I am an optometry student writing a case report on using alphagan to reduce pupil size for night driving after RK. I am trying to figure out why this works. I have read several articles reporting a decrease in norepinephrine release which would cause constriction. However, my confusion stems from the fact that I also read in my pharmacology notes that brimonidine can also be used to test for Horners syndrome....which in this case is a mydriasis issue. So my question is how can the same drug be used for both?(to cause miosis and mydriasis?)