Bone Abstracts

Hypophosphatasia (HPP) is a rare genetic disease characterised by low tissue-nonspecific alkaline phosphatase activity, causing defective mineralisation of bone and teeth. There is limited data on the measurement of bone mineral density (BMD) and body composition in these children.

Objectives: To assess whether BMD and lean body mass (LBM) in treatment naïve children with HPP correlate with functional outcomes using the 6-minute walk test (6MWT).

Methods: A retrospective review of children ≥5 years with a diagnosis of HPP who underwent dual-energy x-ray absorptiometry (DXA) ± peripheral quantitative computed tomography (pQCT) of the radius at a tertiary paediatric centre was conducted. Bone mineral apparent density (BMAD), LBM and bone mineral content (BMC) were measured by DXA and volumetric BMD (vBMD) at distal radius (4% site) by pQCT.

Results: A total of 13 treatment naïve patients with HPP were identified (mean age 7.9±2.8 years, 9 female), of whom 9 were childhood onset, 3 infantile and 1 odonto HPP. Mean z-scores for total body less head BMD and Lumbar Spine BMAD were within normal range (−0.9±0.8 and −0.2±1.3, respectively). The mean centiles for LBM for height and BMC for LBM (64±29 and 38±33, respectively) were also normal. Three children had mild (<25%) vertebral height reductions in a maximum of 3 vertebrae. Seven HPP children had pQCT measured at distal radius with normal mean trabecular vBMD Z-score (−0.6±1.0). Age, sex and height specific 6MWT centiles were determined in all 13 children with a substantially low mean z score of −2.48±1.06. Mean 25 hydroxy vitamin D level at the time of DXA scan was 44.5 nmol/l.

Conclusion: Despite normal BMD, trabecular vBMD on bone imaging and LBM, our cohort of HPP patients demonstrated substantial reduction in functional exercise capacity. Proximal myopathy with microscopic muscle-fibre abnormalities has previously been reported in children with HPP which may explain the dissociation between bone imaging and functional outcomes.

Disclosure: The authors declared no competing interests.