I'm in my 50s now, but I've had these almost all my life. The first time was probably when I was about 14, and scared the crap out of me. Funnily enough I used to suffer from migraine headaches (steel balls in the head, nausea) when I was young, but they stopped about the same time as these scotomas started. I don't get headaches any more, just the flashing zig zag across both eyes.
Anyway, this morning at about 5am I woke up from a dream in which I was having an episode. The dream was very vivid and involved me sitting at a table with a large sheet of paper on it, trying to explain to someone else what I was seeing, and attempting to draw it. While this was going on, I could see (in the dream) the typical wide shimmering rainbow "C-shape" across my vision. Now the dream went on for quite a while, involving me going off and doing other things, while this thing shimmered in the air in front of me.
Then I woke up...and it was still there, exactly as it had looked in the dream. This tells me that mine are not caused by anything dietary (sugar, caffiene, whatever) because I'd been asleep for 5 hours and I don't tend to eat or drink while I'm asleep. More likely to be caused by dehydration. But what it did prove to me is that no matter how long a dream appears to be subjectively, it's only about a minute in real time. The scotoma was the same size all the way through the dream as it was when I woke up, but of course in real life they start out small and spread out beyond the field of vision in about 15 minutes.
I guess that although I was in darkness with my eyes closed, when I got my scotoma my brain could "see" it and so constructed a dream around it. This is the first time this has ever happened. I never thought it possible.
Although I was scared at first as a child, my optician just told me it was a "retinal migraine" and nothing to worry about. I've had them ever since and they no longer worry me, although I do find them irritating if they stop me from doing something like watching TV, reading a book or driving.
-Dave