There have been a variety of thresholds that I have reached as my disease has progressed. They have included the very first symptoms, the first times when the disease started to impact my normal life, when I started to show an external symptom (limping), when I started to use a cane, and so on. It these were positive events I could call them milestones but I associate more positive things with a milestone where as the first thing that comes to mind with threshold is "threshold of pain".
In any case the latest is the installation of a chair lift. It was really hard to mess up the house with it but it had to be done as I was finding it increasingly difficult to get up and down the stairs. Sara was also find it difficult to ignore my progress up and down as she was constantly on edge waiting for a big thump. Any way, it is done and it really does make a big difference.