Mazfab,
Any applicant or any family member who is outside of Canada, has to re-do his/her medical when it expires. If you are an in-land applicant, and remaining in Canada they forward your medical extention to health branch in your province.
The doctors have 10 working days to make a decision, if you need to re-do your medical or not. If doctor sees a health problem or if you have gone a country which may cause a health issue then they ask for re-do.
That's the new rule released on (about) March 13, 2013...
Hope this helps!