I've been having these same questions about my son, Michael. Here is our experience.
Michael and his twin were born 2 months premature. They were induced due to severe preeclampsia.
As for the cause of his hypotonia, I don't know. I asked his pediatrician if we should have testing to find out why he had hypotonia. He said he thought we would do a lot of expensive tests, but would still be doing the same treatment: lots of therapy. He didn't think it would reveal much. I know other people have pursued metabolic studies, etc. I don't know if its prudent/necessary for us or not. I do know Michael is very social, easy-going, and cognitively sharp. He doesn't seem to have any health problems that aren't attributable to hypotonia. I'd be interested in people's thoughts on this.
Continuing with the "cause" question, preeclampsia does cause oxygen to shut off to the babies because of vascular spasms. That's why babies die so quickly with it. I'd never considered if oxygen deprivation was a cause of his hypotonia. $We had an MRI because he had intermittent tremors, which turned out to be benign genetic tremors. The MRI showed no abnormalities. He did have some spasticity in his lower extremities when little. I asked the neurologist if he had cerebral palsy. She told me it was "just a word" and wouldn't affect what we did: lots of therapy. She said they wouldn't diagnosis CP at that age. (He was like 4 months adjusted).
Now I'll turn to milestones. Michael laid plastered to the ground, unable to lift his hands to midline at 5 1/2 months chronological age. He couldn't lift his head.
I threw the book at this little boy, therapy-wise. I put his in OT twice a week, one through EI and one privately. I had him in PT. We did aquatherapy 2x/week. I got him into hippotherapy. I CONSTANTLY worked with him and tried to incorporate his therapies into our life. For instance, I would encourage him to pull up on me to be held, while being careful not to be cruel or let him get overly frustrated.
With all this, my little boy ended up looking like a miracle child. He walked at the equivalent of 12 months. At slightly over age 3, he jumped, which is a huge accomplishment for a hypotonic kid. He runs, but his gait is immature. He doesn't really lift his knees. He can balance in a flamingo pose on one leg with arms extended for a second. He can go downstairs, but not in a reciprocal pattern.
Feeding has been a huge issue for us. He couldn't latch on until he was 12 weeks old. His suck was ineffective. He ended up a huge chunk below the growth chart. After a zillion consultations, including feeding evaluations, a feeding specialist observed that he would only eat soft fruits and soft vegetables because he was swallowing them whole. One or two bites of unchewed food and he was full. They also included the hypotonia left him too exhausted to eat and chew. We went through MONTHS of him crying from hunger but being too exhausted to eat. It was so painful.
We still have lots of feeding issues, though I've finally got his little toe on the chart, as opposed to a shocking chunk below the chart. I never in a million years would have thought I, a health conscious, vegetarian, would be feeding my child McDonald's double cheeseburgers and french fries just to get as many calories and fat into him as possible. He seems to have little to no desire to eat. I primarily give him strawberry milk, kefir, yogurt, chocolate milk, juice, in short, caloric liquids.
EI ended and with it our luxurious aquatherapy 2x/week. I took him for a PT evaluation. She is proposing 6 months of PT and then reevaluate whether to continue, considering if he is simply being frustrated and cannot do much better or is making progress.
I would love to know how this looks for children as they get older. My sense is that Michael has done fantastically for where he started, but he will continue to be affected. He is TINY for his age. He hasn't needed speech therapy for language issues, but it seems like his speech is harder to understand. I wonder if the decrease in services his making his speech harder to understand. I'm curious what potty learning looks like for these children. My son is 40 months and hasn't shown a hint of any readiness to potty learn.