Examination of the Visual Motor System




CHAPTER OUTLINE



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  • OVERVIEW AND TERMINOLOGY



  • GENERAL OBSERVATIONS



  • OCULAR MOTILITY




    • Fixation



    • Ocular versions and ductions



    • Vergence



    • Pursuit



    • Saccades



    • Ancillary methods



    • Assessing ocular alignment



    • Forced duction testing



    • Nystagmus



    • Eye movement recording



  • PUPILS



  • EYELIDS




    • Examination techniques



    • Ptosis



    • Eyelid retraction



    • Facial nerve disorders



  • ORBIT AND ADNEXA




    • Inspection



    • Palpation



    • Auscultation



  • ADDITIONAL ASSESSMENTS




    • Cranial nerve V



    • Cranial nerve VII



  • KEY POINTS





INTRODUCTION



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This chapter expands the discussion of examination techniques summarized in Chapter 1 and is a companion to Chapter 2, which discusses examination of the afferent visual system. In this chapter, methods for examining the pupil and cranial nerves (CNs) III, IV, V, VI, and VII are presented, with greater detail provided in other chapters regarding examination of the pupil (chapter 11), CN VII (chapter 12), and CN V (chapter 13).




OVERVIEW AND TERMINOLOGY



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The purpose of the ocular motor system is to aim the visual axis of each eye at an object of interest. This function places an image of the object on the area of greatest retinal sensitivity: the foveola. Ideally, the motor system keeps the eyes steady and aligned with the fixation object even when the object is moving in three-dimensional space. The smooth tracking of a moving object is called pursuit. When a new object of interest is encountered, the motor system can quickly redirect gaze with a rapid movement to fixate on a new object of interest. This rapid refixation movement is called a saccade. Generally, when following or finding objects, both eyes move in the same direction (conjugate gaze). However, objects that move toward or away from the observer require a disconjugate eye movement (vergence). For example, an object approaching the observer’s nose from a distance requires the eyes to converge, or turn in (right eye turns to the left, left eye turns to the right) to maintain fixation. Divergence occurs in the opposite setting: When the fixation object moves from near to far the eyes move outward—from a converged state to a more parallel alignment. Additional complexity is introduced when the observer is moving or when both the observer and the object are moving in space. Even in this complex situation, eye movement systems can provide stability of fixation, compensating for movement of the observer as an object is tracked.



When binocular fixation occurs, the brain can synthesize depth information from the slightly disparate views of the object (stereopsis). If the motor system fails to provide alignment, the brain cannot reconcile the two images, potentially resulting in confusion and diplopia (Box 7–1).



BOX 7–1. COMMONLY CONFUSED OCULAR MOTILITY TERMS


Abduction/adduction Abduction is horizontal movement of an eye laterally, away from the nose. (A person who is abducted is taken away.) Adduction is horizontal movement of an eye medially, toward the nose.


Comitance/incomitance Ocular misalignments (strabismus) that maintain the same deviation in all gaze positions are said to be comitant (or concomitant). Comitance is a common feature of congenital strabismus. When the angle of deviation changes with the direction of gaze, the strabismus is said to be incomitant. Incomitance is the hallmark of cranial mononeuropathies, with increasing ocular misalignment in the direction of the palsied muscle.


Confusion/diplopia Diplopia refers to seeing the same image twice in a given field of view. However, patients are most often symptomatic with diplopia because it is accompanied by confusion. In confusion, two superimposed images occupy the central vision, causing confusion as to which is the real “straight ahead” image.


Ductions/versions Ductions describe the movement of a single eye, usually with the nontested eye covered. Common prefixes include Ab, away from the nose; Ad, toward the nose; supra, up; infra, down. Versions refer to the movement of both eyes together in the same direction (conjugate gaze). Vergence is movement of the two eyes in opposite directions (convergence and divergence).


Ocular motor nerves/oculomotor nerve The term ocular motor nerves refers to all three motor nerves, involved in ocular movement: cranial nerves (CNs) III, IV, and VI. The oculomotor nerve is CN III alone.


