Locked-in syndrome (LIS) results from bilateral ventral pontine infarction, most commonly caused by basilar artery occlusion. The basis pontis (ventral pons) contains the corticospinal tracts (controlling voluntary movement of the body) and corticobulbar tracts (controlling voluntary movement of cranial nerve-innervated muscles including the face, tongue, and pharynx). Bilateral destruction of these tracts produces complete quadriplegia, bilateral facial paralysis, inability to speak (anarthria), and inability to swallow. However, the dorsal pons is spared, preserving the reticular activating system (consciousness), sensory pathways (all sensation including pain, temperature, and touch), and the vertical eye movement centers. The patient is therefore fully conscious and aware with intact cognition, hearing, and vision but is completely paralyzed -- essentially locked inside an intact mind. The only preserved voluntary movements are vertical eye movements and blinking, controlled by the oculomotor nucleus (CN III) in the midbrain, which lies above the pontine lesion. Communication is established through eye blink codes (one blink for yes, two for no) or letter-board scanning with vertical eye movements. Other causes include central pontine myelinolysis (osmotic demyelination from overly rapid correction of hyponatremia),...