Bone mineral density and its influencing factors in children with idiopathic nephrotic syndrome: a prospective observational study

Abstract

Glucocorticoids are first-line therapy for children with idiopathic nephrotic syndrome (INS). These children are at risk of deranged bone metabolism and low bone mineral density (BMD). We studied 60 children with INS and divided them into two groups. Group 1 included 21 children (initial and infrequent relapsing) and group 2 included 39 children (frequent relapsing, steroid dependent and steroid resistant). Dual-energy X-ray absorptiometry of the lumbar spine was performed to assess BMD. Mean BMD Z-score was compared in both groups; this correlated significantly on univariate analysis with cumulative steroid dose, serum vitamin D levels and calcium supplementation. However, on multivariate analysis, serum vitamin D level was the only factor significantly predictive of low z-score.

Keywords

Bone mineral disease osteopenia osteoporosis glucocorticoids nephrotic syndrome dual-energy X-ray absorptiometry

Background

Nephrotic syndrome (NS) is a common renal disorder in children. It usually presents with nephrotic range proteinuria (> 40 mg/m²/h), hypoalbuminaemia (<2.5 g/dL), hyperlipidaemia and generalised oedema; the majority respond to steroid therapy,^1,2 with which >90% achieve disease remission; however, 80% will subsequently develop disease relapse.³ Owing to the prolonged use of steroids in children with idiopathic NS (INS) and the nature of the disease itself, they are at increased risk of metabolic bone disease (MBD).^3,4 In addition, calcium and vitamin D metabolism abnormalities have been observed in children with INS.^5,6 Despite having preserved glomerular function, they often have alterations in mineral homeostasis evidenced by low levels of vitamin D, calcium and elevated levels of parathyroid hormone (PTH), as children with chronic kidney disease (CKD).³ Because of the disease process, there is urinary loss of vitamin D metabolites and their binding proteins as well as malabsorption of calcium in the intestines.⁷ In addition, it has been documented that children with NS have decreased bone mineral density (BMD)⁸ with evidence of abnormal bone histology.⁹ Bone alteration ranges from osteomalacia to bone resorption resembling secondary hyperparathyroidism.⁹ The incidence of bone mineral disease in children with long-term steroid use is 30–50%.¹⁰ Some progress to CKD; therefore, early recognition and management may prevent bone changes and growth retardation.¹¹

Various adverse effects such as growth disturbances, obesity, diffuse lipomatosis¹² and increased risk of fractures are associated with steroid therapy in children with INS. Osteoblast activity and differentiation is also affected, which in turn decreases bone formation¹³ and causes increased bone resorption owing to increased apoptosis of osteoblasts and osteocytes.¹⁴ Steroids may also promote calcium and vitamin D metabolite loss through the gut and kidney. The resultant negative calcium balance enhances osteoclastic activity and bone remodelling.¹⁵

Children with NS are at increased risk of MBD even before steroid administration owing to loss of calcium, phosphate and vitamin D binding proteins in the urine; this is further aggravated by prolonged steroid use.^16,17

Changes in BMD due to steroid therapy are seen earliest in the lumbar spine where trabecular bone content is highest.¹⁸ Studies on BMD changes in patients with INS are few^19–21 and describe small numbers of children with short duration of steroid therapy, with opposing conclusions.^19–21 We therefore undertook this study to assess the influence of high dose and long duration of steroid therapy and influencing factors on BMD in children with NS.

Methods

We prospectively studied 60 consecutive children, aged 2–14 years with INS from January 2011 to March 2012, seen in the Paediatric Nephrology Clinic of a tertiary care hospital in western India. Institutional Ethical Committee approval was obtained.

After taking informed consent from parents, a detailed history and clinical examination was undertaken (specifically to look for clinical evidence of MBD as bone pain, tetanic spasms and history of fractures). Children were included who fulfilled International Study of Kidney Disease in Children criteria for the diagnosis of INS and had normal renal function. Those with abnormal renal function (glomerular filtration rate [GFR] < 90 mL/min/1.73 m²), who were taking immunosuppressive therapy other than steroids, with endocrine disorders, or on furosemide or thiazide therapy, were excluded from the study.

Children were treated with oral prednisolone according to protocol.^22,^a([1]) They were put in two categories, based on their response to steroid therapy on follow-up and divided into two groups: Group I = Initial episode, infrequent relapse (IFRNS); and Group II = Frequent Relapse (FRNS), Steroid-Dependent (SDNS) or Steroid-Resistant (SRNS) according to standard case definitions.¹ All children were assessed for daily additional supplementation with calcium and vitamin D (apart from dietary intake). The cumulative steroid dose was estimated.

The following tests were performed to confirm the diagnosis of INS: serum albumin; spot urine protein/creatinine ratio; and serum cholesterol. GFR was calculated using the Schwartz formula.²³ Serum calcium, phosphorus, vitamin D (25OH), PTH and alkaline phosphatase levels were taken to assess biochemical evidence of MBD.

