Background/objective: Reduced bone mineral density is a feature of patients with reduced mobility. The aim of this study was to assess bone mineral density in children with spinal muscular atrophy (SMA) and to evaluate bone mineral density in relation to age and motor disability.
Patients and methods: We analysed bone mineral density measurements on twelve patients (4 with SMA type II, mean age 8.2 years [range 6.2 - 11.8]; 8 with SMA type III, mean age 11.8 years [range 5.5 - 20]). Dual-energy X-ray absorptiometry (DXA) was used to determine total body bone mineral density. The results were matched with published normative data for age and sex for a white Caucasian population.
Results: The total body bone mineral density values were in the normal range in 10 out of the 12 SMA patients studied, all below the age of 17 (mean age 8.8 years [range 5.5 - 16.33]). Four of them had SMA II and six had had SMA III and were still ambulant. Total body bone mineral density was, however, below 2 SD in the remaining 2 patients aged 19.7 and 20 years, respectively. Both had SMA III but had lost independent ambulation for a period of 3.5 years.
Conclusion: Our results suggest that bone mineral density was surprisingly normal in most of the young SMA children studied. This is in contrast to what is reported in other conditions characterised by reduced mobility such as Duchenne muscular dystrophy. However, there was a tendency to decreasing bone mineral density with increasing age, not always related to the ability to walk, with the two eldest patients having the lowest values in spite of a relatively good mobility. These findings suggest that factors other than mobility are likely to have an effect on SMA bone mineral density.