What Caused This Woman to Faint?

 Dear Dr. Brayer;
My mother, age 66, fainted while she was at work.  She was taken to the emergency room and was evaluated for hours, including a CT scan of her head.  They could not find a reason and sent her home.  Could it have been a stroke that didn't show up?  
Thanks, Heidi

Dear Heidi,
The medical term for fainting or sudden loss of consciousness is syncope.  It occurs when the blood flow