The pain can shoot down the spine into the arms and/or legs anytime the head and neck are bent forward or extended backward. Lhermitte's is an indication that pressure is being placed on the spinal cord itself.
There may be other signs and symptoms noticeable only to a physician examining you. For example there may be weakness in the hands or feet, decreased muscle tone in the muscles, and changes in deep tendon reflexes (DTRs).
You may be familiar with DTRs such as the knee or ankle jerk. The doctor taps your tendon below the knee and your leg automatically straightens. Or a tap is applied to the Achilles' tendon above the ankle and the foot points downward.
Further tests are needed to find out why there's pressure on the spinal cord. It could be a condition called spinal stenosis. This is a narrowing of the spinal canal where the spinal cord is located. Degenerative changes from aging are the most likely cause of spinal stenosis.
It's best to report this symptom to your doctor and find out what's causing it. Early intervention may help reduce long-term problems.