Ординатура / Офтальмология / Английские материалы / The Neurology of Eye Movements_Leigh, Zee_2006

poral sulcus.35a>132'196 In area MST, the cells seem more likely to participate in coarse stereopsis and may function in signaling self motion and/or initiation of vergence. In another region in the caudal part of the lateral bank and fundus of the intraparietal cortex, there are neurons that discharge in relationship to the threedimensional orientation of objects221 or to objects moving toward an animal.21 Whether neural activity in these various cortical areas is used to trigger activity in the premotor staging areas for vergence commands (and if so, exactly how) is unknown. As discussed above, however, lesions in homologous regions in the cat lead to abnormalities of vergence. An attractive hypothesis is that area MST, shown to be so important for generating pursuit movement, also commands tracking eye movements in three dimensions. In this way it could drive both pursuit and vergence premotoneurons within the brain stem and cerebellum and thus ensure that images of targets moving across the visual field and in depth are kept stable on the fovea.

The widely distributed nature of processing of information about three-dimensional space is reflected in PET studies of hu-

mans performing a binocular disparity discrimination. There are increases in blood flow in the polar striate and neighboring peristriate cortex, the parietal lobe, the dorsal lateral and mesial prefrontal cortex, and the cerebellar vermis.72 Impairment of stereopsis (tested with ran- dom-dot stereograms) can be induced by

repetitive magnetic stimulation of occipital cortex in humans.230

Westheimer and Mitchell248 studied vergence movements in a split-brain patient who had undergone section of the corpus callosum and anterior commissure. A near-target light located on either side of the sagittal plane induced vergence eye movements, but a near target located exactly in the midsagittal plane did not. In this latter circumstance, images lay on the temporal retina of each eye, and therefore did not gain access to the same cerebral hemisphere. This evidence suggests that fusional vergence movements require that

visual information from each eye reach the same hemisphere.

CONCEPTUAL MODELS OF SUPRANUCLEAR CONTROL OF VERGENCE

The organization of vergence premotoneurons has many parallels with that of the saccadic system. Thus it will be useful to compare the functional roles of these various types of neurons in the generation of saccadic and vergence movements. Likewise, smooth tracking of targets moving slowly in depth is in some ways comparable to smooth pursuit of targets moving across the visual field. Accordingly, we will use a conceptual framework for the supranuclear control of vergence analogous to current ideas about the control of saccades and pursuit. Although this scheme is speculative, we believe it useful for understanding vergence.

VergenceIntegrator

Both the saccadic system and the vergence

system must provide the appropriate posi- tion-coded information to hold the eyes

steady at the end of each movement. This involves maintaining the eyes in a particular orbital position after saccades and at a particular vergence angle after vergence. Because the eyes are held in position reasonably well even in darkness, immediate visual feedback cannot account for the perseveration of tonic activity in the dark. One way to obtain the necessary position information is to integrate (in the mathematical sense) the prior velocity command that brought the eyes to their present position. Models for generating conjugate eye movements incorporate such a velocity to-position integrator (see The Need for a Neural Integrator of Ocular Motor Sig-

nals, Chap. 5). Models of the vergence system have also incorporated an integrator to explain vergence input-output relationships.118 This vergence position integrator is presumably distinct from the

conjugate position integrator, although hy-

pothetical constructs suggest some commonality.30'144 It has yet to be established

experimentally, however, that the premotoneurons that carry eye position signals during conjugate movements also carry eye position signals during pure vergence.

Commands forSaccadic

Vergence Movements

The source of input to the vergence integrator may be the output of vergence burst cells.141 The vergence tonic cells may then carry the output of the vergence integrator. The vergence burst-tonic cells seem to combine both vergence velocity and vergence position information. A parsimonious interpretation of these observations is that the vergence system uses a direct (velocity) pathway from vergence burst neurons, in parallel with a vergence integrator (position) pathway; the combined signal may be the input to the ocular motoneurons. The finding that ocular motoneurons discharge not only in relation to the angle of vergence but also to the velocity of vergence is consistent with this idea.60

