In a lawsuit is the other party allowed to check any and all of your medical records?

If someone is trying to slander another persons name by saying they have been a psychiatric patient with multiple illnesses, and because it is a defamation of character as well as having a provable negative impact on the other parties career, the person being accused of this wants to take it to court and sue for defamation ? Would the other party, the one who is slandering be allowed to check every hospital for the other person's personal medical records and obtain all their personal info and private date from the hospitals etc?