L'hermitte's phenomenon, or sign, is the sensation of a very sudden electric shock that spreads from the neck and down the arms, spine and legs. It's usually triggered by flexing the neck - that is, bending your head down, chin towards chest. The sensation is short-lived, usually no more than a second.

How often the symptom occurs, and what other symptoms develop along with it, depends on the underlying cause, of which there are several.

Linked to damage of spinal cord in the neck

L'hermitte's sign simply shows that something is damaging the spinal cord (especially in the part of it that's composed of white matter, at the back of the cord). This damage is usually in the neck or region of the spine known as the cervical spine. But the symptom is very non-specific and says nothing about exactly where in the spinal cord the problem is, or what is damaging it.

The most common cause is arthritis of the small joints of the vertebra in that part of the spine, also known as cervical spondylosis. This can cause abnormal pressure on the spinal cord or the nerves coming out of it.

Other causes include:

• Multiple sclerosis
• Vitamin B12 deficiency (pernicious anaemia)
• Tumours
• Compression of the discs in the cervical spine following trauma

But in many cases the cause can't be found.

Tests rule out main causes in many cases

It's important that L'hermitte's sign is investigated by a specialist to pick up and treat these causes if possible, and limit or prevent further damage. But in many cases the tests (which may include X-ray of the cervical spine, MRI scans of the brain and cervical spinal cord, lumbar puncture and nerve signal tests known as visual evoked potentials) all come back negative.

When this happens, you should keep an eye on the problem and repeat the tests if necessary or if symptoms worsen.

This article was last medically reviewed by Dr Trisha Macnair in April 2008