Finally, one may ask what determines the moment when vergence burst neurons cease discharging. A scheme analogous to that for saccades (see Fig. 3-7, Chap. 3) using internal signals proportional to desired vergence position, actual vergence position (based on efference copy), and vergence motor error has been proposed.254 Vergence motor error would serve as the necessary error signal to drive the vergence burst neurons to provide the appropriate vergence velocity command for the correct duration. A similar scheme has been used to account for the interaction between saccades and vergence during combined shifts of gaze that move the

point of regard both across the visual field and in depth.253 It is not settled whether a

desired vergence position or a desired change in vergence position (analogous to the change in eye position signals that drives saccades) is the critical input signal to the premotor vergence generator.121

Vergence Eye Movements 301

When both saccades and vergence are commanded together, the relationship between their latencies of initiation and target selection suggests common signal processing, probably in the cerebral hemispheres, at an early stage of saccade and vergence initiation.18^227 Downstream,however, their trigger signals must diverge, since each can be influenced separately by the conditions of fixation (see, for example, the gap effect discussed above).

Commands for Pursuit

Vergence Movements

One can also propose a scheme, analogous to models of smooth pursuit (see Chap. 4), for pursuit vergence tracking of slower, smoothly moving stimuli. In this case, a desired vergence velocity would be recreated and used to generate a vergence velocity error signal and, in turn, a vergence acceleration command. Thus, we speculate that there may be separate vergence premotor networks for generating saccadic and pursuit vergence. One can envision that both systems work in concert, just as they do for pursuit and saccades in the conjugate system. One movement brings images to the fovea, and the other attempts to keep them there. In fact, such a combination of saccadic and pursuit vergence has been reported when

the velocity of the disparity change is high.215

One implication of this scheme for vergence is that it may be possible to classify vergence disorders in a way similar to that for disorders of saccades, pursuit, and conjugate gaze-holding. For example, holding of positions of convergence may be impaired, perhaps with consequent divergent drift. This would be analogous to the impaired holding of eccentric positions of conjugate gaze after saccades,with consequent centripetal drift. Instability of the conjugate integrator leads to slow phases with an increasing velocity waveform. Similarly, the vergence integrator might become unstable, leading to excessive convergence and convergence spasm (see Abnormalities of Vergence,below).

ADAPTIVE MECHANISMS TO MAINTAIN OCULAR ALIGNMENT

As has been emphasized in previous chapters dealing with the conjugate eye movement systems, a robust and versatile adaptive capability is essential if an organism is to maintain optimal visuomotor function throughout its life. Most research has focused on conjugate adaptive mechanisms, especially those of the saccade and vestibular systems. Usually, however, muscle weakness is unilateral and asymmetric, so that many of the needed adaptive corrections are disconjugate and may have to vary with the position of the eye in the orbit. Such a capability implies that Hering's Law of equal innervation is not immutable, although the exact mechanisms by which this adaptation takes place are unknown.

Phoria Adaptation

Phoria is the relative deviation of the visual axes when a single target is viewed with one eye. If a disparity is introduced by placing a wedge prism in front of one eye,

the subject's phoria changes by an amount equal to the strength of the prism. Tropia,

the relative deviation of the visual axes when the target is viewed binocularly, does not occur if the new disparity is within the range in which fusional mechanisms can cope, but the residual fixation disparity, or steady-state vergence error during binocular viewing, is increased. In seconds to minutes, however, the subject undergoes phoria or prism adaptation, so both the phoria and the fixation disparity (measured with the prism on) revert to their preprism values. Thus, there has been a resetting of the alignment of the two visual

axes by an amount equivalent to the prismatic demand.17,28,122,149,167,168,200,202,217

It has been suggested that the reduction in fixation disparity after prolonged wearing of a prism is accomplished by a slow fusional adaptive mechanism.168'200'202 The output of the slow fusional mechanism resets the level of tonic vergence so as to reduce fixation disparity. This relieves the

stress from the increased disparity demands of the prism on the fast fusional mechanism (or what we commonly think of as disparity-induced vergence).

