Conjugate gaze is the ability of the eyes to work together or in unison. It refers to the motion of both eyes in the same direction at the same time. The eyes can look laterally (left/right), upward, or downward. Disorders in conjugate gaze refer to the inability to look in a certain direction with both eyes.
Conjugate gaze is mediated in the brain stem by the medial longitudinal fasciculus, which is a nerve tract that connects the abducens, trochlear, and oculomotor nuclei. These nuclei, in turn, are responsible for the muscles that control eye movements. The left pontine center connects with the right frontal center for conjugate gaze to the left, and the right pontine center connects with the left frontal center for conjugate gaze to the right. If extraocular muscles are not working properly, dysconjugate gaze can result, which can then cause diplopia. The mechanisms for horizontal eye movements are better understood than vertical eye movements. As individuals age, the ability to look upward tends to decline. Cerebral structures control when and where the eyes move, and the brain stem structures control how they move.
The centers for lateral conjugate gaze are in the frontal and occipital cortices. In the frontal lobe, this area is in the posterior aspect of the frontal lobes, referred to as the frontal eye fields. This area is close to the motor strip. The function of the frontal centers is to control voluntary conjugate eye movements to the opposite side. The frontal eye fields (FEF) receive inputs from peristriate, parietal and superior temporal cortex, medial pulvinar, and the dorsomedial nucleus of the thalamus. Stimulation of FEF results in contralateral saccades. In strokes that affect this area, one may see an eye deviation toward the side of the lesion and away from the paralyzed limb. This usually occurs only in the acute phase of an infarct. At some point, the patient may be unable to move the eyes away from the lesion on command. However, they may be able to follow an object to the opposite side if the occipital lobe center is not damaged. The occipital lobe centers for lateral conjugate gaze control eye movement when an individual is following an object to the opposite side. Lesions of the occipital lobe that control lateral conjugate gaze are less common than lesions of the frontal centers.
Conjugate gaze can be disrupted by stroke or trauma, depending on the location of the damage. For instance, an intracerebral hemorrhage in the caudate nucleus or putamen will cause conjugate deviation of eye movements to the side of the lesion. Pineal tumors, which can press upon the midbrain, can cause paralysis of upward gaze. Deeper damage can affect downward saccades. Given the extensive anatomy of control of the visual system, damage or dysfunction along any of the nuclei or tracts integral to eye movements can result in abnormalities. Eye movements utilize the basal ganglia and cerebellum in their planning and coordination. Patients with basal ganglia disorders may have involuntary, small, or slow eye movements. Patients with parkinsonian symptoms may have a restriction of upgaze. In patients with progressive supranuclear palsy, a cardinal feature is the restriction of upgaze, and these patients initially complain of difficulties with reading. The flocculus of the cerebellum is important for suppression of vestibular reflex and for smooth pursuit movements. Parts of the vermis help to coordinate saccades, and damage can result in dysmetric saccades. Patients with long-standing frontal lobe lesions cannot inhibit inappropriate saccades from a fixation to a peripheral, visually attractive stimulus that appears suddenly. Frontal seizures with a focus in or near one eye field may manifest by turning of the eye and head away from the side of the lesion.
Evoked eye movements (doll’s eye test or oculocephalic reflex) and the caloric response can be used in the examination of an unresponsive patient. However, if a patient is in a comatose state due to drug intoxication or hypothermia, these tests may show no response. The assessment is as follows:
Doll’s eye test – This test can be used in either the comatose or conscious patient. Hold the patient’s eyes open and rapidly move/rotate the head to one side and hold it there. If the brain stem reflexes are intact, the eyes will move conjugately in the direction opposite to the head rotation. If the injury is in the brain stem, the eyes do not move. If a patient is conscious and can follow commands, have them fixate on an object. This test should not be used in a patient who has possible cervical injuries.
Caloric test – This test can be done in the comatose patient. The patient’s ear canal is irrigated with 20 ml of ice-cold water. The eyes should move toward the ear that is irrigated. If the patient’s eyes do not move, the lesion is in the brain stem. This test should not be used in a patient who has possible cervical injuries, or who has blood in the ear canal or a perforated eardrum.
Conjugate eye movements allow the eyes to get an image onto the fovea and keep it there. Fast movements or saccades allow images onto the fovea and slower movements keep them there. Smooth pursuit movements compensate for target movement. There are several ocular motor systems:
Saccadic system moves the eyes rapidly (up to 800°/s) in order to fixate on new targets in the visual fields. These can be voluntary or a response to a verbal command. Reflex saccades can also occur to stimuli that are threatening or to a sound in the periphery.
Pursuit system enables the eyes to track slowly moving targets (approximately 70°/s) so the image is stable on the fovea.
Vestibular eye movement subsystem maintains a stable image on the retina during head movements. The vestibulo ocular reflex maintains the eyes in the same direction in space during head movements. This is controlled by the semicircular canals, which respond to rotational acceleration of the head.
Optokinetic system complements the vestibular eye movement system. It uses reference points in the environment to maintain orientation. This system uses fixation and pursuit in humans.
Vergence system enables the eyes to move dysconjugately (converge and diverge) in the horizontal axis to maintain fixation on a moving target toward or away from the individual. These are critical for binocular single vision and depth perception.