faculty of Medicine, University of Bergen, Bergen;
Medical Birth Registry of Norway, University of Bergen and Norwegian Institute of Public Health, Bergen;
Department of Medicine, Haukeland University Hospital, Bergen; || Norwegian Renal Registry, Institute of Immunology, Rikshospitalet, Oslo; ¶ Department of Medicine, St. Olav University Hospital, Trondheim; and ** Department of Cancer Research and Molecular Biology, Faculty of Medicine, Norwegian University of Science and Technology, Trondheim, Norway
Correspondence: Dr. Bjørn Egil Vikse, Renal Research Group, Institute of Medicine, Haukeland University Hospital, 5021 Bergen, Norway. Phone: +47-55975000; Fax: +47-55975890; E-mail: bjorn.vikse{at}med.uib.no
Received for publication February 28, 2007. Accepted for publication August 16, 2007.
| Abstract |
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| Introduction |
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In the 1980s, Brenner and colleagues6 hypothesized that intrauterine growth restriction might cause a low nephron number, which could predispose to hypertension and renal disease. Later, studies showed that low birth weight and intrauterine growth restriction were associated with mild to moderate elevations of BP,2,7 a reduced number of compensatory hypertrophied glomeruli,8–10 and lower GFR and higher urine albumin-to-creatinine ratio.11,12 Furthermore, it has been shown that low birth weight is associated with more frequent relapses of nephrotic syndrome in children with minimal-change nephropathy and higher rates of progression in children with IgA nephropathy.13,14 Two case-control studies have shown that birth weight <2.5 kg seems to be associated with a 40 to 60% increased risk for development of chronic renal failure.3,15
No cohort studies have investigated the association between birth weight and ESRD. In Norway, data on all births have been recorded in the Medical Birth Registry since 1967, and since 1980, data on all patients who have developed ESRD have been recorded in the Norwegian Renal Registry. We linked these two registries to investigate whether birth weight or other birth-related variables were associated with development of ESRD during adulthood. We also investigated whether birth weight was associated with specific causes of ESRD.
| RESULTS |
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Analyses for birth weight were repeated using 2.5 and 4.5 kg as cutoffs. In these analyses, birth weight <2.5 kg had a RR of 2.0 (95% CI 1.4 to 2.8) for development of ESRD: RR 1.5 (95% CI 0.89 to 2.4) for men and 2.8 (95% CI 1.8 to 4.4) for women. Birth weight
4.5 kg was associated with ESRD for women but not for men: RR 2.3 (95% CI 1.2 to 4.5) for women and 0.48 (95% CI 0.22 to 1.1) for men (of the nine women with ESRD and birth weight
4.5 kg, five developed ESRD as a result of glomerular disease and two as a result of diabetic nephropathy).
From 1980 to 2005, complete registration of end points was obtained; we therefore performed separate analyses of this subgroup (including 1,417,036 children among whom 138 developed ESRD). These analyses yielded virtually identical results as those presented in the previous paragraph. Separate analyses of children without any congenital abnormalities registered at birth also yielded almost identical results as those presented in Table 2.
Risk Factors for ESRD, Noncongenital
There is a possibility that congenital urinary tract malformations and hereditary renal disease might be a cause of low birth weight. We therefore chose to analyze risk factors for noncongenital ESRD. In these analyses, we excluded 1741 individuals with recorded congenital urinary tract or kidney malformations at birth and 142 individuals who developed ESRD as a result of congenital urinary tract malformations and hereditary renal disease. Overall, risk factors for noncongenital ESRD were virtually identical with those for total ESRD with similar estimates of RR. Because of a lower number of outcomes, P values did, however, decrease in strength for most variables. Birth weight <10th percentile was associated with an RR of 1.5 (95% CI 1.1 to 2.0): 1.5 (95% CI 1.02 to 2.2) in men and 1.5 (0.93 to 2.3) in women. Birth weight for gestational age <10th percentile was associated with an RR of 1.5 (95% CI 1.1 to 1.9): 1.6 (95% CI 1.1 to 2.3) in men and 1.3 (95% CI 0.86 to 2.1) in women.
Risk Factors for ESRD, Specific Causes
Among the 526 who developed ESRD, 238 developed ESRD as a result of glomerular disease, 52 as a result of interstitial nephritis, 142 as a result of congenital or inherited causes, 42 as a result of diabetic nephropathy, and 52 as a result of other causes. Table 4 shows that low birth weight seems to be similarly associated with different causes of ESRD. The numbers of cases in several subgroups, however, were low with imprecise estimates of RR.
