Supranuclear Eye Movements

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Cranial nerves III, IV, and VI serve together with the extraocular muscles as a final mechanism that executes all eye movements. Supranuclear pathways initiate, control, and coordinate various types of eye movements. Several types of eye movements are briefly mentioned here (Table 5-1), but a detailed and lucid synthesis of current concepts of the neural control of eye movements can be found in many other sources.288

Physiology and Clinical Assessment

The vestibular apparatus drives reflex eye movements, which allow us to keep images of the world steady on the retinas as we move our heads during various activities. The eyes move in the opposite direction to the movement of the head so that they remain in a steady position in space. The semicircular canals are the end organs that provide the innervation to the vestibular nuclei, which in turn drive cranial nerves III, IV, and VI to compensate for rotations of the head. In contrast, the otoliths respond to linear accelerations of the head and to gravity when the head is tilted. You can easily test the effectiveness of input from the semicircular canals by testing the vestibulo-ocular reflex (VOR). First, hold your head still and observe an object such as your index finger as you move it side to side at about 1 to 3 cycles/s. The image is a blur. However, if you hold your finger steady and rotate your head from side to side at the same frequency, you are able to maintain a clear image.

Several forms of saccades, fast eye movements, can be clinically observed. Voluntary saccades may be predictive, in anticipation of a target appearing in a specific location; command-generated, in response to a command such as "look to the right"; memory-guided; or antisaccades, in which a reflexive saccade to an abruptly appearing peripheral target is suppressed and, instead, a voluntary saccade is generated in the equidistant but opposite direction. Involuntary saccades consist of the fast phase of nystagmus due to vestibular and optokinetic stimuli; spontaneous saccades, providing repetitive scanning of the environment, although also occurring in the dark and in severely visually impaired children; and reflex saccades, occurring involuntarily in response to new visual, auditory, olfactory, or tactile cues, suppressable by antisaccades.83

TABLE 5-1. Types of Eye Movements.

Type of eye movement

Function

Stimulus

Clinical tests

Vestibular

Maintain steady fixation during head rotation

Head rotation

Fixate on object while moving head; calorics

Saccades

Rapid refixation to eccentric stimuli

Eccentric retinal image

Voluntary movement between two objects; fast phases of OKN or vestibular nystagmus

Smooth pursuit

Keep moving object on fovea

Retinal image slip

Voluntarily follow a moving target; OKN slow phases

Vergence

Disconjugate, slow movement to maintain binocular vision

Binasal or bitemporal disparity; retinal blur

Fusional amplitudes; near point of convergence

OKN, optokinetic nystagmus.

OKN, optokinetic nystagmus.

The pathway of saccades originates in the visual cortex and projects through the anterior limb of the internal capsule, through the diencephalon. It then divides into dorsal and ventral pathways, the dorsal limb going to the superior colliculi, and the ventral limb (which contains the ocular motor pathways for horizontal and vertical eye movements) to the pons and midbrain. The superior colliculus acts as an important relay for some of these projections (Fig. 5-2).

The neurons responsible for generating the burst, or discharge, for saccades are classified as excitatory burst neurons (EBN); inhibitory burst neurons (IBN) function to silence activ-

FIGURE 5-2. The superior colliculi are a pair of ovoid masses composed of alternating layers of gray and white matter; they are centers for visual reflexes and ocular movements, and their connections to other structures in the brain and spinal cord are varied and complex. Some of these other structures include the retina, visual and nonvisual cerebral cortex, inferior colliculus, paramedian pontine reticular formation, thalamus, basal ganglia, and spinal cord ventral gray horn. The fibers of the medial longitudinal fasciculus form a fringe on its ventrolateral side: 1, superior (cranial) colliculus; 2, brachium of superior (cranial) colliculus; 3, medial geniculate nucleus; 4, brachium of inferior (caudal) colliculus; 5, central gray substance; 6, cerebral aqueduct; 7, visceral nucleus of oculomotor nerve (Edinger-Westphal nucleus); 8, nucleus of oculomotor nerve; 9, medial lemniscus; 10, central tegmental tract; 11, medial longitudinal fasciculus; 12, red nucleus; 13, fibers of oculomotor nerve; 14, substantia nigra; 15, basis pedunculi.

