Which phenomenon is the sensation of an electric shock that quickly travels down the back and into the limbs with flexure of the neck and is associated with Multiple Sclerosis?

• A. Lhermitte's phenomenon
• B. Uhthoff's phenomenon
• C. Romberg sign
• D. Nystagmus

Answer: (A)

This is Lhermitte's sign. In multiple sclerosis, demyelination of the cervical spinal cord makes the dorsal columns vulnerable; when the neck is flexed, the stretched demyelinated fibers generate a sudden electric-shock sensation that travels down the spine into the limbs. The sensation is brief, reproducible with neck flexion, and serves as a clue to cervical spinal cord involvement in MS. Other options describe different phenomena: heat-related worsening of symptoms (Uhthoff's phenomenon), a balance test reflecting proprioceptive loss (Romberg sign), or involuntary eye movements (nystagmus), none of which capture the characteristic electric shock down the spine with neck flexion.