The above poster is exactly right - the advisor is there to help you through school. They want what's best for your and they will understand that taking care of your health comes before anything else.

Is your advisor strictly an academic advisor or is he/she from some sort of disability office or student services office? I ask this because while a strictly academic advisor will be concerned primarily with documentation and what a leave of absence will look like in terms of academic progress, it doesn't necessarily mean that they have no knowledge of mental illness. For instance, at my university, while yes academic registration is done through the university registrar and academic advising office, I work closely with a disabilities specialist in the student services center. She has laid out the process for me (as a formality) what options are available for me should I feel the need to take a reduced course load or take a medical leave of absence. If that came to be, I would meet with her to file the necessary paperwork and she would sign off on it. That way, I wouldn't have to deal with the registrar and dean directly - I could worry about my health and my treatment and not all of the little details. So she has the authority to deal with academic course work and whatnot but she isn't exactly an advisor.