I have never heard of muscle relaxants causing shortening of new muscle fibers, and can find no information to suggest it.
Some muscle relaxants do have a potential for abuse, so this could cause of increased discomfort when they are discontinued,
JMK MD
Hey Terri,
I'm not a nephrologist, so I can't say for sure, but that makes no sense to me either. Especially with it being your opposite kidney showing hydronephrosis. I would definitely at least mention it to the nephrologist and see what they say. I really hope you don't have to go through another kidney biopsy.
Hang tough,
JMK.