all I can say is my interpretation of what her Mom and Dad, and my sister, have told the doctors have said.
There are many 'forms' of MD, as I understand it... some far more debilitating than others. My niece, apparently, does not have the type that 'normally' causes severe degeneration and also.. it's a 'type' that responds pretty well to various treatments... no cure but helpful in fighting the effects and impeding the disease's progression.
I have a friend close to my age that has had it his entire life.. diagnosed when he was a child. His hands shake... and that's about it. Doesn't seem to have gotten any worse in many decades.
A neighbors young boy I knew had it and in what seemed overnight he went from walking, to a wheelchair, to completely invalid. Sad story that for more than one reason...
His Dad was, is, as alcoholic was as non-supportive as it gets... eventually his wife left him, (though it was several years), moved away and I have not heard from her or how her son's doing in many, many years. BUT... while she was here you have never seen a more caregiving mother. One could say she completely 'surrendered' her life to care for him but that's not accurate... he was her life. - JamesJM