Monday, February 14, 2022

Eye Movements Case File

Posted By: Medical Group - 2/14/2022 Post Author : Medical Group Post Date : Monday, February 14, 2022 Post Time : 2/14/2022
Eye Movements Case File
EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 27
A 6-month-old female is brought to the pediatrician by her mother. The mother states she has noticed the child moving her eyes back and forth for no apparent reason over the past several weeks, and as a result, is concerned that her daughter may have vision problems. The patient’s chart shows that she had an uncomplicated pregnancy and delivery and has been meeting her developmental milestones. On examination the pediatrician notes involuntary, rhythmic, horizontal eye movements. The remainder of the examination is normal. Based on the patient’s history, the physician decides that the patient’s disorder is most likely congenital.
  • What are the eye movements called?
  • Are there any treatment options available?

ANSWERS TO CASE 27: EYE MOVEMENTS

Summary: A 6 months old female presents with several weeks of abnormal eye movements. Physical examination demonstrates horizontal nystagmus and is otherwise normal.
  • Eye movements: Nystagmus which are involuntary, rhythmic, horizontal eye movements.
  • Treatment options: In most cases there is no treatment for congenital nystagmus. Some patients may, however, qualify for surgical intervention intended to mitigate the effect the nystagmus has on visual acuity.

CLINICAL CORRELATION

Congenital nystagmus is the most common form of nystagmus. Typically, the patient’s visual acuity is only mildly affected and the patient has little awareness of the movements. The etiology of congenital nystagmus is unknown, although it may be caused by a disruption in the nuclei which influence eye movements. Nystagmus may also be acquired and present after neurological trauma, ischemic episodes, or cerebrovascular accidents.


APPROACH TO EYE MOVEMENTS

Objectives
  1. Know the muscles that move the eye and the types of movements they control.
  2. Describe the various types of eye movements.
  3. Know the centers for eye movement control found in the brain stem, cerebellum, and cortex.

Definitions

Convergence: The medial rectus muscles contract to move both eyes toward the midline in order to keep the image in each eye focused.
Saccades: Rapid conjugated eye movements that move the visual axis from one point of fixation to another.
Smooth-pursuit movements: Slow conjugate eye movements that track a moving object and keep it on the fovea.
Optokinetic reflex movements: Stabilizing movements which occur after a retinal slip, when the entire visual field moves with respect to the head.
Vestibular-ocular reflex: Eye movements that occur in response to vestibular signals from movement of the head that move the eyes in an equal and opposite direction in order to keep a stationary image.
Vergence: Converging or diverging the eyes to keep both foveae aligned to a target that moves closer or farther away.


DISCUSSION

Several different types of movements are used to provide the delicate control of the eyes necessary to keep the image from a viewed object placed on the macula. The macula lies near the center of the retina and has the fovea at its center. This is the area of the retina with the highest visual acuity.

The extraocular muscles facilitate three basic movements: horizontal eye movements, vertical eye movements, and rotary eye movements. Horizontal eye movements are principally controlled by the medial rectus and lateral rectus muscles. Vertical eye movements are mainly mediated by the inferior and superior rectus muscles. Rotary eye movements are primarily controlled by the inferior and superior oblique muscles. These muscles are innervated by cranial nerves leaving the brain stem. The abducens nerve or sixth cranial nerve innervates the lateral rectus muscle for abduction of the eye. The trochlear nerve or fourth cranial nerve innervates the superior oblique muscle, pulling the eye down and out. The remainder of the eye muscles is innervated by the oculomotor nerve, or third cranial nerve.

All normal eye movements are synchronized, allowing both eyes to focus on the same image. To achieve this level of control, a delicate balance of excitatory and inhibitory stimuli must be sent to the synergistic and antagonistic muscle groups. The varying degrees of tension between the extraocular muscles permits smooth, coordinated movement. Damage to the extraocular muscles or the neurological mechanisms which control their movement can lead to a deficit between the coordination of the eyes, resulting in diplopia, or double vision.

There are five types of eye movements that interact in order to place a viewed object on the fovea for best visual resolution, and to keep it there as either the object or the observer moves. These five eye movements may be either voluntarily or reflexively controlled and include the following: saccade movements, smooth-pursuit movements, vergence movements, vestibuloocular movements, and optokinetic movements.

