By segments I refer to the the spinal cord segment that the spinal nerve that is damaged. The wasting is not due to pain; it is due to damage to the nerves resulting in either a loss of nerve cell function or nerve cell death. If you are having that amount of weakness something seriously has happened to your spinal nerve(s) - most likely L5 and possibly also S1.

Since you have had a previous fusion I presume you had severe back pain with nerve damage back then? Was it on the same side?

While it is good that the xray shows the spinal fusion is ok one would have to suspect something has gone wrong. Something in there is most likely severely pressing on the spinal nerve(s). Alternatively segments above or below a fusion do have a tendency to have problems as they now take a lot more shearing forces and due to abrupt change from normal movement in the other spinal segments and the sudden loss of movement. However this usually takes a several years to develop if it is going to.

One question: did the weakness develop quickly after the nerve root block or was it there before?

I don't know how much interaction you have had with your surgeon in recent weeks but I think it is important that he knows you are having severe recent nerve damage.