Bone Mineral Density and its Influencing Factors in Chinese Children with Spinal Muscular Atrophy types 2 and 3

Background: Patients with spinal muscular atrophy (SMA) are at risk of decreased bone mineral density (BMD). The bone status of Chinese patients with SMA has been poorly studied. We aimed to describe bone health of children with SMA types 2 and 3 in mainland China and to investigate the associated factors that influence BMD. Methods: : Forty patients with a mean age of 5.5 years affected by SMA types 2 and 3 (n = 22 and n = 18, respectively) were enrolled between September 2017 and May 2019. Total body less head (TBLH) BMD, lumbar spine (LS) BMD, and body composition were measured using dual-energy X-ray absorptiometry. Serum bone metabolism markers and complete spinal radiographs were assessed. We utilized a linear regression model to explore the correlations between BMD and its related factors. Results: : A total of 67.5% (27/40) patients were diagnosed with low BMD, 2.5% (1/40) were diagnosed with osteoporosis, and 5.0% (2/40) had low-trauma fractures. The mean Z-score was -3.0 ± 1.8 for TBLH BMD and -1.3 ± 1.4 for LS BMD. The TBLH BMD and LS BMD Z-scores in children with SMA type 2 were significantly lower than those with SMA type 3 (t = 3.344, P = 0.002; t = 3.266, P = 0.002, respectively). Vitamin D insufficiency and deficiency was found in 37.5% (15/40) of the patients. Phenotype and serum parathormone (PTH) levels were the factors associated with the TBLH BMD Z-scores (t = 2.847，P = 0.008; t = 2.572, P = 0.015, respectively), and the phenotype severity was the factor associated with the LS BMD Z-scores (t = 2.762, P = 0.009). Conclusions: : Low BMD was highly prevalent in mainland Chinese children with SMA type 2 or 3. Phenotype and serum PTH levels were the factors associated with BMD. More than one-third of our patients also had vitamin D insufficiency or deficiency, which indirectly affected BMD despite their serum PTH levels being normal. We consider it to be necessary to carry out bone health screening in this population.