In this patient, occlusion of the basilar artery just distal to the superior cerebellar arteries had produced infarction of the base of the rostral pons bilaterally. Interruption of the corticobulbar and corticospinal tracts caused the complete paralysis of all voluntary movements except certain eye movements. This neurologic condition is termed the locked-in state.
Voluntary horizontal eye movements were impossible because they require intact projections from the frontal lobe (via superior colliculus) to structures in the caudal pons. Vertical eye movements were spared since the entire pathway for voluntary vertical eye movements is contained in the midbrain (the location of brainstem vertical gaze centers and the 3rd and 4th nuclei and nerves) and forebrain, both of which were spared because they are located rostral to the lesion.
The reticular formation and other structures located in the midbrain continued to get adequate blood flow, accounting for the patient's alertness, awareness of herself and her surroundings and ability to perceive sensory stimuli. It is likely that the size and arrangement of vessels in her circle of Willis allowed enough blood to flow from the internal carotid arteries through the posterior communicating and into the posterior cerebral arteries to supply these areas.