FIGURE 5-2. The superior colliculi are a pair of ovoid masses composed of alternating layers of gray and white matter; they are centers for visual reflexes and ocular movements, and their connections to other structures in the brain and spinal cord are varied and complex. Some of these other structures include the retina, visual and nonvisual cerebral cortex, inferior colliculus, paramedian pontine reticular formation, thalamus, basal ganglia, and spinal cord ventral gray horn. The fibers of the medial longitudinal fasciculus form a fringe on its ventrolateral side: 1, superior (cranial) colliculus; 2, brachium of superior (cranial) colliculus; 3, medial geniculate nucleus; 4, brachium of inferior (caudal) colliculus; 5, central gray substance; 6, cerebral aqueduct; 7, visceral nucleus of oculomotor nerve (Edinger-Westphal nucleus); 8, nucleus of oculomotor nerve; 9, medial lemniscus; 10, central tegmental tract; 11, medial longitudinal fasciculus; 12, red nucleus; 13, fibers of oculomotor nerve; 14, substantia nigra; 15, basis pedunculi.

Nucleus Iii Inc Cajal Mri

FIGURE 5-3. Brainstem structures controlling eye movements. Parasagittal section of the cerebrum and brainstem shows areas of the ocular motor nuclei and brainstem structures involved with internuclear and supranuclear pathways. PC, posterior commissure; SC, superior col-liculus; IC, inferior colliculus; Pi, pineal; riMLF, rostral interstitial nucleus of the medial longitudinal fasciculus; INC, interstitial nucleus of Cajal; CN III, IV, VI, cranial nerve III, IV, VI; MLF, medial longitudinal fasciculus; PPRF, paramedian pontine reticular formation; VN, vestibular nuclei.

FIGURE 5-3. Brainstem structures controlling eye movements. Parasagittal section of the cerebrum and brainstem shows areas of the ocular motor nuclei and brainstem structures involved with internuclear and supranuclear pathways. PC, posterior commissure; SC, superior col-liculus; IC, inferior colliculus; Pi, pineal; riMLF, rostral interstitial nucleus of the medial longitudinal fasciculus; INC, interstitial nucleus of Cajal; CN III, IV, VI, cranial nerve III, IV, VI; MLF, medial longitudinal fasciculus; PPRF, paramedian pontine reticular formation; VN, vestibular nuclei.

ity in the antagonist muscle during the saccade. In the brainstem, the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the pontine paramedian reticular formation (PPRF) provide the saccadic velocity commands, by generating the "pulse of innervation" immediately before the eye movement, to cranial nerves III, IV, and VI. Horizontal saccades are generated by EBN in the PPRF, which is found just ventral and lateral to the MLF in the pons (Figs. 5-3, 5-4, 5-5), and by IBN in the nucleus paragigantocellularis dorsalis just caudal to the abducens nucleus in the dorsomedial portion of the rostral medulla. Vertical and torsional components of saccades are generated by EBN and IBN in the riMLF, located in the midbrain.

Following a saccade, a "step of innervation" occurs during which a higher level of tonic innervation to ocular motoneurons keeps the eye in its new position, against orbital elastic forces

Rimlf Location

FIGURE 5-4. Transverse section of caudal pons. AbdNu, abducens nucleus; AbdNr, abducens nerve; AMV, anterior medullary velum; CSp, corticospinal tract; FacG, internal genu of facial nerve; FacNr, facial nerve; FacNu, facial nucleus; LVN, lateral vestibular nucleus; ML, medial lemniscus; MLF, medial longitudinal fasciculus; MVN, medial vestibular nucleus; RetF, paramedian pontine reticular formation; SCP, superior cerebellar peduncle; SpTNu, spinal trigeminal nucleus; SpTT, spinal trigeminal tract; SVN, superior vestibular nucleus. (Adapted from Haines DE. Neuroanatomy: an atlas of structures, sections, and systems. Baltimore: Urban & Schwarzenberg, 1983, with permission.)

