Casey Anthony's mom suffered a panic attack while in the hot seat ... and it was so severe, she had to walk away from an interview about her late granddaughter.

Cindy Anthony's anxiety set in during an interview she did for the A&E and Lifetime doc, "Casey Anthony's Parents: The Lie Detector Test," which debuts tonight ... and she says she's convinced her daughter, Casey, knows what happened to 3-year-old Caylee in 2008.

Cindy starts crying during the convo, reaches for her chest and has to step out of the room, claiming to be having a panic attack ... her husband, George, quickly goes with her to help calm her down.

It seems the attack didn't go away, however -- as George recommends heading to a hospital as he lays her on the ground to try and get her to relax. We've only seen a clip at this point, so it's unclear just how serious it got.

As we reported, George got emotional during a lie detector test in another preview of the doc, struggling to answer whether or not he concealed his granddaughter's whereabouts -- something Casey claimed.

Remember, Caylee disappeared in June 2008 and her body was found months later -- Casey said during her 2011 trial that George disposed of Caylee's body, and she said in a 2022 doc that he staged her death to make it look like it was Casey's fault.

Unclear how well the parents passed their lie detector tests ... but safe to say that'll be revealed Thursday night.