Since the discovery that dystrophin is defective in DMD and BMD, but more severely defective in DMD, examination of dystrophin in muscle biopsy samples can be used to distinguish them. DMD is fairly predictable. There is a range of severity, and disability progresses more rapidly in some people than in others. Nevertheless the range of severity is rather narrow. BMD is much more variable.
Some affected people are able to walk only until early adulthood, others to an advantaged age.
Survival in some is to middle age but others have survived more than 80 years. Some develop heart trouble in early adulthood, others never do. There are sometimes big differences in severity of BMD, between affected individuals in the same family. The process is like that described for DMD but the doctor's findings are in all respects generally less severe. The SCK level is increased, the EMG and muscle biopsy show changes consistent with the diagnosis.