Florence,
Actually, it is likely that you don't need this information at all. As an instructor, you really don't need information about the student's psychological disability so much as you need information about how to support the student and/or manage the student's behavior in class. WHY the student is manifesting certain behaviors is less an issue that what the behavior is and how it impacts on the student and on the rest of the class. Stick to the here and now -- what is the student doing and does it interfere in some way. If not, then the existence of disability is neither here nor there. There should be someone at your institution who is assigned the role of collecting information from the student about disability and letng you know if there is some accommodation necessary.
Dr. Jane Jarrow