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| DISCUSSION |
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Previous studies have shown that low birth weight is associated with a lower nephron number,8–10 larger glomeruli,8,10 mildly to moderately elevated BP,2,7 lower GFR,11 albuminuria,11,12 and adverse clinical course of minimal-change nephropathy as well as IgA nephropathy.13,14 In addition, two case-control studies showed a 40 to 60% increased risk for chronic renal failure in individuals with low birth weight.3,15 The participants in these studies were older and more often black (70% versus none), and chronic renal failure was more often caused by hypertension or diabetes as compared with our study. Subanalyses of white individuals, however, showed similar findings as our study did.3 Our study thus confirms previous findings in a cohort study of a white population up to a maximum age of 38 yr, showing a 70% increased risk for ESRD in low birth weight individuals, independent of other birth-related variables. Birth weight for gestational age <10th percentile was associated with a 50% increased risk for ESRD, significant for men but not for women. Comparing the predictors birth weight and birth weight for gestational age, birth weight was associated with somewhat higher RR, showed similar findings for men and women, and were more important in the adjustment analyses. In our study, birth weight therefore seems to be a more important predictor of ESRD than birth weight for gestational age. It has been argued that gestational age should be taken into account when analyzing effects of birth weight because low birth weight at short gestational age might be physiologically normal, whereas low birth weight at term probably reflects severe placental insufficiency. Our study suggests that birth weight per se might be more predictive for development of ESRD than birth weight for gestational age. Maternal preeclampsia is an important cause of low birth weight children and neonatal complications as well as an important risk factor for cardiovascular and renal disease in the mothers.17,18 It is therefore interesting to note that data from this study indicated that maternal preeclampsia was not associated with development of ESRD for the children.
Further analyses showed that the association between low birth weight and ESRD was stronger during the first 14 yr of life than after 15 yr of age. This age-related difference may be explained by a stronger association between low birth weight and ESRD as a result of congenital or inherited causes that were more common during the first 14 yr of life. Another explanation might be confounding by unrecognized malformations of the kidney or urinary tract; this is also likely to be more important during the first 14 yr of life. Our main finding of increased ESRD risk associated with low birth weight, however, is unlikely to be explained by congenital malformations because low birth weight was significantly associated both with ESRD as a result of noncongenital causes and with ESRD as a result of glomerular disease. The latter is especially unlikely to be associated with congenital malformations. A possible interpretation might also be that a critical shortage of nephrons associated with low birth weight8–10 manifests itself particularly strongly in the first 15 yr of life. The pathophysiologic background for an increased risk for glomerular and hypertensive nephropathy may be that low birth weight is associated with a decreased number of hypertrophied glomeruli8–10 as well as mild to moderate elevations of BP.2,7 We would thus expect a stronger effect with advanced age. Because this study had no data on glomeruli number and BP levels before development of renal disease and had follow-up data only up to a maximum age of 38 yr, these issues could not be investigated further.
In this study, birth weight
4.5 kg was associated with an increased risk for ESRD in women. Previous studies showed associations between high birth weight and later development of autoimmune diseases,19–21 the metabolic syndrome,22 and ESRD caused by diabetic nephropathy.3 In the last study, birth weight
4.0 kg was significantly associated with development of ESRD as a result of diabetes. In this study, a similar association was found for birth weight
4.5 kg and seemed to be associated with an increased risk for both ESRD as a result of glomerular disease and diabetes. These differences might be explained by differences in study sample because the participants in the previous study were older and more often black (70% versus none), and diabetes was more common than in our study.3 Our study also analyzed ESRD as a result of glomerulonephritis as a separate outcome, which was not done in the previous study.3 The association between high birth weight and increased risk for renal failure should be investigated further.
This study included all children who were born between 1967 and 2004 in Norway and examined risk factors for development of ESRD in the period from 1980 to 2005. The major strength of our study is the complete national inclusion and registration of end points, yielding a large number of participants with prospective registration of birth related variables, allowing a cohort design of the study. Furthermore, the study used a hard outcome that is clinically useful and reliable. The main limitation is that it investigated risk factors for development of ESRD only up to a maximum age of 38 yr. Another limitation of the study is that ESRD is a rare end point, and despite the 2.2 million included individuals, only 314 men and 211 women developed ESRD. This is an adequate number for investigation of risk factors for our main outcome but too few to allow reliable analyses of risk factors for specific causes of ESRD. The previously mentioned study on low birth weight and ESRD, however, showed that low birth weight similarly predicted ESRD as a result of diabetes, hypertension, and other causes.3 Together with the results from this study, it therefore seems as though birth weight is similarly associated with different causes of ESRD.
