Have you been tested absolutely positive for enterobius vermicularis? This is more important than you think, because there are other things which can cause chronic itch of the anal region. If pinworm, you should be able to find them without a problem. They are visible to the naked eye. A magnifying glass can help, but they can be found. You could use "clear wrap" wipe the area during itch, especially after falling asleep, then hold the wrap up to the light, and try to observe small movements.