Maria Luisa Bianchi1, Francis H. Glorieux2, in Pediatric Bone Second Edition , Duchenne muscular dystrophy DMD, an X-linked recessive disorder due to a mutation of the dystrophin gene at locus Xp21 in the short arm of the X chromosome , is the most frequent muscular disease in childhood.
The severe reduction or absence of dystrophin, depending on the type of mutation, leads to the progressive degeneration of striated muscle, which is slowly substituted by fat and connective tissue. The process is characterized by a massive elevation of creatine kinase levels in the blood. The prevalence of DMD is estimated in 1: Proximal muscle weakness becomes evident in early childhood usually before 3 years of age and after a few years the child becomes progressively unable to walk.
Generally, ambulation is completely lost at around 15 years of age. There is no definitive cure and, among the different drugs used to treat DMD, treatment with glucocorticoids GCs has clearly changed the course of the disease. GCs improve muscle strength, maintain cardiopulmonary function, prolong ambulation by 2 to 5 years, reduce the severity of scoliosis, and significantly extend life [,].
In recent years, different types, doses, and schemes of GC administration have been evaluated in clinical trials with the aim of reducing side effects such as weight gain, cataracts, impaired growth, and behavioral changes , but no consensus has ever been reached. In clinical practice, GCs are currently started at a much earlier age than in the past, since the sooner they are given, the greater their effectiveness in slowing the disease progression.
Fractures have been recognized as a complication of DMD many years ago, and since the mids, fractures of the long bones have been considered a serious obstacle in maintaining independent ambulation in these patients . In recent years, the presence of low bone mass has been frequently reported, leading to a greater awareness of bone problems.
However, prospective epidemiological studies on fractures in large cohorts of patients on GC treatment or not are still lacking. In a year long study, Biggar et al. However, vertebral fractures were not evaluated in these two studies. This study showed two other relevant aspects: Another study on a large DMD population patients, age 1—25 years, median age 12 years found that Falling was the most common cause.
Data on vertebral fractures are even less. In a retrospective study on DMD boys, the GC-treated subjects had an increased fracture rate, long-bone fractures being 2.
Vertebral fractures were also observed in a study on 33 boys with DMD: Significant decrements in bone mass occurred over the study period . On the contrary, a recent study, that used DXA to evaluate 25 boys mean age 7. Regarding bone density in DMD, there are only a few published studies. Finally, the relationship between bone density, mobility, and fractures has been poorly studied. Regarding the pathogenesis of osteoporosis in DMD, many factors have been considered but it is not clear whether the muscle dystrophy itself or other factors contribute to the weakening of bone.
Muscle impairment significantly reduces weight-bearing activities during the crucial period of growth, and this can profoundly influence bone development and bone density. Animal studies have demonstrated that lean mass muscle is a significant determinant of BMD and bone strength, even when accompanied by a dystrophic phenotype . In a study on 32 DMD boys, there was a significant BMD reduction at lower limbs and LS with respect to healthy controls, interpreted as the consequence of reduced weight-bearing and muscular activity on bone .
An increased frequency of fractures has been observed with GC treatment , even if short-term studies on deflazacort suggested that deflazacort may have a bone sparing effect in children . On the other hand, GCs may also have an indirectly positive influence, by preserving muscle function and its stimuli on bone for a longer time. Recently, some preliminary studies discovered an involvement of cytokines, in particular TGF-beta and connective tissue growth factor CTGF , in the replacement of the dystrophic muscle with fibrotic tissue [,].
Knowing the relevant role of cytokines in bone metabolism, it will not be a surprise if, within a few years, a cytokine involvement in the pathogenesis of bone loss in DMD is discovered. Considering the relatively recent interest in the bone complications in DMD and the lack of randomized placebo-controlled trials, the treatment of osteoporosis and fractures in DMD is currently based only on observations from clinical practice, general rules applied in other forms of secondary osteoporosis, and the few available studies, all on small patient samples .
A recent review of nutrition in DMD highlighted the importance of calcium and vitamin D supplements . For the moment, only one study demonstrated that 25OHD calcifediol plus adequate dietary calcium intake seems to be effective in controlling bone turnover, correcting vitamin D deficiency, and increasing BMC and BMD in patients with DMD.
Currently, BP treatment with different drugs pamidronate, risedronate , dosages, and administration routes i. Considering the relevance of bone problems in DMD, and the fact that fractures are a cause of premature loss of ambulation, fracture prevention in these patients is now recognized as a primary health issue. The development of novel therapies such as mesenchymal stem cell therapy , and the continuous progress in understanding the pathogenetic mechanisms of bone loss underlines the necessity of specially designed controlled trials.