L'hermitte's sign is the term used to describe an electric shock-like sensation which radiates down the back and into the legs when someone flexes their neck. This is a sign of nerve damage which shows up when the neck is flexed and the nerve stretched.

Though associated with early multiple sclerosis, this symptom is not in itself diagnostic. L'hermitte's sign may also be caused by radiation injury or as a side effect of radiotherapy in the region of the neck. Trauma to the cervical portion of the spinal cord, cervical cord tumour, cervical spondylosis, or even a simple vitamin B12 deficiency may also cause the nerve damage which results in this symptom. It is also be experienced in patients with a particular type of brain tumour - a meningioma - in the base of the skull.