QJM Advance Access originally published online on June 27, 2005
QJM 2005 98(8):575-579; doi:10.1093/qjmed/hci092
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Hyperhomocysteinaemia and aortic calcification are associated with fractures in patients on haemodialysis
From the 1Division of Endocrinology and Metabolism, St Michael's Hospital, Toronto, Canada, and 2Department of Medicine, Epidemiology and Biostatistics, University of California, San Francisco, USA
Address correspondence to Dr S.A. Jamal, St Michael's Hospital, 61 Queen Street East, Suite 6113, Toronto, ONM5C 2T2, Canada. email: sophie.jamal{at}utoronto.ca
Received 8 February 2005 and in revised form 17 May 2005
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Background: Fractures and atherosclerosis are common in patients with renal failure; this may be due to hyperhomocysteinemia.
Aim: To examine the relationships between fractures, vascular calcification and homocysteine levels in haemodialysis patients.
Design: Cross-sectional survey.
Methods: We enrolled 37 men and 15 women who had been on haemodialysis for at least 1 year. We identified prevalent spine fractures by radiographs. Non-spine fractures were identified by self-report and confirmed by review of radiographs or radiology reports. We classified the presence and severity of lumbar aortic calcifications with lateral lumbar radiographs. We measured serum homocysteine in all subjects within 30 days of study entry.
Results: After adjusting for age and weight, increased levels of homocysteine were associated with an increased risk fracture (OR per mmol/l increase in homocysteine 1.6, 95%CI 1.2–2.0), as was the presence of aortic calcification (OR 1.6, 95%CI 1.2–2.1). Homocysteine and lumbar aortic calcification were highly correlated (r = 0.86).
Discussion: Hyperhomocysteinaemia may explain the relationship between fractures and atherosclerosis in patients with renal failure.
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Introduction |
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Vascular calcification is a recognized process in atherosclerosis. In healthy men and women, the association between vascular calcification and osteoporosis is consistent, strong and possibly causal.1 This relationship appears to be even stronger in men and women with renal failure. Compared with the general population, event rates for myocardial infarction and stroke are 5- to 10-fold higher, and risk of hip fracture is 17 times higher among patients with renal failure.2 One factor that may explain the high prevalence of fractures and extensive vascular calcification is elevated levels of homocysteine.
Hyperhomocysteinaemia has been consistently demonstrated in patients with renal failure.3 Homocysteine is an atherothrombotic amino acid, and hyperhomocysteinemia is a robust and independent risk factor for cardiovascular disease in patients with renal failure.4,5 In addition, hyperhomocysteinemia may, through interference with collagen cross-linking, weaken the bone and increase the risk of fracture.6 Indeed, in recent studies in otherwise healthy men and women, hyperhomocysteinemia was associated with an increased risk of fracture.7,8
The purpose of this study was to evaluate the cross-sectional relationships between fractures, vascular calcification and homocysteine levels in men and women on haemodialysis.
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Methods |
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Participants
We studied men and women aged 50 years and older with adult-onset renal failure who had been receiving haemodialysis for at least 1 year. We excluded subjects with prior renal transplants and women using hormone replacement therapy. Subjects were recruited over 5 months, from two haemodialysis centres in Toronto, Canada. We identified potential subjects by chart review. All measurements were obtained at a single site. All subjects gave informed consent. The study was approved by the appropriate institutional review boards.
Measurements
Chart review
We reviewed dialysis unit medical records for demographic data, dialysis frequency (3, 4, or 
5 days per week), aetiology of renal failure, and use of calcium salts, vitamin D sterols, and Replavite, a multivitamin that contains 1 mg folate, 10 mg B6, and 6 mcg B12. We recorded the most recent (within 1 week of study entry) post dialysis weight. Height was obtained at the time of BMD assessment. We also recorded values for: serum calcium, phosphate, intact parathyroid hormone (PTH), alkaline phosphatase and thyroid-stimulating hormone (TSH), obtained within 30 days of study entry.
Interviewer-administered questionnaire
We asked about age, race, current alcohol consumption, current caffeine consumption, current smoking, physical activity, and family history of osteoporosis (height loss during adulthood, dorsal kyphosis, or fractures in a first-degree relative). We inquired about fractures (see below), falls in the past 12 months, the presence of diabetes, vascular disease and previous parathyroid surgery. The questionnaire has been validated in several populations, including HD patients.9,10 We compared interviewer-elicited medical histories with charted histories; discrepancies were resolved by contacting the dialysis or primary care physician after obtaining the participant's consent.
Bone mineral density by dual X-ray absorptiometry
BMD at the lumbar spine (L1–4) and hip (total hip and femoral neck) was assessed by dual energy X-ray absorptiometry (DEXA) using a DPX-L absorptiometer (Lunar Corp.).
Low trauma fractures
We classified fractures as low trauma or traumatic, according to the World Health Organization definition.11 Low-trauma fractures are typically due to osteoporosis, and are the fractures we considered in our study. We confirmed all self-reported low-trauma fractures either by review of radiographs or review of the radiology report.
Vertebral fractures
Anteroposterior and lateral radiographs of the thoracic and lumbar spine were reviewed for prevalent vertebral fractures, using quantitative morphometry, by a single experienced radiologist who was blinded to the clinical history and results of BMD testing. A prevalent fracture was defined as a decrease of at least 20% and 4 mm or greater in any vertical dimension, compared with the vertebra above or below one or more thoracic and lumbar vertebrae.12
Lumbar aortic calcifications
We reviewed the lateral lumbar radiographs to classify the extent of lumbar aortic calcifications. The severity of the anterior and posterior aortic calcifications were graded individually on a 0–3 scale for each lumbar segment (L1–L4) and the results were summarized to develop the anteroposterior severity score (range 0—24).13
Serum homocysteine
We measured non-fasting, pre-dialysis plasma homocysteine with high performance liquid chromatography, using a standard method, in all subjects within 30 days of study entry.14 The normal range for serum homocysteine in our laboratory is 4–15 µmol/l.
