Eye MovementControl Systems
For the eyes to move together (conjugate gaze), the oculomotor nuclei and abducens nuclei are interconnected by the medial longitudinal fasciculus (MLF).
Horizontal gaze is controlled by 2 gaze centers: 1 . Frontal eye field (contralateral gaze)
2. PPRF (paramedian pontine reticular formation, ipsilateral gaze)
Nystagmus
Nystagrnus refers to rhythmic oscillations of the eyes slowly to one side followed by a rapid reflex movement in the opposite direction. Nystagmus is defined by the direction of the rapid reflex movement or the fast phase. It is usually horizontal, although rotatory or vertical nystagmus may also occur.
Unilateral vestibular nerve or vestibular nucleus lesions may result in a vestibular nystagmus. In a pathologic vestibular nystagmus, the initial slow phase is the re sponse to the pathology, and the fast phase is the correction attempt made bythe cortex in response to the pathology. Consider this example: ifthe left vestibular nerve or nuclei are lesioned, because of the loss of balance between the 2 sides, the right vestibular nuclei are unopposed and act as if they have been stimulated, causing both eyes to look slowly to the left. This is the slow phase of a pathologic vestibular nystagmus. Because the head did not move, the cortex responds by moving both eyes quickly back to the right, the direction of the fast phase of the nystagmus.
Tests for Nystagmus
The integrity of the vestibulo-ocular reflex can be an indicator of brain-stem in tegrity in comatose patients. To test this reflex, a vestibular nystagmus is induced byperforming a caloric test in which an examiner introduces warm or cool water into an external auditory meatus. Warm water introduced into the external ear stimulates the horizontal semicircular duct and causes the eyes to move slowly in the opposite direction. Because the head did not turn, the eyes are moved quickly back by the cortex (if intact) toward the same ear where the warm water was introduced, producing a fast phase of nystagmus to the same side. Intro duction of cool water into the external ear mimics a lesion; the horizontal duct activity is inhibited on the cool water side, and the opposite vestibular complex moves the eyes slowly toward the cool-water ear. The corrective or fast phase of the nystagrnus moves the eyes quickly away from the ear where the cool water was introduced. A mnemonic which summarizes the direction of the fast phase of vestibular nystagmus in a caloric test toward the warm-water side and away from the cool-water side is COWS; cool, opposite; warm, same.
HORIZONTAL CONJUGATE GAZE
The eyeballs move together in conjugate gaze. The ocular muscles function to move and position both eyes as a unit so that an image falls on a corresponding spot on the retina of each eye. The slightest weakness in the movements of one eye causes diplopia, the presence of a double image, indicating that the image has been shifted to a different position on the retina of the affected side. Although gaze in allplanes is possible, the muscles and cranial nerves involved in horizon tal conjugate gaze, or abduction and adduction ofboth eyes together, are the most important eye movements (Figure IV-5- 1 1).