Saccades are rapid, conjugate movements that place the desired image on the fovea. These movements are rapid and accurate to allow for object fixation. Saccades allow the entire visual field to be assessed by compensating for the decrease in visual acuity that occurs as an image moves away from the fovea. There are several types of saccades. Volitional saccades direct the gaze to either a remembered location or a location where a target will likely appear. A reflexive saccade can be stimulated by a nonvisual stimulus such as a sound. The frontal eye fields in the precentral sulcus initiate volitional and reflexive saccades. The parietal eye fields in the posterior parietal cortex mediate visually guided saccades of both volitional and reflexive types. There are, however, interconnections between the frontal and parietal eye fields, with each influencing the other. Input from the cerebral cortex travels to the contralateral paramedian pontine reticular formation (PPRF). These inputs come directly from the frontal eye fields, and come by way of the superior colliculus from the parietal eye fields.

During horizontal saccades, the PPRF sends signals to the abducens nucleus of cranial nerve VI to activate both abducens neurons and interneurons. These signals then travel to the lateral rectus muscle and through the interneurons to the medial longitudinal fasciculus in order to signal the contralateral oculomotor nucleus of cranial nerve III for coordinated movement of the medial rectus muscle. The nucleus prepositus hypoglossi projects information to the abducens nucleus about the current position of the head and eyes in order to hold the eyes on target until the end of the saccade.

Vertical saccades also use the PPRF pathway, however, impulses from the PPRF relay through eye movement centers in the midbrain before reaching the motor nuclei. The tegmentum of the rostral midbrain contains the rostral interstitial nucleus of the medial longitudinal fasciculus and the interstitial nucleus of Cajal. These nuclei regulate vertical and torsional saccades through connections to the oculomotor and trochlear motor nuclei.

Large, acute lesions of the cerebral hemispheres can affect saccade movements in the direction contralateral to the side of the lesion. The eyes will often deviate toward the side of the lesion. These deficits are usually temporary because of the ability of other pathways to compensate. Lesions of the pons will result in similar deficits, but affect eye movements toward the ipsilateral side of the lesion. A cerebellar lesion can cause small, dyssymmetric saccades and difficulty maintaining off-center gaze.

Smooth-pursuit movements allow an object to be tracked as it moves slowly through a visual field. These movements keep the desired image on the retina. The movements themselves are involuntarily; however, they do require the observer’s attention to be focused on the object. If pursuit movements fail, saccades are needed to catch up with the target. Visual inputs to the temporooccipital junction carry information about the speed and direction of movement of a target and initiate pursuit movements. Information is projected via corticopontine fibers to the ipsilateral dorsolateral pontine nuclei and from there to the cerebellar vermis, flocculus, vestibular nuclei, and nucleus prepositus hypoglossi. From here the contralateral abducens nucleus and the medial rectus subnucleus of the oculomotor nerve are activated via the medial longitudinal fasciculus.

Vestibulo-ocular movements are reflexive movements of the eye in the opposite direction of a head movement, preventing an object from moving away from the fovea as the head moves. These eye movements occur with the same velocity as the head, allowing the image to remain stable on the retina. These movements can occur along any visual axis. The pathway for these movements begins with signals from the semicircular canals that travel to the vestibular nuclei. The fibers of the vestibular nerve enter the medulla oblongata and pass between the inferior peduncle and the spinal tract of the trigeminal. They then divide into ascending and descending fibers. The latter end by arborizing around the cells of the medial nucleus. The ascending fibers either end in the same manner or in the lateral nucleus. Some of the axons of the cells of the lateral nucleus, and possibly also of the medial nucleus, are continued upward through the inferior peduncle to the roof nuclei of the opposite side of the cerebellum, to which also other fibers of the vestibular root are prolonged without interruption in the nuclei of the medulla oblongata. A second set of fibers from the medial and lateral nuclei end partly in the tegmentum, while the remainder ascends in the medial longitudinal fasciculus to arborize around the cells of the nuclei of the oculomotor nerve.

Vergence movements allow an image to remain in focus when the object’s depth is changing relative to the observer. They occur together with pupillary contraction and accommodation and change the visual axis of the eyes relative to each other through either convergence or divergence. The activation of the vergence system results from a blurred image or from an image falling on noncorresponding retinal areas. Signals from the posterior temporal and prefrontal cortex project to vergence cells in the brain stem and from there to cranial nerves III and VI.

Optokinetic movements occur in response to retinal slip during prolonged head movement at a constant velocity. Initially, the vestibulo-ocular system will produce compensatory eye movements in response to head acceleration, but these movements will fade as the motion of the endolymph in the semicircular canals reaches equilibrium and the vestibular input ceases. The optokinetic movements will then begin to compensate for retinal slip. The indirect optokinetic pathway consists of signals between the temporo-occipital cortex and the accessory optic system nuclei. The direct optokinetic pathway consists of input from retinal ganglion cell axons to the nucleus of the optic tract and nuclei of the accessory optic system, which then project to the cerebellum and vestibular nuclei.