The study is also limited by the fact that individuals were included from 1967 to 2004, whereas outcomes were not registered until 1980. This limitation was compensated for by two approaches and is unlikely to have affected the results significantly. First, those born after 1980 were analyzed separately, yielding similar results as when the whole period was analyzed. Second, the statistical method that was used did not include patients in the analyses until an outcome can actually be registered. Some individuals who developed ESRD may have emigrated after birth; their development of ESRD would therefore not be registered in the Norwegian Renal Registry. Because the total emigration rate from Norway during the study period was only 0.1% (data from the Norwegian Population Registry), we conclude that this is very unlikely to have affected the results. We therefore conclude that despite some limitations of this study, it has a robust design and the results are reliable.
This investigation is the first cohort study on birth weight and development of ESRD. We found that low birth weight was associated with a 70% increased risk for ESRD up to a maximum age of 38 yr. This confirms the hypothesis that low birth weight predisposes for renal failure, but the magnitude of RR is smaller than might be expected. Birth-related risk factors for different causes of ESRD seemed to be similar, but this finding should be confirmed in other studies including more individuals with ESRD.
| CONCISE METHODS |
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We included all children born alive in Norway between 1967 and 2004. Data on these children recorded in the Medical Birth Registry of Norway were linked with the Norwegian Renal Registry and the Cause of Death Registry using the national identification number. The outcome was development of ESRD, defined as starting long-term dialysis treatment or having a kidney transplantation. The study was approved by the regional ethics committee.
Exposure Variables
Birth weight was categorized into three categories using the gender-specific 10th and 90th percentiles (2.87 and 4.27 kg for men; 2.80 and 4.11 kg for women) as cutoffs. From 1967 through 1998, gestational age was based on the last menstrual period and from 1999 onward on routine ultrasonographic examination in gestational weeks 17 through 20. Gestational age was categorized into three categories: <37, 37 to 41, and
42 wk. Birth weight for gestational age was categorized into three categories (<10th percentile, 10 to 90th percentile, and
90th percentile) using previously published gender-specific reference values in Norway.24,25 We used two categories of congenital disorders: Congenital malformation of the kidney or urinary tract (denoted as congenital urinary tract abnormalities in text) and any other congenital malformation.
Maternal age at time of birth was categorized into three groups using 25 and 30 yr as cutoffs. Marital status was recorded as either single (divorced or not living with partner) or not single (married or living with partner). Three categories of birth order were used: 1, 2, and
3. Preeclampsia was defined as increased BP after 20 wk gestation (BP
140/90 mmHg or increase by 30/15 mmHg from BP before 20 wk gestation) and proteinuria. Maternal diabetes (type 1 or 2), kidney disease (kidney or urinary tract disease), rheumatic disease (autoimmune connective tissue disease or inflammatory arthritides), and essential hypertension were recorded when present before pregnancy.
Outcome Variables
The outcome was development of ESRD, and onset was defined as the date of starting long-term dialysis treatment or undergoing renal transplantation. Causes of ESRD were divided into five categories: Glomerular disease (primary glomerulonephritis, inflammatory vascular disease, and systemic autoimmune disease), interstitial nephritis (upper urinary tract infections and nephropathy as a result of drugs or toxins), congenital disease (congenital kidney or urinary tract malformations, cystic kidney disease, and other heritable causes of renal disease), diabetic nephropathy, and other causes (renal vascular disease, amyloidosis, rare conditions, and idiopathic ESRD). Individuals who did not develop ESRD were followed until December 31, 2005, or date of death.
Statistical Analyses
Data were analyzed in a cohort design with birth-related variables as exposure and ESRD as outcome variables. RR estimates associated with selected risk factors for ESRD were obtained by Cox regression analyses. Because some analyses showed different results for men and women, analyses were stratified for gender. Analyses of the total cohort were adjusted for gender. Because development of ESRD had not been registered between 1967 and 1980, children who were born during this period were left truncated in the survival analyses before 1980. We therefore used the counting process formulation of proportional hazards.26 This method did not include individuals in the analyses until an event could be registered (e.g., an individual who was born in 1975 would be included in the analyses at 5 yr of age and right-censored at 30 yr of age if the individual did not develop ESRD).
The analyses were performed with the statistical packages SPSS 14 (SPSS, Chicago, IL) and S-Plus 7.0 (Insightful Corp., Seattle, WA). Values are means ± SD or estimate (95% CI); P < 0.05 was considered statistically significant, and all tests were two tailed.
| DISCLOSURES |
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| Acknowledgments |
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Statistician Stein Atle Lie contributed with important statistical advice.
| Footnotes |
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| REFERENCES |
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