Origin
Dennis Hoxie was one of many heroes who joined the Anti-Registration Underground. He was presumably killed when S.H.I.E.L.D. discovered the location of the underground (as his body was seen) but he may have only been unconscious.
Creation
Brain Spasm was created by Paul Jenkins and Ramon Bachs, with his first (and only) appearance being in Civil War: Front Line #4 – Embedded, Part 4.