What Causes Severe Depression?

My daughter was hospitalized w/a multitude of problems as a baby (4 open heart and 3 digestive surgeries)...all complete by 18 months old. She is 19 yoa now, and in great health physically, but can t get a grasp on her depression like symptoms. She has tried a multitude of anti-depressants, to no avail. Having said all that, my question is this: I remember her neurologist (as a baby) saying she had brain atrophy as a result of her multiple hospitalizations. He described this, basically, as spaces between portions of the brain. Does atrophy correct itself? And/or, could that be a reason for her depression/anxiety symptoms?