FIGURE 5-4. Transverse section of caudal pons. AbdNu, abducens nucleus; AbdNr, abducens nerve; AMV, anterior medullary velum; CSp, corticospinal tract; FacG, internal genu of facial nerve; FacNr, facial nerve; FacNu, facial nucleus; LVN, lateral vestibular nucleus; ML, medial lemniscus; MLF, medial longitudinal fasciculus; MVN, medial vestibular nucleus; RetF, paramedian pontine reticular formation; SCP, superior cerebellar peduncle; SpTNu, spinal trigeminal nucleus; SpTT, spinal trigeminal tract; SVN, superior vestibular nucleus. (Adapted from Haines DE. Neuroanatomy: an atlas of structures, sections, and systems. Baltimore: Urban & Schwarzenberg, 1983, with permission.)

FIGURE 5-5. Schematic of brainstem pathways coordinating horizontal saccades. The PPRF, after receiving input from the ipsilateral cortical centers and superior colliculus, stimulates two sets of neurons in the abducens nucleus: (1) those that send axons to innervate the ipsilateral lateral rectus and (2) those whose axons join the MLF and subsequently activate the medial rectus subnuclei of the contralateral third nerve. PPRF, paramedian pontine reticular formation; VI, sixth cranial nerve nucleus; III, third cranial nerve nucleus.

FIGURE 5-5. Schematic of brainstem pathways coordinating horizontal saccades. The PPRF, after receiving input from the ipsilateral cortical centers and superior colliculus, stimulates two sets of neurons in the abducens nucleus: (1) those that send axons to innervate the ipsilateral lateral rectus and (2) those whose axons join the MLF and subsequently activate the medial rectus subnuclei of the contralateral third nerve. PPRF, paramedian pontine reticular formation; VI, sixth cranial nerve nucleus; III, third cranial nerve nucleus.

that would restore the eye to an anatomically "neutral" position. For horizontal saccades, the step of innervation comes from the neural integrator (see following), primarily from the nucleus prepositus-medial vestibular nucleus complex. The eye is held steady at the end of vertical and torsional saccades by the step of innervation provided from the interstitial nucleus of Cajal in the midbrain.288

In addition to burst neurons, omnipause neurons, located in the nucleus raphe interpositus in the midline of the pons, between the rootlets of the abducens nerves, are essential for normal saccadic activity. Continuous discharge from omnipause neurons inhibits burst neurons, and this discharge only ceases immediately before and during saccades.288

Other burst neurons termed long-lead burst neurons (LLBN) have also been identified that discharge 40 ms before saccades, whereas the previously mentioned burst cells discharge 12 ms before saccades. Some LLBN lie in the midbrain, receiving projections from the superior colliculus and projecting to the pontine EBN, medullary IBN, and omnipause neurons. Other LLBN lie in the nucleus reticularis tegmenti pontis (NRTP), projecting mainly to the cerebellum but also to the PPRF. It appears that LLBN receiving input from the superior colliculus may play a crucial role in transforming spatially coded to temporally coded commands, whereas other LLBN may synchronize the onset and end of saccades.288

If an abnormality of saccadic eye movements is suspected, the quick phases of vestibular and optokinetic nystagmus (OKN) can be easily evaluated in infants and young children. To produce and observe vestibular nystagmus, hold the infant at arm's length, maintain eye contact, and spin first in one direction and then in the other. An OKN response can be elicited in the usual manner by passing a repetitive stimulus, such as stripes or an OKN drum, in front of the baby first in one direction and then in another. In addition, reflex saccades are induced in many young patients when toys or other interesting stimuli are introduced into the visual field. Older children are asked to fixate alternately upon two targets so that the examiner can closely observe the saccades for promptness of initiation, speed, and accuracy.

