I've been idly thinking about this one a lot, it's something that I had noticed as Davids condition progressed, and since he has gone I've had a little more time to analyse.
Every time that David had a hard fall, the PSP seemed almost instantly to move on to the next level. It was almost as if the impact knocked it along as in between falls, David always seemed pretty stable with no deterioration.
I know there is no science behind this and it could be the that the deterioration happened and showed itself in a fall when he suddenly couldn't do something that we had always assumed was ok.
But the biggest thing for me was as I've just mentioned in another post, we were involved in an accident in a patient ambulance on our way to an appointment, and immediately that evening David was more tired than usual, and from then on for his final five weeks was tired like that every day. That's a very strange coincidence!