Palsy/paralysis/paresis These terms refer to impairment of function of a motor nerve. Palsy and paralysis suggest that the nerve is not working at all, whereas paresis denotes any level of weakness short of absolute. In practice the terms are often used interchangeably. These terms are also often applied to the nerve or the muscle it innervates: A CN IV palsy and a superior oblique palsy are the same thing.


Pursuit/saccades Pursuit ocular movements are slow, smooth, binocular ocular rotations that allow precise tracking of slow-moving objects (or movement of the observer). Pursuit is tested by having the patient track the examiner’s slowly moving finger in the horizontal and vertical planes. Saccades are fast eye movements that redirect the eyes to a new object in the visual field. Saccades are tested by having the patient look from one of the examiner’s fingers to the other in the horizontal and vertical plane. Optokinetic nystagmus (OKN) testing evaluates both pursuit and saccades. Pursuit and saccades are discussed in detail in Chapter 10.


Tropia/phoria A phoria (heterophoria) is an ocular deviation that occurs only when binocular fixation is disturbed, such as when one eye is covered. When viewing an object with both eyes, a subject with a phoria is capable of aligning the eyes to achieve fusion (single binocular vision). A tropia (heterotropia) is an ocular misalignment that is present even when both eyes are viewing, and may result in diplopia. Descriptive prefixes include eso, inward deviation (toward the nose); exo, outward; hyper, upward; hypo, downward—as in esophoria or hypertropia.


Right and left designations of tropias and phorias When a horizontal strabismus is incomitant and one eye is obviously the culprit, then it makes sense to say right esotropia or left exophoria. In comitant horizontal deviations, both eyes contribute equally to the problem and left or right is not designated. However, with vertical deviations, an eye must be designated and called hypo or hyper. Usually, the eye suspected of being the weakest is designated, but left hypertropia and right hypotropia say the same thing: The left eye is higher than the right.




Six extraocular muscles insert on the globe and act in concert to rotate the eye horizontally and vertically. In addition, the globe can be rotated to a limited extent around the visual axis (torsion) to provide limited compensation for head or environmental tilt.



The extraocular muscles are controlled by CNs III, IV, and VI, which originate in the brainstem. CN III also innervates the levator muscle (for elevating the upper eyelid) and carries parasympathetic input to the pupillary sphincter. The CNs are in turn coordinated by supranuclear regions in the brain. Other components of the efferent visual system include CN VII (the facial nerve), which innervates the orbicularis muscle that closes the eye and the sympathetic system that controls pupillary dilation. A brief discussion of the examination of CN V (trigeminal nerve) is included in this chapter, although it is primarily a sensory (afferent) cranial nerve.




GENERAL OBSERVATIONS



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The fine art of observation cannot be detailed in a textbook. The examiner should realize that the examination actually begins when the physician first encounters the patient—observing how the patient ambulates to the examination room, sits in the chair, and shakes the examiner’s hand. Initial observations of the patient’s facial movements, eyelids, and eye movements should be made during the history. Such observations are important because they may be more objective than when the patient is aware that his or her eyes and face are being scrutinized (see Figure 6–12).




OCULAR MOTILITY



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A general guide for evaluating ocular motility is presented in Table 7–1.




TABLE 7–1.

EXAMINING OCULAR MOTILITY





FIXATION



Fixation describes how the eyes remain trained on a single, stationary distant or near object. Fixation is normally central, with the eye pointed directly at the target object. Patients with central scotomas or anomalous retinal correspondence may display eccentric fixation. Fixation cannot occur if the patient’s poor vision precludes seeing the fixation object. Patients with ocular misalignment may adopt a head turn or tilt, or may close one eye to avoid diplopia when both eyes are attempting to fixate.



Fixation is normally steady, without interruption. Abnormalities include square-wave jerks (brief, back-and-forth, horizontal saccadic diversions) from brainstem or cerebellar disease, or nystagmus.



Normal fixation is also maintained, that is, both eyes remain trained on the target, even if fixation is momentarily broken by a blink or brief occlusion of one eye. In young children, the determination of central, steady, and maintained fixation may be the only clues of central visual function that the examiner can determine. In adults, observations about fixation are usually made during visual acuity testing and throughout the motility examination.



