Has anyone been diagnosed with Polymyositis?

I am not sure if autoimmune disease in the right group. Within the past 60 days I have been experiencing severe weakness in my shoulders / upper arms and both thighs in my legs. I initially thought that it could be related to a cervical spine issue from a November 2024 surgery, but my Orthopedic surgeon ruled it out and indicated the fusion was successful. I searched AI and the number one item is AI returned was polymyositis. I was not looking for this specifically, but my brother was diagnosed with it three years ago. I also read it is rare for siblings to both have it. I am a 68 year old male and my brother is 69.

If you have or know someone that has had polymyositis, what were your symptoms and how is it managed. I see my Neurologist at Wash-U on 9/18. It looks like a myositis blood panel can be ran, but I read that a muscle biopsy is the gold standard. Thank you for input.