Lhermitte sign

A sensation similar to an electrical shock radiating from the back of the head
down the spine as the neck is bent forward. Flexing of the neck produces
electric shock-like sensations that extend down the spine and shoot into the
limbs caused by trauma to the cervical portion of the spinal cord, multiple
sclerosis, cervical cord tumor, cervical spondylosis, or even vitamin B12
deficiency.