OCULAR VERSIONS AND DUCTIONS



Ocular version testing looks for defects in the full range of conjugate gaze. The test is performed with both eyes open, observing each eye and comparing the two eyes at the extremes of gaze in the cardinal positions (Figure 7–1). The test is performed in a manner that allows observation of the action of each of the extraocular muscles. The actions of the vertically acting muscles are best tested when the eye is in adduction (superior and inferior oblique muscles) or abduction (superior and inferior rectus muscles). Ocular version testing also includes testing straight up and down to look for vertical gaze palsies.




Figure 7–1.


Observing and recording ocular versions.

The cardinal positions of gaze are demonstrated. Note that six of the locations (outer columns) allow relative isolation of the action of individual extraocular muscles. Straight up and down assess vertical gaze. One method of recording observations of ocular versions is shown in the lower half of the illustration: 0 is normal motility and −4 means that the eye cannot get past primary position, with intermediate underaction graded −1, −2, or −3. A −5 designates that the eye cannot even achieve midposition, suggesting contracture of the opposing muscle. A “+” denotes an overaction (not shown in this example). The patient presented in this example has a right oculomotor nerve palsy. Observe how the numbers correspond to the action of individual muscles in each eye (arrows).





In horizontal gaze, the normal eye can be abducted and adducted such that little or no sclera is visible in extreme gaze. This ability varies somewhat among individuals (especially with age) but normally should be symmetric between eyes. Vertical gaze is more difficult to judge because the eyelids move with vertical eye movements. Upgaze is particularly dependent on age, with increasing deficiency in normal patients with age. The upper eyelids need to be held up by the examiner to fully observe the eyes in the lower gaze positions. (However, it is important at some point to observe the position of the eyelids in downgaze.) The results of the motility examination can be recorded as shown in Figure 7–1.



Ocular duction testing is a monocular test, usually performed with the fellow eye covered, that examines the range of motion of a single eye. Ocular versions are performed first, followed by duction testing when necessary.



VERGENCE



Vergence testing examines the ability of the eyes to track an object from distant to near. It can be measured (in centimeters from the bridge of the nose) as the point at which an approaching accommodative target breaks down and is seen as double, and the point at which fusion is regained when the path of the target is reversed. However, these measurements are entirely dependent on the effort exerted by the patient, making interpretation difficult. Convergence effort can be maximized by having the patient focus on a near-card letter, rather than the examiner’s finger. Another method is to use the patient’s own finger as the convergence object, guided by the examiner from distant to near. Even blind patients can be tested in this manner because proprioceptive clues help the patient achieve convergence.



Convergence occurs by activation of both medial rectus muscles through a different supranuclear pathway than conjugate gaze. Disparities in medial rectus function between convergence and ocular versions may help to localize the lesion as supranuclear. For example, in an internuclear ophthalmoplegia (INO), an adduction deficit is present on attempted lateral gaze, but adduction is usually normal with convergence. This finding usually helps to distinguish the INO from an orbital restrictive process or partial CN III palsy.



Testing convergence is also important when convergence insufficiency is suspected. These patients have an exophoria or intermittent exotropia at near only and complain of vague discomfort (and occasionally diplopia) after extended near tasks. Usually a benign idiopathic condition, convergence insufficiency can also be associated with head trauma.



Convergence testing is also performed to observe the pupillary response to near (see Table 11–2) and should be performed whenever the pupillary light reaction is poor.



PURSUIT



Pursuit testing checks the brain’s supranuclear smooth-tracking mechanisms (discussed in Chapter 10). Pursuit is tested by asking the patient to follow the examiner’s finger or other target as it slowly and smoothly moves between the extremes in the horizontal plane and then in the vertical plane. The pursuit target should be at least an arm’s length from the patient to minimize confounding factors from convergence and accommodation. Abnormal pursuit may be saccadic (choppy) rather than smooth in one or both directions. Pursuit testing can be performed as an integral part of testing ocular versions—observations of how smoothly the patient can track the examiner’s moving finger evaluates pursuit, and the ability of each eye to reach the endpoint at extreme gaze evaluates ocular versions.



