Woke up Blind!!

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my friend will be taken tomorrow to hospital to undergo an operation on his left eye becuase he lost the vision there over a night because of increased interocular pressure.. he is 20 YO who never seemed to have any trouble of any kind that is related to health.. he had a headache, took medicine for headache, slept, and then he simply woke up noticing that he can't see with his left eye! just like that he lost his sight! the medical reason that he was given was increased interocular pressure! do you have any idea what that is???? and how could it happen to a healthy person??