I have had this problem recently when I passed out while cycling and broke my shoulder and a rib. It happened during a morning group ride. I got this buzzing sensation in my head and then i blacked out . I woke up in the emergency room. I had my head and heart checked out and the results were all good . The docs said " it could be one of those thing , just hydrate and you will be fine." well a few months later it happened again , I passed out , crashed and had injuries. I wonder if anyone can throw some light on this.