Statistical analysis
We used bivariate analyses (t-test, 
2, or Fisher's exact test) to examine differences in demographics, lifestyle factors, medical history, cause of renal failure, duration of dialysis, serum biochemistry, bone mineral density, serum homocysteine and lumbar aortic calcification among subjects with and subjects without fractures. Subjects were classified as having fractures if they had a prevalent vertebral fracture by radiograph and/or a history of a non-spine fracture since starting dialysis. We analysed data separately, by gender, and compared the coefficients between models. We also included an interaction term for gender and tests of bone strength in all models.
We used logistic regression models, adjusted for age and weight, to examine the relationships between fractures and homocysteine levels and fractures and aortic calcification. We used receiver operating characteristic (ROC) curves to evaluate how well each of our models could predict the presence of fractures. All analyses used STATA Version 7.0 (STATA Corp.). Statistical tests were considered significant at a two-tailed level of 0.05, and were not adjusted for multiple comparisons.
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Results |
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We identified 118 eligible subjects, of whom 66 declined to participate (17 subjects had difficulty obtaining transportation, 49 were ‘too busy’), leaving 52 subjects. Of the 66 who declined, 61 (92%) allowed us to conduct a chart review to compare demographic characteristics of non-participants and participants.
The mean age was 65.8 ± 9.0 years and the mean weight was 72.3 ± 15.6 kg. Most subjects had been on dialysis for just under 3.5 years, had renal failure due to diabetic nephropathy, and received dialysis three times a week. Just over half reported a fall in the past year. Most subjects did not report angina, a history of coronary artery bypass surgery, balloon dilatation or stroke, and most took calcium, vitamin D and Replavite. None of these characteristics were associated with fractures. Corrected calcium, phosphate and alkaline phosphatase were in the normal range, and were not associated with fractures. Serum PTH was above the normal range (1.3–5.4 pmol/l) in all subjects. The mean level of PTH was not different among subjects with and without fractures, and PTH was not associated with lumbar aortic calcification. There was no difference in BMD at the lumbar, femoral neck, or total hip, among subjects with and without fractures. Compared to subjects without fractures, subjects with fractures had significantly higher levels of serum homocysteine and greater lumbar aortic calcification (Table 1). Subjects who declined to participate were more often women (35/61, 57%) than those who agreed (15/52, 29%; p < 0.001). There were no other significant differences.
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Serum homocysteine and aortic calcification were highly correlated (r = 0.86) and were not included in the same regression models. After adjusting for age and weight, every 1 mmol/l increase in serum homocysteine increased the risk of fracture by 60% (OR 1.6, 95%CI 1.2–2.0); there was no evidence of a threshold relationship between serum homocysteine and fractures. After adjusting for age and weight, every 1 unit segment increase in lumbar aortic calcification increased fracture risk by 60% (OR 1.6, 95%CI 1.2–2.1). Further adjustments for diabetes, the presence of vascular disease, serum PTH and BMD (lumbar spine and total hip) did not substantially alter our results (data not shown). There were no differences in fractures by gender, nor were there interactions between gender and any of the predictor variables; combined results are presented.
ROC curves demonstrated that both serum homocysteine and the presence of lumbar aortic calcification were able to discriminate equally well between subjects with and without fractures, across a range of values (area under the curve, AUC, for homocysteine = 0.95; AUC for aortic calcification = 0.96; p = 0.92) (Figure 1).
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Discussion |
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Hyperhomocysteinaemia and lumbar aortic calcification were strongly correlated and were both associated with fractures in patients with renal failure. Our data are consistent with the hypothesis that increased levels of homocysteine may be the precursor to both atherosclerosis and osteoporosis in patients with renal failure. Of note, elevated levels of PTH are also associated with osteoporosis and vascular calcification in patients with renal failure.15–17 However, we did not find an association between serum PTH and either fracture or lumbar aortic calcification.
Our estimates of increased fracture risk in renal failure patients with high homocysteine are consistent with the increased fracture risks recently reported in large longitudinal cohort studies of otherwise healthy men and women with elevated homocysteine.7,8 The lack of association between homocysteine, fracture, and BMD (which reflects the quantity of mineralized bone) is not surprising. Elevated levels of homocysteine are purported to impede collagen cross-linking in bone, which may interfere with bone microarchitecture, impair bone quality and increase fracture risk, independent of bone quantity or BMD.
Our study has limitations. The cross-sectional study design prevents conclusions concerning the causal relationships between hyperhomocysteinemia, aortic calcification and fractures. We did not have measures of serum folate, which may influence homocysteine levels. The small sample size both limits the generalizability of our positive findings and raises the possibility that our negative findings may be due to lack of statistical power.
Longitudinal studies are needed to explore the relationship between homocysteine, fractures and aortic calcification. Studies are also needed to determine whether interventions designed to lower homocysteine levels, such as the administration of folic acid, either alone or combined with vitamin B6 or B12,18–20 result in a decreased incidence of fracture and atherosclerosis in patients with renal failure.
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Acknowledgments |
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This work was supported by grants from the Dean's Fund and the Connaught New Staff Matching Grant, both from The University of Toronto, Toronto, Ontario, Canada.
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