Obviously your senior needs to know you are off work, so that your work can be done by someone else
The senior also needs to know roughly when you might return to work, again to get someone else
scheduled to do your work
So what exactly has been breached here, do you think?
That is not a breach of anything. If you have a serious medical problem then your Senior should know so they can watch over you. it is for your protection, not gossip. This is called Due Dilligence.
UK
What do you want to happen, and how much of your life are you willing to commit to depositions and hearings in order to get it to happen?
I think your first step should be to find a lawyer that specializes in workers rights. It's been my observation that employers walk all over their labor force when it comes to medical privacy laws.
If it impacts your job, I suspect he has a right to do so.
My manager has told my senior about a serious health problem without notifying me he's told them and was on the verge of telling someone else as well before I found out.
I feel this is unacceptable. What can I do and what would happen?