There are important practical implications of such a phoria adaptation mechanism.28 Short-term phoria adaptation influences measures of the range of divergence and convergence as tested with basein or base-out prisms; this influence varies with viewing distance.167'194 Conversely, to fully appreciate the baseline phoria in a symptomatic patient, one must eliminate binocular cues for hours to days, allowing phoria adaptation to dissipate.82'161 Normal subjects also commonly show a change in phoria after prolonged monocular occlusion, often in the pattern of an oblique muscle imbalance.128'160 Rarely, a symptomatic tropia may emerge after prolonged occlusion, requiring treatment with prisms or even surgery.13 A robust phoria adaptation mechanism might act to limit the efficacy of prismatic therapy for ocular motor imbalance. These patients "eat up the prism," as their phoria adaptation overcomes the effect of the prism and defeats its purpose. Elderly individuals show decreased phoria adaptation, but this has the advantage that they often accept a prismatic correction more readily than younger patients.250

An adaptive mechanism has also been demonstrated for accommodation. By opening the visual feedback loop, one can measure the tonic level of accommodation (i.e., the accommodative phoria). Using appropriate lenses, one can demonstrate that the tonic level of accommodation can

be adaptively readjusted, independent of a change in disparity.146'165'193

Using these findings, models for phoria adaptation that incorporate both accom-

modation and vergence have been developed (Fig. 8-6).91'202 The fast fusional sys-

tem uses retinal image disparity, and the slow fusional adaptive system uses the motor output of the fast fusional system as its error signal. The fast fusional vergence system appears to use a slightly imperfect, leaky integrator with a time constant of 10 to 15 sec. This is the vergence position integrator previously described. The slow fusional adaptive system also uses a leaky integrator but with a much longer time

constant (minutes or more). In fact, there are probably multiple mechanisms that subserve phoria adaptation, with different capabilities, degrees of permanency, and time courses of action. In time, the slow fusional mechanism takes over much of the load of keeping the eyes aligned, by resetting the level of tonic vergence. Thus, phoria adaptation resets the resting position of the eyes toward the original phoria and thereby restores the dynamic range (or fusional reserve) in which fast fusional vergence can function. Similar considerations apply to the accommodation system. The fast accommodative system uses retinal blur and the slow accommodative system adjusts tonic accommodation using the output of the fast system as its error signal. One unresolved issue is the stage of central processing at which voluntary vergence and accommodation interject their influences on phoria adaptation.37'146

One may ask if the AC/A or CA/C ratios are genetically fixed or if they can be modified by environmental factors. If subjects wear periscopic spectacles to simulate an increase in the interocular separation,

both the AC/A and the CA/C ratios may change.10'53'100-152 Such a mechanism would be necessary, for example, to optimize visual function as the interpupillary distance increased during growth or to assure an accurate response when accommodation or vergence fatigues.149 Some disorders of binocular ocular motor function (e.g., vergence excess or vergence insufficiency) may have their basis in alterations in the strength of the cross-linkages between accommodation and convergence and/or in the sensitivity of the slow adaptive mechanisms for vergence and accommodation (see Abnormalities of Vergence).

The anatomic substrate underlying phoria adaptation is not known. Physiological recordings indicate that some but not allof the phoria adaptation signal is carried by midbrain vergence-related neurons.155 Patients with cerebellar lesions occasionally show a decrease in phoria adaptation,148 but in most cases it is normal.73 Monkeys with floccular lesions can still undergo phoria adaptation.97 The deep cerebellar nuclei may be the critical cerebellar structure influencing phoria adaptation.

