This is likely from overuse because her left side is weaker and slightly paralyzed. Therapists so far haven't really been effective at helping her with it.
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Answer transcribed from Brightway's interview with Dr. Cristen Gordon (pt.1):
This question really spoke to me. I don't want to get on too much of a soapbox but when you're going to see a physical therapist or any kind of therapist, you want to go to a therapist that specializes in what you need. In this case, if your daughter has had a brain injury, she is a patient that has neurological issues. Neurological issues in this case are affecting her left side, but then she's acquired orthopedic issues on the right side related to the neurological issues. You may think that you could see an orthopedic therapist or a neuro therapist, but I would highly recommend you consider seeing a neuro therapist mostly because the neuro therapist is going to be able to think about the things that have happened on the neurological side that can be affecting the orthopedic side.
I imagine as was stated in the question if the right arm is being overused to compensate for the left arm, my first thought of not having met the patient is what is the patient doing with their left arm. A lot of times, what happens is it's easier to use the right arm, so you use the right arm. However, you could be using the left arm more to take off some of the load from the right arm. The other thing is maybe it's the way you transfer or the way you stand. If you're using a walker or something like that, as a therapist I'd want to evaluate how you're doing those things and see how we can protect the right shoulder more. Then from there, we need to go in and treat the orthopedic issues.
Keep in mind that just because a therapist is a neuro therapist does not mean they can't treat orthopedic issues. The same goes for just because an orthopedic therapist is not a neuro therapist, doesn't mean they can't treat a neuro issue. I'm just saying you want to vet who you're seeing and make sure they can do what you need them to do. Once we have figured out the cause and the culprit for why the right side is being painful or not being able to do its normal routine, we then need to also fix the actual impairment. Do you have tight muscles or muscle spasms from overuse? Do you have any weaknesses? Maybe you've got a muscle imbalance that's causing your right shoulder to not function the way it normally would. So those things have to be addressed orthopedically as well.
Your best bet then, though it can be hard to find, is to find a clinic that has neuro and ortho therapies in the same clinic because then, those therapists can talk to each other and help each other out. I've worked in clinics like that and it's really great because you have the best of both worlds, and those two brains can help each other figure out the entire problem and fix it. I've also worked with therapists where there was an orthopedic therapist that was at a different clinic and they reached out to our neuro therapist. We shared the patient from clinic to clinic and the neuro therapist worked with the patient for two months or so, and then they went to see the ortho therapist for two months with constant communication. It can be challenging to find a neuro therapist, but I would suggest you get on americanphysicaltherapy.com, physicaltherapyassociation.com, that's apta.com, and you can search specifically for neuro therapists.