WD means wallerian degeneration, which is severed nerves or axons (also I think).
Wallerian degeneration? Yes...axons as you know are the long branches out from a nerve cell. If an axon gets severed, the branch that is out on its own is no longer able to get food and nutrients from the nerve cell, so that branch shrivels up and dies. In the peripepheral nervous system, the nerve puts out a new axon branch, things reconnect, all is well. In the brain, glial cells often get in the way and form a blockage and the neuron is not able to grow a new branch.
Why is wallerian degeneration seen in MS? There would be no point of severage. I assume it means the axonal damage or axons dying from the outmost point inward. If we look at what CCSVI might mean in this context, my guess is that the neurons are not receiving enough nutrients and glucose, and what nutrients and glucose they get are not enough to feed the entire nerve cell all the way out to the end of the axons, so the axon goes unfed and undernourished and eventually begins to die from the outmost point inward.
So, to me, wallerian degeneration means that pre-CCSVI treatment, we should keep the level of nutrients in our body high and our glucose levels steady. Post-CCSVI treatment, if the wallerian degeneration process has already started, unfortunately it may continue, if the damage is already too great. This is my best understanding of this. The name "Wallerian" comes from the guy (Waller?) who discovered the phenomenon, but it's specifically most commonly in reference to severed axons, so it's odd that we see the same process in MS where the axons are not severed.