If a member of the Nightfallen goes for too long without any sort of arcane sustenance, they wither even further into a "stage 2" of the Nightfallen state "Withered" - this is the fully corrupted state.
Nightfallen at this point are mindless, shells of their former selves, beings driven purely by instinct. Here there is no chance of recovery, and they become bestial creatures, found throughout the darkest reaches of Suramar.
Most nightfallen in the earlier stages of decline, though suffering from starvation, are known to have all their faculties intact and are never seen to violate other living beings or feed off them - in this case the show the dignity that is often seen within the night elf peoples and a stark contrast to the Blood Elves who followed Illidan in the aftermath of the 3rd war, using Fel energy to satiate their magical pangs till they turned from that dark path.
It is also to be noted none of this progression is within the control of the fallen nightborne, this change is a result of the corruption in the Nightwell itself, and no amount of control or discipline can stop this. The corruption changed the body to only accept the Arcwine magical nourishment from the nightwell to survive. Normal food doesn't work as nourishment. This is the what needs to be fixed at the source. Magical energy from crystals and other sources properly converted can stave off further regression, but until the corruption is cleansed from the Nightwell, only the arcwine can restore the fallen nightborne. However if he crosses over into the stage 2 Withered State, there is no return known as yet.
Withered may look somewhat similar to the blood elven Wretched but they are quite different. The former is mindless, driven only by instinct and corrupt in body but above all, doesn't have a choice in the matter if he is denied food of the Nightwell. The latter on the other hand is a result of lack of self control, a choice and further to that retains a semblance of his/her faculties but has given in totally to his/her greedy lust for more magical energy. You could say it's a need vs greed situation.