Disconjugate Adaptation

A special case of phoria adaptation is illustrated by the response to wearing an anisometropic spectacle correction.77'217 Anisometropia is a difference in the anteriorposterior dimensions of the two eyes and requires a corrective lens of a different power for each eye. Corrective spectacle lenses have a prismatic effect that results in the relative displacement of an image of an object from its actual position. This is also known as the rotational magnification effect because it changes the amount the eye must rotate to fix upon a target located at a given point in space. (The linear magnification effect, on the other hand, describes the relative size of an image of an object. Contact lenses have a linear but not a rotational magnification effect.) The prismatic effect of a spectacle lens is roughly proportional to both its power and the distance from its optical center. This effect will increase continuously toward the lens periphery. With an anisometropic correction, the prismatic effect of each spectacle lens is different. Therefore, the retinal disparity between the images of a given object will change as a function of gaze position. Accordingly, ocular alignment must undergo disconjugate adaptation to produce a new pattern of innervation, as a function of orbital position. When subjects begin wearing an anisometropic spectacle correction, their phoria (as measured while wearing their spectacle correction) soon reverts to the preadaptation state in all positions of gaze (i.e., the resting ocular alignment appropriately varies as a function of orbital position).77 This is the way to assure concomitancy while wearing glasses; ocular alignment becomes correct in all orbital positions. However, to achieve fusion promptly upon changing gaze, a subject wearing an anisometropic spectacle correction must also be able to change the alignment of the visual axes during the saccade.

The most frequent circumstance to which a disconjugate adaptive mechanism must respond is asymmetry in the strengths of the eye muscles themselves. This may occur either during natural de-

velopment and aging or after trauma or disease of the ocular motor nerves or orbital contents. Such asymmetries lead to a noncomitant deviation and consequent diplopia if the disparity-driven fusional mechanisms cannot overcome it. It is due to this visuomotor problem that our disconjugate adaptation mechanisms evolved, certainly not from a need to wear corrective spectacles!

Disconjugate adaptation has been extensively investigated using a number of techniques and covering time frames of adaptation ranging from minutes to days.26 These paradigms include having anisometropic subjects wear newly fitted corrective spectacles47'125'175 or having emmetropic subjects wear optical devices that simulate a spectacle-corrected anisometropia, such as a contact lens-spectacle combination77'254 or an afocal magnifier.124'206 Other techniques used to elicit disconjugate adaptation include wearing

displacing prisms in front of one eye in just one part of the visual field,5'174'251'254

presenting different-sized images (aniseikonia) of a target to each eye,102-103'239

and dissociating the images seen by each eye at the end of a saccade.2'38'103'206

In experimental animals, surgically or botulinum-induced asymmetrical muscle weakness elicits disconjugate adaptation.94'244 Human patients with strabismus

have also been a model group for studying disconjugate adaptation.i2,93,iooa,ioi,i27a

What have we learned from these many experiments? First, clinical experience indicates that the degree to which the innervation to the two eyes can be selectively adjusted to overcome a noncomitant deviation is limited. If the relative degree of weakness is large, disconjugate mechanisms may be overwhelmed. Another factor may the degree of ocular dominance. Patients who strongly prefer to fix with one eye (even with both eyes viewing) may undergo no adaptation at all if the preferred eye is the strong one. If the preferred eye is the weak one, they may undergo conjugate adaptation, which increases the innervation to both eyes.

Second, in some individuals, especially some patients with a long-standing requirement for a disconjugate correction,

disconjugate adaptation can be remarkably robust. As an example, consider the recordings shown from a subject who had been wearing spectacles to correct a large degree of anisometropia (Fig. 8-7). It is important to note that the intrasaccadic and postsaccadic changes in alignment occurred under both binocular and monocular viewing conditions. In other words, the subject learned to preprogram intrasaccadic and postsaccadic disconjugate movements independent of any immediate disparity cues.

Third, and even more remarkable, is the finding that subjects may have more than one motor program of disconjugate

innervation, which can be gated in and out on the basis of context.175 Both the

phoria and the yoking of the eyes can be trained to specific combinations of eye po-

sitions, both across the visual field and in depth.18'209'210-251 Even the angle of head

Figure 8-7. Search coil recordings showing disconjugate adaptation to spectacle-corrected anisometropia.175 The subject habitually wore a spectacle correction of about -10 diopters (myopic correction) in front of the left eye and -0.5 diopters in front of the right eye. (This correctioncalls for divergence on right gaze and convergence on left gaze.) For this recording only the right eye was viewing the target (i.e., there were no disparity cues). Note the change in vergenceduring the saccade and the corresponding change in phoria at the end of the saccade. RE, right eye position; LE, left eye position; VERG, vergence angle; obtained by subtracting the right and left eye position traces.