BMD was estimated by dual-energy X-ray absorptiometry (DEXA) Hologic scan of the lumbar spine in anteroposterior (AP) and lateral views, on remission in relapsers and during alternate day therapy in steroid-resistant and steroid-dependent cases. Z-scores^1,a([2]) were calculated for BMD values of L1–L4 spines (AP view).

Mean and standard deviation were calculated for variables with normal distribution and Student’s t-test was used for parametric data. We compared the different parameters in the group administered with lower doses of steroids (Initial and IFRNS) with higher doses (FRNS, SDNS and SRNS). Pearson’s correlation coefficient test was used to assess the correlation between BMD Z-score and different variables. Subsequently, multivariate analysis was carried out to assess the factors predictive of a low BMD Z-score. Variables with a significant correlation were entered into a stepwise linear regression model. A P value < 0.05 was considered statistically significant.

Results

This prospective observational study was conducted at Sawai Man Singh Medical College, Jaipur, Rajasthan, India. During the 15-month study period, 69 children with NS were assessed for eligibility, of whom nine were excluded on the basis of congenital nephrotic syndrome (n = 1), immunosuppressive therapy (n = 3), secondary NS (n = 2), furosemide use (n = 2) and renal failure (n = 1).

Data are shown in Table 1.

Table 1.

Comparison of demographic, biochemical and radiographic characteristics of the two groups.

Variables (mean ± SD)	Group 1 (n = 21)	Group 2 (n = 39)	P value
Male	16	28
Female	5	11
Diagnosis
Initial episode	14	0
IFRNS	7	0
FRNS	0	21
SDNS	0	16
SRNS	0	2
Age (years)	5.5 ± 2.92	7.2 ± 2.77	0.02
Age at onset (years)	4.95 ± 3.35	3.87 ± 2.14	0.13
Weight (kg)	17.38 ± 6.40	21.92 ± 10.95	0.08
Height (cm)	105.8 ± 17.35	113.8 ± 16.66	0.08
BMI (kg/m²)	15.08 ± 1.89	16.23 ± 3.35	0.15
Calcium intake (mg/kg/day)	11.28 ± 8.93	16.28 ± 9.41	0.053
Cumulative steroid intake (mg)	2172 ± 1036	8191 ± 6721	0.0001
Duration of steroid intake (months)	4.02 ± 2.08	19.46 ± 9.01	0.0001
Serum total protein (g/dL)	4.58 ± 1.11	4.83 ± 0.72	0.36
Serum albumin (g/dL)	2.49 ± 0.88	2.53 ± 0.71	0.83
S. Urea (mg/dL)	25.42 ± 5.03	29.70 ± 10.7	0.04
S. Creatinine (mg/dL)	0.52 ± 0.13	0.58 ± 0.11	0.11
GFR (mL/min/1.73 m²)	104 ± 10	110 ± 18	0.15
Urine protein/creatinine ratio	10.58 ± 11.15	14.61 ± 9.7	0.18
Serum calcium (mg/dL)	7.94 ± 0.59	7.17 ± 0.86	0.001
S. Phosphorus (mg/dL)	4.32 ± 0.35	4.26 ± 0.61	0.62
Serum alkaline phosphatase (U/L)	240.8 ± 118.7	660.2 ± 153.1	0.0001
S. PTH (pg/mL)	52.39 ± 19.52	61.54 ± 31.68	0.17
S. Vitamin D (nmol/L)	24.47 ± 11.66	23.19 ± 13.14	0.71
BMD Z-score	−1.18 ± 0.99	−1.31 ± 1.13	0.66

SD, standard deviation; IFRNS infrequent relapsing nephrotic syndrome; FRNS, frequent relapsing nephrotic syndrome; SDNS, steroid-dependent nephrotic syndrome; SRNS, steroid-resistant nephrotic syndrome; BMI, body mass index; GFR, glomerular filtration rate; PTH, parathyroid hormone; BMD, bone mineral density.

There was no significant difference in mean weight, height, body mass index (BMI), daily calcium supplement, urine protein/creatinine ratio, serum total protein, albumin, creatinine, phosphorus, PTH, vitamin D and GFR between the two groups (P > 0.05), while group 2 had significantly higher cumulative steroid dose, duration of steroid intake, serum urea and alkaline phosphatase (P < 0.05) (Table 1). One patient in group 2 (SDNS) had tetanic spasms of the hands but none of the children in either group had signs and symptoms of bone pain. A total of 58/60 had insufficient vitamin D levels ( < 50 nmol/L).

The lumbar spine mean BMD Z-score was comparable in both groups (group 1: –1.18 ± 0.99, group 2: –1.31 ± 1.13; P = 0.66). A total of 38/60 had a normal Z-score (14 in group 1 and 24 in group 2), 13 had osteopenia (5 in group 1 and 8 in group 2) and nine had osteoporosis (2 in group 1 and 7 in group 2). None of the study participants had fractures.

