The Norwegian Kidney Register, 1 Institute of Medicine and 2 Department of Pathology Gade Institute, Haukeland University Hospital, Bergen and 3 Department of Medicine, University Hospital of Trondheim, Norway
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Abstract |
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Methods. We retrospectively examined 102 patients who were diagnosed by kidney biopsy in Norway between April 1988 and December 1990. All patients were followed by means of registries for 13 years to describe renal and patient survival.
Results. The age of the patients at the time of biopsy was 55±15 years (range 1588 years). Three years after the time of biopsy, 18% had developed ESRF and 24% had died; the corresponding numbers 13 years after biopsy were 32% and 47%. By KaplanMeier analyses, the following variables indicated short-term progression to ESRF: serum creatinine 200 µmol/l, systolic blood pressure
160 mmHg and proteinuria
1 g/24 h. In addition, patients with increased diastolic blood pressure, increased age and decreased serum albumin tended to develop ESRF more often. Long-term predictors of ESRF in KaplanMeier analyses were increased serum creatinine and urinary protein. Independent risk factors for progression to ESRF were increased serum creatinine and increased urinary protein. Independent risk factors for death were increased age and increased serum creatinine.
Conclusions. Benign nephrosclerosis is a common condition that is associated with a high morbidity and mortality. Short-term predictors of ESRF differ from long-term predictors and this may reflect a pathophysiologically meaningful difference.
Keywords: chronic renal failure; hypertension; kidney biopsy; mortality; risk factors
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Introduction |
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Characteristic features of hypertensive nephrosclerosis are hypertension followed by a decrease in glomerular filtration rate. Proteinuria and haematuria are other common findings [10]. Morphologically, BN is characterized by arteriolar hyaline degeneration and sclerosis, especially seen in preglomerular vessels, with narrowing of the vascular lumen followed by glomerular and tubulointerstitial ischaemia [11]. Potential risk factors for progression to ESRF include black race, cigarette smoking, increased age, family history of ESRF, microalbuminuria, increased blood pressure, decreased response to antihypertensive treatment and history of urinary tract abnormalities [3,1214].
The purpose of the present study was to retrospectively describe prevalence and characteristics of patients with biopsy-proven BN in Norway in the years 19881990. We also wanted to investigate short-term and long-term clinical prognostic factors for ESRF and death.
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Subjects and methods |
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The primary endpoint in the present study was ESRF, defined as chronic renal failure necessitating chronic dialysis treatment or renal transplantation. The secondary endpoint was patient death. Two methods of follow-up were used. First, all patients were followed until 1 January 2002 and data for patient survival and ESRF were obtained from the Norwegian Population Register and the Norwegian ESRD Register. Second, we managed to follow 84 (82%) patients by review of the individual patient's hospital records for 3 years after biopsy and information about blood pressure and serum creatinine were obtained. The results presented in this paper were mainly obtained by the first method.
Histopathological diagnosis
Standard methods of processing and staining of biopsy tissue were used. All specimens were examined by light microscopy and immunohistochemistry (staining for IgG, IgA, IgM, C3 and C1q). When considered necessary, electron microscopy was applied to exclude other types of renal disease.
BN was diagnosed in accordance with the criteria given in the WHO monograph of renal disease [11]. Light microscopic observation of hyaline arteriolosclerosis, especially of the afferent arteriole, is characteristic of BN. The arteriolar wall is thickened by homogenous eosinophilic material that narrows the lumen. Arteries may show medial hypertrophy, subsequent intimal sclerosis and reduplication of elastic laminae. There is a variable degree of glomerular capillary wrinkling, mesangial matrix increase, glomerular capillary collapse and glomerular sclerosis associated with focal tubular atrophy with interstitial fibrosis. Areas with clusters of sclerotic glomeruli and tubulointerstitial scarring become more extensive with progression of the disease. Cases complicated by other kidney diseases, like glomerulonephritis, malignant hypertension and interstitial nephritis, were excluded from the study.
Clinical and laboratory investigations
Baseline was defined as the time of biopsy, when standard clinical and laboratory tests were performed. In this paper we have focused on the following baseline variables: age, sex, serum creatinine, urinary protein, blood pressure and serum albumin. Unfortunately, all baseline variables had not been recorded for all patients and, especially, urinary protein and serum albumin were missing in some patients. Dates of death, start of dialysis or kidney transplantation were recorded. For use in statistical analyses, continuous variables were converted to ordinal as shown in the Results' section. Values of serum creatinine were considered normal when <110 µmol/l (1.24 mg/dl) for women and <125 µmol/l (1.41 mg/dl) for men (guidelines at Haukeland University Hospital). They were regarded as moderately increased if they were higher than normal but <200 µmol/l (2.26 mg/dl) and largely increased if >200 µmol/l.