Vergence Eye Movements 305

tilt can be a contextual clue for gating in different adaptive changes in phoria.134'135 The degree of context specificity does have some limits. If two different eye positions used as contextual cues are too close to each other, adaptation at each will interfere with adaptation at the other.174'206 These types of interactions can be successfully simulated using neural network models.142'143 Disconjugate adaptation also can be made selective to one type of conjugate eye movement (e.g., pursuit) and not to another (e.g., saccades).204 Disconjugate pursuit adaptation and phoria adaptation can be trained together, leaving saccade conjugacy unchanged.205 This last finding suggests that the velocity (pulse) and position (step) components of conjugate innervation to each eye may be differentially adapted.

The exact mechanisms underlying both the static and dynamic changes in ocular alignment that occur with dis-

conjugate adaptation are not presently known.5'3*'102'124'125'174'175'205'239 Presumably,

the retinal disparity that occurs at the end

to overcome it, is the necessary error signal used to readjust the relative innervation to the eyes during and after eye movements. Afferent cues from orbital proprioceptors may also be important.127 Patients with microstrabismus and lack of bifoveal fixation can still undergo disconjugate adaptation, but only if some degree of binocular function is present.12'101 The beneficial effect of corrective surgery in childhood strabismus is aided by disconjugate adaptive mechanisms that may come into play once some binocular function is restored.93

With respect to the motor learning itself, there could be an adjustment of the innervation to the two eyes independently, or it could perhaps reflect a modification of the normal interaction between saccade and vergence eye movements.5 Recall that even under normal circumstances, changes in ocular alignment are facilitated when vergence movements are combined with an ongoing saccade (see Fig. 8-2). Whatever the precise mechanisms, such a disconjugate adaptive capability is exceed-

ingly important. It will make adjustments not only for acquired abnormalities but also for the small, inherent asymmetriesin ocular muscle strength and in other orbital mechanical properties that exist in all humans.

EXAMINATION OF

VERGENCE MOVEMENTS

As with the interpretation of all ocular motor function, it is important to measure the corrected visual acuity of each eye, for both near and far viewing. In addition, it is useful to measure stereopsis as a prelude to evaluating vergence. Appendix A contains a summary of the examination.

Conventionally, the examiner tests fusional and accommodative vergence together by asking the patient to fix upon an accommodative target (one that requires bringing its image into focus) as it is slowly brought in along the sagittal plane to the bridge of the nose. More quantitative estimates of a near point of convergence can be made using both objective and subjective tests. Such measurements are helpful in evaluating patients with visual fatigue

(asthenopia) or horizontal diplopia due to convergence insufficiency. The neurolo-

gist should always keep in mind that presbyopia, the loss of accommodation that be-

comes symptomatic when humans reach their early forties, is often the cause of a number of visual complaints. These include episodic diplopia, visual fatigue, and difficulty with reading.

Testing the vergence responses to pure fusional or pure accommodative stimuli usually requires use of prisms and lenses, and in some cases, laboratory facilities. Nevertheless, if the patient has a phoria but not a tropia, one can infer that fusional vergence mechanisms are working.

The fusional vergence system may be tested directly by asking the patient to fix upon a distant target. Insertion of a horizontal prism before one eye will then induce a fusional vergence movement, often in combination with a saccade. By slowly and progressively increasing the amplitude of the prism (for example, using a ro-

tary prism) until diplopia occurs (the break point of vergence), one can gain a measure of the range of fusional amplitudes for both convergence and divergence. Fusional capabilities depend upon the stimulus.For example, disparities seen in the periphery aid fusion of central targets.96 Measures of fusional vergence amplitudes can only be properly interpreted if the patient's underlying phoria is known. The recovery point of vergence (when fusion is restored as the prism strength is decreased) is also an important measure, and may be different from the break point in patients with, for example, intermittent deviations (see Von Noorden245 for a discussion of these testing techniques).

The accommodative vergence system may be tested using the procedure of the Miiller experiment (the heterophoria method). One eye is covered and the other eye changes fixation from a far to a near target, both of which lie along the visual axis of the viewing eye. (Alternatively a plus or a minus lens may be placed in front of the viewing eye to change the depth of focus.) The vergence movement of the covered eye is recorded or measured using prisms when the occluder is switched to the other, uncovered eye.

