Why is Stevie so absent on stage?

[SisterNightroad]

For what I know Stevie is practically almost blind, I remember her and Richard Dashut saying that she has only 4 dioptres on each eye and that even with contacts she doesn't see beyond 4 metres ahead of her and at concerts the audience is mostly a blurred mass.

About her movements, I think the age really took its toll on her, sometimes when I watch video on youtube I see her holding her back or joints and walking very tiredly.