The 29-year-old has undergone a series of tests this week which have eventually found the cause of the problem.
Cassano suffered an ischemic attack which was triggered by a restriction of blood supply to his brain, caused by a slight perforation in the heart, but fortunately without any lasting damage.
"Antonio Cassano has experienced ischemic cerebral damage," a club statement read. "The player will undergo surgery in the next few days and is expected to be out of action for the next few months.
"The instrumental and neuroradiological examinations required 72 hours for their course and have highlighted the problem in the circumscribed area of the brain that did not result in persistent neurological deficits.
"The cause was identified as the presence of a patent foramen ovale septal heart, detectable only with sophisticated specialized investigations.
"The timing of therapy established has allowed a rapid recovery and improvement of clinical conditions that are good.
"In the coming days the player will undergo a small surgical interventional cardiology (closure of the foramen ovale).
"The recovery time for return to playing will be better defined after the surgery, but probably will be a few months.
"Milan gratefully acknowledges the facilities of the Polyclinic of Milan for what they are doing with great promptness and professionalism."