When the muscles involved in breathing become very weak, lung function becomes inadequate so that there is not enough oxygen and too much carbon dioxide in the blood. This causes drowsiness, headaches and a general lack of well-being. When this happens, assistance with breathing through a face mask, used during sleep, may return the blood oxygen and carbon dioxide levels to normal and relieve the symptoms. A small number of affected people choose to have mechanically assisted breathing for 24 hours a day when their breathing muscles are so weak that they could not otherwise support life. An arbitary means of distinguishing the two disorders depends on whether the affected person can still walk at age 16 years.
Muscle biopsy tends to show more or less severe changes, related to the severity of disability.
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.