Statistical analysis
The SPSS package was used for statistical analysis. Data are presented as means±SD for normally distributed continuous variables and as median (range) for continuous variables not having a normal distribution. Significance testing was performed by Student's t-test and MannWhitney test. Differences of proportions between patient groups were compared by the chi-square test. A P-value of <0.05 was considered statistically significant and all tests were two-tailed.
Renal survival for subgroups of patients was described with the KaplanMeier method and the analyses were censored for death. Log-rank tests were performed to estimate P-values of differences between subgroups. These were performed separately for renal and patient survival and separately for the first 3 years after biopsy and for the entire follow-up period. The latter was performed to estimate both short-term and long-term P-values of differences. The Cox proportional hazard regression model was used to evaluate the effects of the individual baseline variables on progression to ESRF or death. These analyses were applied only to the entire follow-up period. The analysis yields estimated relative risks (hazards ratio) for each unit change in the included variables. This model controls for differences in other included variables possibly affecting disease progression, generating independent risk factors for progression [15]. The following variables were entered into the Cox model: continuous values of age, proteinuria, serum albumin, systolic blood pressure, diastolic blood pressure and a logarithmically transformed serum creatinine and the dichotomous variable sex. The analyses were done in a backward stepwise manner. The assumption of time-independent covariates was tested with covariate adjusted loglog plots and the assumption was met. The maximum number of patients was included in each step and at the last step all variables were significant (P<0.05).
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Results |
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Thirty-eight patients (37.6%) received antihypertensive treatment at the time of biopsy. Patients receiving antihypertensive treatment at the time of biopsy had significantly higher diastolic blood pressure than patients not receiving any antihypertensive treatment. By the second method of follow-up (median follow-up of 30.1 months), information about antihypertensive treatment and blood pressure measurements were available for 76 patients. Sixty (79%) of these received antihypertensive treatment during follow-up. Patients that received antihypertensive treatment during follow-up were significantly older and had higher baseline proteinuria and higher baseline serum creatinine, but had similar baseline blood pressure levels compared with patients not receiving antihypertensive treatment. Compared with the patients who did not receive antihypertensive treatment, patients that received treatment had a significantly reduced systolic blood pressure during follow-up (P<0.01) and a non-significant lowering of their diastolic blood pressure. The risk of development of ESRF or death was similar whether the patients received blood pressure treatment or not.
At baseline, values of logarithmically transformed serum creatinine correlated significantly with urinary protein. Values of systolic blood pressure correlated with values of age, diastolic blood pressure and urinary protein. Values of serum albumin correlated inversely with values of logarithmically transformed serum creatinine, urinary protein, diastolic blood pressure and age.
Overall prognosis
As shown in Figure 1, both renal and patient survival showed a steep decline during the first 3 years; thereafter the decline diminished. We therefore decided to study separately the first 3 years after biopsy and the entire follow-up period.
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Predictors of progression to ESRF
Cumulative probabilities of renal survival for all subgroups of patients are given in Table 2. Patients with increased serum creatinine and urinary protein had a significantly higher risk of progression to ESRF during the entire follow-up period. Patients with increased systolic blood pressure significantly more often developed ESRF during the first 3 years after biopsy but had similar risk of developing ESRF after 13 years of follow-up. Increased diastolic blood pressure, increased age and decreased serum albumin also tended to predict ESRF during the first 3 years of follow-up, whereas after 13 years they showed no effect.
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Discussion |
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The major strength of our study is the fact that we studied all Norwegian patients who were diagnosed with BN on biopsy and that we followed these patients for 13 years. Similar studies have, to our knowledge, never been performed before. We managed to follow up all patients for 13 years by means of registry data. In addition, we followed 84 patients by means of hospital medical records. No endpoints have been missed. Missing baseline variable values decreased the number of patients available for the analyses. The effect of this was probably of minor importance.
