What autoimmune disease could cause a significant decrease in sensation from head to toe and if not why would this not be possible?

Autoimmune: Complex question. Any autoimmune disorder that can alter the central nervous system could cause sensory changes. Ms, lupus, sjogren's and many diseases that are not exactly autoimmune but cause inflammation or demyelination can cause this (b12 deficiency, et al). There are some peripheral neuropathies that can be autoimmune and cause numbness. See a good neurologist.