L'Hermitte's phenomenon, or sign, describes a sensation of pain that is triggered by flexing the neck (bending the head forward). This pain travels from the neck down the spine and into the arms or legs. The pain is usually brief but intense and is often compared to a sharp electrical shock.

Another name for this symptom is the "barber's chair" phenomenon, since it can occur when a person leans forward while getting a haircut.

L'Hermitte's phenomenon appears to be triggered by damage to the spinal cord as it travels through the neck. While this symptom is often linked to multiple sclerosis, it can also be found in a number of other conditions, including:

• Severe arthritis or disc disease affecting the neck
• Vitamin B12 deficiency, or other metabolic conditions that affect nerve function
• Tumors or growths of the spinal cord
• Inflammation of the spinal cord, from conditions such as transverse myelitis
• As a consequence of radiation therapy

L'Hermitte's phenomenon can also occur in some people without any clear cause, despite an intensive medical investigation.

On a historical note: The phenomenon is named for Dr. Jacques Jean L'Hermitte (1877-1959), a noted French neurologist who described this symptom in the medical literature in 1924.