Fig. 4-6. Diagrammatic representation of the major regions involved with conjugate vertical and convergent gaze and eye opening. Eye findings with lesions in different region are illustrated. Broken arrows represent loss of function; solid arrows represent normal function. (a) Diffuse bilateral cerebral cortex disease or severe bilateral involvement of the basal ganglia, as in parkinsonism, is associated with difficulty in upward gaze, convergence and later, downward gaze, essentially a deafferentation of the pretectal vertical gaze zone. Unilateral cortical or basal ganglia disease does not cause clinically apparent problems with vertical or convergent gaze. (b) Pretectal destruction (e.g., compression by pineal tumor) causes more severe difficulty with vertical upgaze and convergence and, possibly, complete loss of these functions. Deeper involvement in the region medial to the red nucleus is necessary to cause significant loss of downgaze. Involvement of the pupillary light reflex region in the pretectum and also the descending sympathetic pathways results in mid-position, fixed-to-light pupils, frequently beginning with small reactive pupils, because the sympathetic pathways are more dorsal and possibly more sensitive to compression. Lesions, usually ischemic restricted to the periaqueductal region of the midbrain, cause similar abnornalities and, in addition, a peculiar nystagmus that is convergent or involves all the eye muscles simultaneously and causes a jerky retraction of the eyes into the orbit (nystagmus retractorius) on attempted voluntary gaze. (c) A central lesion in the lower pons transecting both the basis pontis and the tegmentum (most often infarction or hemorrhage) leaves a person quadriplegic and mute with loss of all horizontal gaze function. Vertical gaze and convergence (also eye opening) are the only remaining means of communication. If this goes unrecognized, the patient will be essentially locked in. A higher bilateral lesion (e.g., midpons) interrupts the reticular formation necessary for the maintenance of consciousness and causes coma. Lesions that transect the basis pontis or the basis pedunculi, sparing the tegmentum, do not depress consciousness but leave the patient quadriplegic with no voluntary horizontal gaze (see Fig 4-5, lesion 3, but bilateral); reflex vestibular-oculomotor connections are preserved because they are located entirely within the tegmentum and therefore caloric irrigations or the oculcephalic maneuver gives reflex conjugate horizontal and vertical gaze of the eyes (see the section on nerve VIII).