We have shown that patients with biopsy-proven BN constituted 8.7% of the total number of patients having a renal biopsy in Norway, representing an annual incidence of about nine patients per million. These numbers seem to be higher than those reported from Denmark where BN constituted 2.1% of the total number of biopsies, an incidence of 0.8 per million per year [16] and from the UK where BN constituted 2.5% of the biopsies [10]. This difference may be due to different indications for kidney biopsy in Norway and one possible explanation could be that most patients in Norway will become biopsied if they have a proteinuria >1 g/24 h or a serum creatinine >150 µmol/l and a renal parenchymal cause is suspected. During the follow-up period, 18% of the patients who progressed to ESRF in Norway suffered from renal vascular disease (EDTA codes 7072); an annual incidence of 12 per million (unpublished data). During the same period, essential hypertension was assumed to be the cause of ESRF in 1315% of English and Spanish patients [1] and 28% of American patients [17]. The higher number of patients reported to develop ESRF from hypertension than actually diagnosed with BN at biopsy seems reasonable when taking into account that patients with presumed hypertensive nephrosclerosis traditionally are not biopsied. It should be remembered that our patients were not selected for kidney biopsy because hypertensive nephrosclerosis was the most probable cause, but because a glomerular disease was suspected. Our patients may therefore represent cases with more advanced hypertensive nephrosclerosis.
The present study has shown that BN is associated with severe morbidity and mortality. BN is recognized as a disease with steady progression towards ESRF and the fact that patients with largely increased serum creatinine develop ESRF earlier than patients with normal or moderately increased serum creatinine is, therefore, of no surprise. However, the progression towards ESRF seems to be slow for patients with BN, considering that no patients progressed from a serum creatinine <200 µmol/l to ESRF during the first 5 years after biopsy. The baseline serum creatinine value seems, therefore, to be a very important indicator of how far the disease has progressed and for the potential of further renal damage. Increased urinary protein independently predicted ESRF. This observation may support the theory that proteinuria is toxic to the kidney and may be an important mediator for development of ESRF [18].
The most important risk factors for death were older age and increased serum creatinine. The fact that older patients have an increased mortality is of no surprise and the increased mortality associated with renal failure is well known.
Whereas the effects of increased serum creatinine and urinary protein on ESRF seemed to increase during follow-up, increased blood pressure, increased age and decreased serum albumin all tended to predict ESRF during the first 3 years, after which the predictive effect of these variables was lost. There may, therefore, be a qualitative difference between long-term predictors and those only predictive during the first 3 years. As both increased systolic blood pressure and decreased serum albumin were associated with older age, one explanation might be that older patients develop ESRF at a higher rate because of less functional renal reserve [19], more advanced arteriolosclerosis [20] and decreased response to antihypertensive treatment [14]. The latter may be supported by the fact that the effect of short-term risk factors seemed to disappear after 3 years and the general knowledge that older patients with systolic hypertension are bad responders to antihypertensive medication. The qualitative difference between short-term and long-term risk factors suggested above may also represent the fact that whereas increased proteinuria and serum creatinine may reflect increased renal damage, hypertension, old age and hypoalbuminaemia may reflect decreased defence mechanisms against progression of renal failure. This remains to be explored further.
Antihypertensive treatment was given to 37% of the patients at the time of biopsy and this increased to 79% during follow-up. The fact that 62% of the patients did not receive antihypertensive treatment at baseline despite being diagnosed with BN may have at least two explanations. First, BN may arise in susceptible persons even with near-normal levels of blood pressure, due to renal susceptibility genes [12], age [20] or atherosclerosis. Second, patients with neglected or badly controlled blood pressure probably have an increased risk of developing BN. Follow-up data for antihypertensive treatment were only available for 76 patients during the first 3 years. This severely limited the possibility of discussing the effect of treatment in our study and this was therefore omitted. However, one comment should be made: although the 60 patients receiving antihypertensive treatment were older, had higher proteinuria and higher serum creatinine, they had similar risks of progression to ESRF or death compared with the 16 patients not receiving antihypertensive treatment. These data may indicate a benefit of antihypertensive treatment.
In conclusion, BN is associated with high mortality and morbidity and the vascular changes seen in BN are markers of a generalized disease in these patients. It seems reasonable to recommend close clinical follow-ups of patients with high blood pressure, particularly if these patients have the risk factors demonstrated in this study, such as increased serum creatinine, increased urinary protein, old age and decreased serum albumin. Early start and vigorous treatment of hypertension may improve prognosis. We have also demonstrated the difference between long-term and short-term risk factors and this should be considered in other studies of prognostic factors for ESRF.
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Acknowledgments |
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Notes |
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References |
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