SACCADES



Saccades are tested by asking patients to look rapidly from primary position (looking at the examiner’s nose) to a target (the examiner’s finger positioned in right and left gaze in the horizontal plane or straight up and down in the vertical plane). Supranuclear abnormalities may cause hypometric saccades, in which the eyes do not line up on the new fixation target in one quick movement, but require additional smaller saccades to finally arrive on target. Hypermetric saccades, in which the eyes overshoot the target, also occur. Unilateral slow saccades are identified by comparing the two eyes: The slow eye is still moving toward the target after the normal eye has completed its saccade. Unilateral slow saccades are seen in an eye with a neurogenic palsy, in contrast to saccades that are abruptly halted in restrictive disorders such as thyroid eye disease. Slow movement of the adducting eye alone suggests an internuclear ophthalmoplegia (INO). Attempted upward saccades produce convergence-retraction nystagmus in patients with dorsal midbrain syndrome (see Chapter 10).



ANCILLARY METHODS



Optokinetic Nystagmus Testing


Optokinetic nystagmus (OKN) testing was discussed briefly in the context of afferent visual system testing as an objective physiological response to test gross vision (see Table 6–2). The afferent limb of this physiological response begins with a moving visual stimulus, usually alternating black and white stripes on a rotating drum, but any interesting stimuli that are equally spaced at appropriate intervals moving across the visual field will work (Figure 7–2). The efferent limb consists of pursuit of the moving target in one direction and a refixation saccade in the opposite direction to acquire the next target. This test has value as a physiological stimulus that allows observation of saccade and pursuit movements, even in patients who may be incapable of cooperating with other examination techniques. In addition, the rapidly repeating saccades and pursuit greatly increase the clinical sensitivity of detecting abnormalities. When performing an optokinetic test, the amplitude and frequency of the induced nystagmus should be observed, particularly when compared to the response from stimulus movement in the opposite direction. Box 7–2 lists the important clinical uses of the optokinetic response.




Figure 7–2.


Optokinetic nystagmus test.

(A) As the drum is rotated from left to right across the patient’s field, pursuit movements are generated to follow the stimulus to the right, with rapid refixation saccades to the left. (B) A plot of the eye movements in response to the optokinetic stimulus is shown.





BOX 7–2. UTILITY OF OPTOKINETIC TESTING


AFFERENT VISUAL SYSTEM


Objective determination of vision in patients with nonorganic vision loss


EFFERENT VISUAL SYSTEM


Dorsal midbrain syndrome. Upward saccades (downward moving optokinetic nystagmus [OKN] stimulus) generate convergence retraction nystagmus.


Internuclear ophthalmoplegia. The slowed adducting saccades (and abducting nystagmus of the opposite eye) are accentuated when the OKN stimulus is rotated toward the affected eye.


Progressive supranuclear palsy (early). The patient’s eyes follow the stimulus but cannot generate a return saccade (the “drift” sign).


Congenital nystagmus. When an OKN stimulus is presented at the null point, the nystagmus induced is opposite in direction from the normal response in about two-thirds of patients.


Parietal lobe lesions may affect the pursuit response when the target is moved toward the side of the lesion, diminishing the amplitude of the nystagmus (when compared to rotation to the opposite side).




Oculocephalic (Doll’s Head) Maneuver


The oculocephalic maneuver helps determine the level of a lesion in a gaze palsy because it uses the vestibular system to move the eyes. The test is performed by rotating the patient’s head in the plane of the gaze palsy while the patient fixates on a stationary object. Patients who are unable to produce a gaze movement with this maneuver have a lesion at or below the level of vestibular input to the efferent visual motor system. Patients with a gaze palsy who can achieve full horizontal gaze with this maneuver but cannot do it volitionally have a supranuclaear palsy. The test can be performed for both horizontal and vertical gaze palsies.

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Jan 2, 2019 | Posted by in OPHTHALMOLOGY | Comments Off on Examination of the Visual Motor System
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