it was too early to tell if it were merely a minor side effect of the dehydration , the beginnings of a common headache , or the first hint of something worse-the migraines that had plagued him since adolescence and that at various times could come so strongly they could temporarily rob him of vision in one eye , turn light into a hailstorm of hot nails , and had once-only once , thank god-left him partially paralyzed for a day and a half .