I've had migraine's for years and the first sign that one is coming is the vision disturbances you describe. First I have zig zaggy lines at the edges of vision and then it moves to blackness in the center of whatever I'm trying to focus on. That lasts around 20 minutes and then the pain starts. The vision changes are a good warning so that I can take tylenol right away.

Here's hoping you've had your last migraine!