On univariate analysis, we found that lumbar spine BMD Z-score correlated with cumulative steroid dose (r = –0.33, P = 0.01) (Figure 1a), serum vitamin D levels (r = 0.65, P = 0.0001) (Figure 1b) and daily calcium supplementation (r = 0.271, P = 0.018), but not with other parameters (Table 2). Multivariate analysis (Table 3), however, showed that serum vitamin D level was the only factor significantly predictive of low Z-score (P = 0.0001, r = 0.738). The variance inflation factor (VIF) was assessed for multicollinearity and found to be insignificant (<5). Keeping BMD Z-score as a dependent variable, the VIF and tolerance for phosphorus, PTH, alkaline phosphatase and vitamin D was 1.104 and 0.906, 1.108 and 0.902, 1.044 and 0.957, and 1.041 and 0.960, respectively.

Figure 1.

Correlation of lumbar spine BMD Z-score with (a) cumulative steroid dose (r = –0.33, P = 0.01) and (b) serum vitamin D levels (r = 0.65, P = 0.0001). ^a([1])First episode: prednisolone, 60 mg/m² daily for six weeks, followed by 40 mg/m² on alternate days for six weeks. Relapses were treated with 60 mg/m² until remission for three days, followed by 40 mg/m² on alternate days for four weeks.

Table 2.

Correlation of BMD z-score with various quantitative variables.

	BMD z score
Variables	Pearson correlation coefficient (r)	P value
Age	−0.006	0.96
Age at onset	0.06	0.63
Total calcium supplement	0.271	0.018
Cumulative steroid dose	−0.33	0.01
Steroid duration	−0.17	0.17
S. Calcium	0.24	0.06
S. Phosphorus	−0.11	0.41
S. Alkaline phosphatase	−0.25	0.052
S. PTH	0.11	0.4
S. Vitamin D	0.65	0.0001

PTH, parathyroid hormone.

Table 3.

Multivariate analysis to find out the predictors of low BMD Z-score.

Coefficients*
Predictors	Unstandardized coefficients (β)	t	Significance (p)	95% CI for β
Predictors	Unstandardized coefficients (β)	t	Significance (p)	Lower	Upper
(Constant)	−2.289	−1.350	0.183	−5.695	1.116
Age	−0.008	−0.785	0.438	−0.03	0.14
Age at onset	0.022	0.228	0.821	−0.169	0.213
Calcium supplement	0.027	1.795	0.079	−0.003	0.058
Cumulative steroid dose	−2.670 E-05 (−0.0000266)	−1.585	0.119	−0.00014	0.000091
Steroid duration	0.032	0.966	0.339	−0.034	0.097
S. Urea	0.012	0.812	0.421	−0.018	0.042
S. Calcium	0.090	0.482	0.633	−0.291	0.472
S. Phosphorus	−0.253	−0.968	0.34	−0.787	0.280
S. Alkaline phosphatase	−0.0001	−0.346	0.732	−0.001	0.001
S. PTH	0.006	1.232	0.227	−0.004	0.016
S. Vitamin D	0.049	4.97	0.0001	0.029	0.07

R value for model = 0.738; F = 2.642; P = 0.049 (analysis of variance).

*Dependent variable: lumbar spine BMD z-score.

CI, confidence interval; PTH, parathyroid hormone.

Discussion

Since steroids are the mainstay of treatment in children with INS, the side effects on bone mineralisation have to be considered of significant importance. Vitamin D deficiency is universal in some places¹⁷ and common in India;^25–29 its association with osteoporosis is suggested³⁰ though not always confirmed.³¹ The major factors are geographical location, poor dietary intake due to low socioeconomic circumstances, lack of outdoor activities and extremes of weather. Vitamin D deficiency is also prevalent in children with NS.³² In children with INS, loss of vitamin D binding protein further contributes.

We found that higher cumulative dose of steroids was associated with decreased BMD Z-scores and osteoporosis, as in similar studies,^19,33 in addition to low total calcium intake and older age of onset.³⁴ Other studies did not find such association,^20,21 which may be due to small sample size, variable characteristics of participants and disease, wide age range and variable exposure to steroids.

The Z-score is the best parameter to assess the BMD in growing children as it is matched for age, sex and height. Compared to cortical bone, trabecular bone is affected early by steroid-induced bone mineral disease. Thus, lumbar spine DEXA is a better indicator, compared to whole-body DEXA, to detect steroid-induced demineralisation.

The role of vitamin D and changes in BMD has been well documented in the literature.³⁴ Vitamin D deficiency in India is known to be widespread, and is an essential nutrient for optimal bone growth; potential adverse effects of over dosage supplementation must be kept in mind.^35,36

Our study supports the evidence that children with INS are at increased risk for osteoporosis. Therefore, regular monitoring of children with INS and estimation of BMD is important for early diagnosis and intervention.

A limitation of our study, despite its large study size, was that dietary vitamin D intake was not assessed and no control population was taken to compare children with INS and apparently healthy children. A large multi-centric, randomised control trial is needed to validate our observations.

Conclusion

Nearly all children with INS have insufficient vitamin D levels. Although cumulative steroid dose and daily calcium intake affect BMD in children with INS, serum vitamin D level is an independent predictor of BMD. We suggest regular BMD evaluation in children with INS, especially those on higher cumulative dose of steroids (FRNS, SDNS or SRNS), so that early diagnosis and appropriate intervention can be planned to prevent the complications related to steroid-induced loss of bone mass.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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