Breathing difficulties may become progressively worse in such cases.
[rarediseases.org]
Some cases may be mild and progress very slowly over a normal lifespan, while others produce severe muscle weakness, functional disability, and loss of the ability to walk
[ninds.nih.gov]
In most cases, the arms, legs, and spine become progressively deformed, and there may be some cognitive impairment.
[webmd.com]