Smooth pursuit permits us to maintain a steady image of a moving object on our foveas and thereby to track moving targets with clear vision. The pathways for smooth pursuit have not been fully elucidated, but it appears that frontal and extrastriate visual cortex transmits information about the motion of both the target and the eyes to the dorsolateral pontine nuclei (DLPN). This complex signal travels from the DLPN to the cerebellum (paraflocculus, flocculus, and dorsal vermis), and from the cerebellum via the vestibular and fastigial nuclei to its final destination, the ocular motor nerve nuclei III, IV, and VI. Unilateral lesions in the cortex and cerebellum affect smooth pursuit toward the side of the lesion.

Vergences are eye movements that turn the eyes in opposite directions so that images of objects will fall on corresponding retinal points. Two major stimuli are known to elicit vergences: (1) retinal disparity, which leads to fusional vergences, and (2) retinal blur, which evokes accommodative vergences. Convergence of the eyes, accommodation of the lens, and constriction of the pupils occur simultaneously when there is a shift in fixation from distance to near; together, these actions constitute the near triad.

The neural substrate for vergence lies in the mesencephalic reticular formation, dorsolateral to the oculomotor nucleus. Neurons in this region discharge in relation to vergence angle (vergence tonic cells), to vergence velocity (vergence burst cells), or to both vergence angle and velocity (vergence burst-tonic cells). Although most of these neurons also discharge with accommodation, experiments have shown that some do remain predominantly related to vergence.32 Like versional movements, a velocity-to-position integration of vergence signals is necessary. The nucleus reticularis tegmenti pontis (NRTP) has been shown to be important in the neural integration, that is, velocity-to-position integration, of vergence signals. The cells in NRTP that mediate the near response appear to be separate from the cells which mediate the far response. Lesions of NRTP cause inability to hold a steady vergence angle. NRTP has reciprocal connection with the cerebellum (nucleus interpositus) and receives descending projections from several cortical and subcortical structures.32,288

The cerebellum plays an important role in eye movements. Together with several brainstem structures, including the nucleus prepositus and the medial vestibular nucleus, it appears to convert velocity signals to position signals for all conjugate eye movements through mathematical integration. Because of this, all the structures involved in this process are often referred to as the neural integrator. The role of the neural integrator in horizontal saccades was mentioned earlier.

To test the neural integrator clinically, observe fixation, fixation in eccentric gaze, saccades, pursuit, and OKN and also test for rebound nystagmus and VOR cancellation. To examine for rebound nystagmus, first ask the patient to fixate on a target from the primary position, then to refixate on an eccentric target for 30 s, and finally to return to the original primary position target. A patient with rebound nystagmus will show transient nystagmus with the slow phases toward the previous gaze position. To evaluate a child's VOR cancellation, it is easiest to place your hand on top of the patient's head to control both the head and a fixation target that will extend in front of the child's visual axis. You may use a Prince rule with a picture attached. Ask the child to fixate on the target as you passively rotate both the head and the target side to side. If the child is unable to cancel the VOR, you will observe nystagmus instead of the steady fixation expected in normal subjects.

Patients with faulty neural integration may show gaze-evoked nystagmus, impaired smooth pursuit, inability to cancel the vestibulo-ocular reflex during fixation, saccadic dysmetria, defective OKN response, or rebound nystagmus. Most frequently, gaze-evoked nystagmus is seen in conjunction with use of anticonvulsants or sedatives. However, because 60% to 70% of brain tumors in children are subtentorial, acquired eye movement abnormalities suggesting defective neural integration, whether isolated or associated with other neurological deficits, alert the examiner to investigate for a serious central nervous system abnormality.39,110,132 Structural anomalies affecting the brainstem and cerebellum, for example, the Arnold-Chiari malformation, as well as metabolic, vascular, and neurodegenerative disorders, may also produce abnormalities of the neural integrator.

Reflex eye movements such as the vestibulo-ocular reflex and Bell's phenomenon are easy to elicit clinically and are very useful for gross localization of neural lesions. When both saccades and smooth pursuit in a certain direction are limited, the examiner tries to stimulate eye movements in that same direction with a doll's head (oculocephalic) maneuver, spin test, or forced lid closure. If any of the reflex eye movements are intact, the appropriate cranial nerve(s) and extraocular muscles(s) are clearly functioning, and the defect is necessarily supranuclear.

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