source: Bone Abstracts
Nicola Crabtree, Wolfgang Hogler, Helen Roper, Nicholas Shaw
Current guidelines recommend annual assessments of bone densitometry in boys with Duchenne muscular dystrophy (DMD). However, this recommendation is based on the assumption that bone density is a predictor of fractures in this patient group. The aim of this study was to evaluate the relationships between long-term changes in bone density, corticosteroid exposure and mobility with vertebral and long bone fractures.
Twenty-four DMD boys (mean age 10.1 (S.D. 2.4) years) with at least six annual DXA assessments were included in the study; each boy had three measures whilst ambulant and three measures once ambulation had ceased. A repeated measures model was used to compare size adjusted lumbar spine BMD (BMAD), total body less head BMD (TBLH BMD), lean body mass (LBM) and corticosteroid (CS) cumulative exposure with fractures and mobility.
Over 5 years, nine long bone fractures were reported in 8 boys and 41 vertebral fractures in 14 boys, of which 6 and 4 respectively, occurred after loss of ambulation; only 7 boys (29%) remained fracture free. At baseline, no differences were seen between the fracture and non-fracture groups for height, LBM and BMAD. Boys who developed vertebral fractures were heavier (P=0.04) and had a higher CS exposure (P=0.02) whilst those who developed long-bone fractures were lighter (P=0.04) and had lower TBLH BMD (P=0.05). BMAD, TBLH BMD, & LBM Z-scores declined consistently over the measurement time frame but the rate of decline was greatest once ambulation ceased (P<0.001). There was a significant positive interaction between CS exposure and vertebral fracture but this was not seen in those who developed long bone fractures.
The only distinguisher of long-bone fractures was low TBLH BMD whereas vertebral fractures were not associated with low BMAD, TBLH BMD or rate of loss of bone density. Cumulative CS exposure was associated with vertebral fractures but not long-bone fractures. Both fracture types were more likely after loss of mobility. This dichotomy between bone density as assessed by DXA and fractures may be potentially misleading when monitoring bone health in boys with DMD. Current guidelines should be revised to reflect these issues.
Birmingham Women’s and Children’s NHS Foundation Trust, UK; Institute of Metabolism and Systems Research, UK; Heart of England NHS Foundation Trust, UK