Blog entry by Arif Khwaja

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by Arif Khwaja - Friday, 13 July 2012, 10:34 AM
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Both chronic kidney disease (CKD) and osteoporosis are emerging as major public health problems associated with an ageing population. Osteoporosis is characterised by reduced bone strength resulting in an increased predisposition to fragility fracture. Bone strength itself is a reflection of both bone mineral density (BMD) and bone quality. There is significant co-prevalence of osteoporotic fractures and CKD in the elderly population. The qualitative abnormalities in bone in CKD are reflected in the fact that fracture risk is much higher in both the dialysis and the non-dialysis CKD population and the outcomes of fractures are significantly worse with a 2 to 3 fold increase in mortality after hip fracture in the CKD population compared to the general population.

However the management of bone disease in CKD remains uncertain - the fundamental problem being that we simply do not know whether achieving biochemical 'targets'  (say PTH, calcium phosphorus) or whether the use of therapeutic agents (such as bisphosphonates, calcimimetics or vitamin D)  have any impact on outcomes that matter to patients - like death or fracture rates. Furthermore apart from bone biopsy (which is rarely performed) we don't have any tools to assess bone quality and predict fracture risk.

A recent paper from Yenchek and colleagues is published in this months cJASN and is the first prospective, longitudinal study to specifically test the hypothesis that bone mineral density (BMD) predicts fracture risk irrespective of CKD status. In a cohort of 2754 (of whom 587 had CKD stages 3-5) patients aged between 70-79 years old data was collected on incident fractures over a follow up of 11 years. There were 384 incident fractures and even after adjustment for factors such as age, sex, race, BMI, vitamin D and PTH, low femoral neck BMD as measured by DXA (Dual Energy X-Ray Absorptiometry)was associated with fracture risk irrespective of CKD status. The authors state "This argues against the current KDIGO guideline recommendations and suggests that there may be a role for DXA screening in CKD."

So should we start using DXA to identify fracture risk in CKD and prescribe anti-resorptive therapy to those with reduced BMD? Well I'm not sure I share their view and it is worth considering the following:

i) The study by Yenchek and colleagues is important but only 4% of patients had CKD4/5 so the jury remains out on whether DXA will be useful in CKD4/5

ii) We know from post-hoc analysis from the large registration studies that both Bisphosphonates and Denosumab can reduce fracture risk in osteoporotic patients with CKD3 and CKD4 (down to an eGFR of around 20mls/minute). However, it is worth noting the all patients with abnormal PTH or vitamin D levels were excluded from these studies. Therefore whilst these drugs probably reduce fracture risk in patients with CKD3/4 with 'pure' osteoporosis we simply do not know whether they have any impact in those with evidence of coexistent renal osteodystrophy.

iii) Whilst DXA may identify fracture risk in CKD3 a careful cost/benefit analysis study would need to be performed before such screening could be recommended as routine - indeed it may be cheaper just to focus on non-pharmacological strategies such as reducing risk of falls and improving neuromuscular strength.

iv) A key challenge is how to develop the evidence base for reducing fracture rates in CKD 4 and 5. In the general population huge studies with thousands of patients were required to show a beneficial effect of anti-resorptive therapy. The reality is that it will be almost impossible) to carry out such large studies in the CKD4/5 population - therefore perhaps a more realistic goal is to develop validated surrogate markers of fracture risk in this population (be they biochemical markers of bone turnover combine with bone biopsy data or imaging such as high resolution CT) and then evaluate whether modifying these factors can impact of fracture rates.

 

References

 

  1. Yenchek RH, Ix JH, Shlipack MG, Bauer DC, Rianon NJ, Kritchevsky SB, Harris TB, Newman AB, Cauley JA, Fried LF, for the Health, Aging, and Body Composition Study: Bone mineral density and fracture risk in older individuals with CKD. Clin J Am Soc Nephrol 7: 1130–1136, 2012
  2. Miller PD, Roux C, Boonen S, Barton IP, Dunlap LE, Burgio DE: Safety and efficacy of risedronate in patients with age-related reduced renal function as estimated by the Cockcroft and Gault method: a pooled analysis of nine clinical trials. J Bone Miner Res 20: 2105–2115, 2005
  3. Jamal SA, Bauer DC, Ensrud KE, Cauley JA, Hochberg M, Ishani A, Cummings SR: Alendronate treatment in women with normal to severely impaired renal function: an analysis of the fracture intervention trial. J Bone Miner Res 22: 503–508, 2007
  4. Jamal SA, Ljunggren O, Stehman-Breen C, Cummings SR, McClung MR, Goemaere S, Ebeling PR, Franek E, Yang YC, Egbuna OI, Boonen S, Miller PD. J Bone Miner Res. 2011 Aug;26 (8): 1829-35
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