At the end, Denzel Washington's character has a Grand Mal Seizure. But he is supposed to have a spinal cord injury. He can still have seizures in that the electrical impulses in his brain go haywire, but if he has a spinal cord injury that prevents him from moving, the seizure action in his brain wouldn't translate to his body jerking and shaking.
The movie makers really wanted the audience to believe that Michael Rooker was the killer, rather than Leland Orser. So much so that Rooker actually played the killer in almost all the scenes where the killer wears a mask. When you see the eyes of the killer through the mask the eyes are Rooker's.