I would definitely talk to them. Panic attacks are serious things and you can't function and think about school when you can't breathe and are thinking you are going to die. If you are being seen either by a doctor or a therapist, even though they didn't physically witness a particular panic attack, they can certainly write that you are being treated for panic attack disorder, and give specifics of treatment, like the therpist, any meds or other therapies. I really think it is enough that you just tell the professors that you are deeply committed to the class and how can you make up missed lectures (can you borrow notes or have an hour session with them). That is the best proof of all, that you are committed to making up the work and getting a good grade.