COMPREHENSION QUESTIONS

[27.1] A 29-year-old woman comes in to the emergency department after falling off a ladder. She did not lose consciousness at the time, and does not recall hitting her head as she fell. Nonetheless, you do a screening neurological examination to make sure she does not have any gross deficits. As part of this examination, you ask her to follow your finger with her eyes as you draw an invisible H in front of her. In addition to testing cranial nerve III, IV, and VI, and the integrity of the extraocular muscles, what type of eye movement is this testing?
A. Vergence
B. Visual saccades
C. Vesibulo-ocular movements
D. Smooth pursuit

[27.2] In addition to having your patient follow as you trace an invisible H in the air, you ask her to keep looking at your fingers as you move them closer to her face in the midline. Her eyes appropriately cross as your finger approaches her nose. What is the stimulus for this type of eye movement?
A. Activation of the frontal eye fields
B. Blurred images perceived by the visual cortex
C. Angular motion detected in the semicircular canals
D. Movement detected by accessory optic nuclei

[27.3] A 43-year-old woman comes into your office with the complaint of inability to look to the left. This problem has been going on for several months, with increasing frequency. She is now totally incapable of looking at objects on her left side without turning her head. On examination, she is unable to voluntarily direct either of her eyes toward the left side. If she follows a finger with her eyes, however, beginning in the right visual field, she can track the object as it crosses midline all the way to the lateral extent of normal ocular movement. She finds this quite distressing. Which of the following structures is most likely damaged resulting in this woman’s symptoms?
A. Right frontal eye field
B. Left frontal eye field
C. Right PPRF
D. Left rostral interstitial nucleus of the MLF


Answers

[27.1] D. Tracking a moving object with the eyes with a stationary head is known as smooth pursuit. This seemingly simple action is in fact quite a complicated action requiring the appropriate interaction of numerous levels of the brain and spinal cord. Smooth pursuit movements are initiated in the visual association areas of the parietooccipital cortex, which transmit information about the speed and direction of object motion to dorsolateral pontine nuclei. These in turn project to the cerebellum (particularly the flocculus) and the vestibular nuclei, which project to the abducens and oculomotor nuclei for final control of the extraocular muscles.

[27.2] B. The type of eye movement described is called vergence, and it is initiated by blurred images perceived by the visual cortex or by images falling on noncorresponding parts of the retinas. The exact pathway responsible for vergence has not been elucidated, but it is thought to involve the visual association cortex sending projections to vergence nuclei in the midbrain which then project to the oculomotor nuclei to control the final eye movements. Frontal eye fields initiate voluntary saccades, angular motion is the stimulus for vestibulo-ocular reflexes, and the accessory optic nuclei control optokinetic movements.

[27.3] A. This woman has a defect in voluntary saccadic movements toward left, which are initiated by the right frontal eye field. That she can track an object past midline indicates that the nuclei responsible for ocular movement and the extraocular muscles are all intact, and that the defect is higher than that. The frontal eye field projects to the contralateral PPRF, directly and via the parietal eye field and the superior colliculus. The PPRF then sends signal to the ipsilateral abducens nucleus, which control the lateral rectus and sends impulses to the contralateral oculomotor nucleus to control the medial rectus. This causes both eyes to deviate away from the frontal eye field initiating the movement. The right PPRF is involved in voluntary saccades toward the right side, and the rostral interstitial nucleus of the MLF is involved in vertical saccades.


NEUROSCIENCE PEARLS

Six muscles control the movement of the eye within the orbit: the inferior rectus, superior rectus, medial rectus, lateral rectus, inferior oblique, and superior oblique.
The muscle groups are innervated by the third, fourth, and sixth cranial nerves.
Damage to the extraocular muscles or the neurological mechanisms, which control their movement, can lead to a deficit between the coordination of the eyes, resulting in diplopia, or double vision.
The five types of voluntary and reflexive eye movements include saccades, smooth-pursuit movements, optokinetic reflexive movements, vergence movements, and vestibulo-ocular movements.


REFERENCES

Brodal P. Eye movements. The Central Nervous System: Structure and Function. 3rd ed. New York, NY: Oxford University Press; 2004. 

Haines DE. Visual motor systems. Fundamental Neuroscience. 2nd ed. Philadelphia, Pennsylvania, PA: Churchill Livingstone; 2002. 

Martin JH. The olfactory system. Neuroanatomy, Text and Atlas. 2nd ed. Stamford, CT: Appleton & Lange. 

Ropper AH, Brown RH. Disorders of ocular movement and pupillary function. Adam’s and Victor’s Principles of Neurology. 8th ed. New York, NY: McGraw-Hill; 2005.

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