The Lhermitte phenomenon: variant forms and their significance.

Typical Lhermitte phenomenon (tingling sensations moving down the limbs or trunk on neck flexion) is a sign of intrinsic spinal cord pathology, most commonly cervical spinal cord demyelination. The phenomenon has several variant forms, and each has a different pathological significance. A delayed typical Lhermitte phenomenon can follow contusion of the spinal cord from neck trauma. Reverse Lhermitte phenomenon induced by neck extension is usually produced by extrinsic compression of the cervical spinal cord. Upward moving paraesthesia with neck flexion (inverse Lhermitte phenomenon) is relatively rare, and can be a sign of myelopathy from nitrous oxide inhalation.