Mei Mei .....
I think I said this somewhere on an earlier post but I, also, do not feel that headaches are automatic with the retrosigmoid approach. I have had two skull based surgeries with retrosigmoid approach (one on each side). Following the first one, in 1994, I did have significant headaches for about two weeks afterwards and that was it. Following my AN surgery in 2008 I had absolutely no headaches. The two surgeries were done in two different medical facilities in different states, with different neurosurgeons.
For most of my life I have had what I refer to as "barometric headaches" because there is a direct correlation with weather system fronts. This was the case long before either of my surgeries and on an infrequent basis. Since 2008 I have had even fewer of these type of headaches. As with any normal person, I can get a tired headache or bright sunlight exposure headache, or now prolonged very loud noise induced headaches ..... but they are few and far between.
I certainly hope you find some relief for yours ..... it has to be very difficult for you.
Best thoughts and prayers. Clarice