Lhermitte sign: Sudden transient electric-like shocks extending down the spine triggered by flexing the head forward. Due to a disorder such as compression of the cervical spine (the portion of the spinal cord within the neck).
One patient who experienced Lhermitte sign wrote that: "Looking down to sign a hotel register, I felt a spasm like an electric current move along my back from the base of my skull to my toes. Every time I repeated the movement, the spasm recurred."
The causes of Lhermitte sign include multiple sclerosis (MS), radiation damage to the spinal cord, cervical spondylosis (degeneration of the disc spaces between the vertebrae), herniation of a cervical disc, a cervical spinal cord tumor, and subacute combined degeneration (caused by vitamin B12 deficiency).
Shocks radiating up the spine are sometimes referred to a reverse Lhermitte sign.