Holly underwent pioneering surgery at Great Ormond Street Hospital (GOSH) after a brain scan showed she had moya-moya syndrome.
Moya-moya syndrome is a very rare disorder that causes multiple mini-strokes.
To prevent her from suffering a serious stroke, surgeons created a new blood supply to her brain by rerouting arteries which normally supply the skin of the scalp.
It involved joining arteries that measure about one millimeter in diameter. Holly, 12, had two operations under general anaesthetic.
"The first time it was a bit scary, the second time I understood what was going on," she says.
GOSH is the first hospital in Europe to offer this operation.