Although I think you should include why you got poor grades (because you have a valid excuse), I don't think you need to narrate your academic history throughout the entire essay. Too many excuses comes off as whiny or sheepish. Consider making the story more about your passion, inspiration or reason for pursuing a degree. It will be a more positive storyline.
I was watching Project Runway and a contestant didn't show the judges one of her "wow" designs because she wanted it to be a surprise at the runway show. The judges shared some good advice with her: wow us NOW because you aren't guaranteed to go to the runway show without impressing us now. I think it is a good metaphor for applying to grad school. Wow the judges with your application because you don't know whether you'll have the chance to wow them at a later stage.
