Lhermitte’s sign, or the "barber’s chair syndrome", is a painless but unpleasant tingling or electric shock-like sensation in the back and spreading instantaneously down the arms and legs following neck flexion (active or passive). It is associated with pathology within the cervical spinal cord. Although most commonly encountered (and originally described in) demyelination, it is not pathognomonic of this condition, and has been described with other local pathologies, such as:
subacute combined degeneration of the cord (vitamin B12 deficiency); nitrous oxide (N2O) exposure
traumatic or compressive cervical myelopathy (e.g., cervical spondylotic myelopathy)
epidural/subdural/intraparenchymal tumor radiation myelitis
inflammation, e.g., systemic lupus erythematosus, Behçet’s disease cervical herpes zoster myelitis
cavernous angioma of the cervical cord
Pathophysiologically, this movement-induced symptom may reflect the exquisite mechanosensitivity of axons which are demyelinated, or damaged in some other way.
A "motor equivalent" of Lhermitte’s sign, McArdle’s sign, has been described, as has "reverse Lhermitte’s sign", a label applied either to the aforementioned symptoms occurring on neck extension, or in which neck flexion induces electrical shock-like sensation traveling from the feet upward.
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