The connection between nerves and Progestin is this:
Progestin inhibits the production of natural progesterone. When progesterone has been inhibited in mice, impairs remyelenation. Myelin is the layer of fat that lines nerve cells, which help transmit the signals along the nerves. In other words, without progesterone, we may not be able to heal our nerve cells very well. This would explain, at least in my case, why it took so long for the numbness to begin, because it wasn't that mirena damaged the cells, but prevented healing with any natural damage over time. When progesterone was applied to their nerves, myelanation occured. I am hoping this is correct and that once I get off BCP (in 3 months, husband just had vasectomy), which also prevents progesterone production, things will improve. However, nerve cells are the slowest cells in our body to heal.
Having said all this I have had 3 MRI's and I have no scarring of my nerves in my neck or head (they only tested this because numbness is in hand and leg on one side, which indicates that damage would be high up in the neck or above). I also had nerve impulse tests to see if there was a delay in the signal and there was not. Doctors cannot find anything wrong.