This procedure is often used to measure the AC/A(accommodative convergence/accommodation) ratio. Conventionally, mea surements of the phoria are made when viewing a distant target and one at 33 cm. Then, the AC/Aratio is given by the equation

AC/A =IPD +(phoria[n] -phoria[d])/d

where IPD is the interpupillary distance (cm); phoria[n] is the phoria in prism diopters (exodeviations are negative, esodeviations are positive) when viewing the near target; phoriafd] is the phoria when viewing the distant target, and d is the fixation distance of the near target in sphere diopters (in this case, 3.0).

The dynamic aspects of vergence eye movements can be judged at the bedside by asking the patient to change fixation abruptly between near and far targets aligned along the midsagittal plane (sac-

cadic vergence) and to follow a target moving slowly in depth (pursuit vergence). The dynamic responses of vergence movements elicited by pure disparity or pure accommodation stimuli can be elicited with prisms and lenses, as already described.

Vergence movements, which are characteristically slow, should be differentiated from abnormal rapid disjunctive movements, such as the quick phases of convergent or divergent nystagmus. Eye movement recordings can often help make the distinction.

ABNORMALITIES

OF VERGENCE

Inborn defects of the vergence mechanisms are common. Abnormalities of the accommodative-convergence synkinesis (high AC/A ratio) accompany some forms of childhood strabismus (see Diagnosis of Concomitant Strabismus, Chap. 9).245 Common disorders of binocular function include convergence insufficiency, convergence excess, divergence insufficiency, and divergence excess.189 In these conditions, "excess" refers to a high AC/A ratio, and "insufficiency" refers to a low ratio; "convergence" and "divergence" refer to the viewing distance (near or far) at which the largest phoria exists.

The cause of these disorders may be related to an inability to adjust correctly the level of tonic vergence and tonic accommodation, as well as the values of the cross-links between accommodation and

convergence, as reflected in the AC/A and CA/C ratios.166,201,207 Specifically, patients

with unusually high AC/A ratios (vergence excess) usually have a poor ability to adaptively adjust their level of tonic accommodation. Patients with unusually low AC/A ratios (vergence insufficiency) usually have a poor ability to adaptively adjust their level of tonic vergence. High AC/A ratios are associated with low CA/C ratios, and vice versa. Orthoptic exercises designed to restore normal vergenceaccommodation interactions might be therapeutic.71

Vergence Eye Movements 307

The neurologist is sometimes asked to evaluate patients with diplopia due to convergence insufficiency. This is a common disorder among teenagers and college students (often those with an increased visual workload and stress), the elderly, and individuals who have suffered even mild head trauma.120 Convergence insufficiency is usually treated by orthoptic exercises,70 although prisms may be necessary. Occasionally, acquired cerebral lesions (especially of the nondominant cerebral hemisphere and probably the parietal lobe) may lead to both impaired stereopsis and poor fusional vergence.8'55'170'237 Anecdotal reports attest to the efficacy of orthoptic exercises in treating disorders of fu-

sional convergence following head trauma and cerebral ischemia.110'111

Many acquired neurologic disorders cause disturbances of vergence often associated with abnormalities of vertical gaze. In some of these conditions, such as Parkinson's disease or progressive supranuclear palsy, vergence is impaired or absent. In others, such as tumors of the pineal region and infarction of the rostral midbrain and thalamus, excessive disjunctive eye movements appear as convergence-retraction nystagmus and spasm of convergence.

Convergence and Nystagmus

Convergence-retraction nystagmus is discussed in detail in Chapter 10 (see VIDEO: "Convergence-retraction nystagmus"). It is primarily a disorder of saccades and occurs as part of the dorsal midbrain syndrome. In these patients, excessive convergence drives may also appear during horizontal saccades; the abducting eye moves slower than the adducting eye. This has been called pseudoabducens palsy,34 and it often leads patients with pretectal lesions to complain of difficulty in reading because of the break of fusion that occurs when changing lines. Pretectal pseudobobbing, another disorder of saccades associated with lesions in the midbrain, is nonrhythmic and rapid and has combined downward and adducting movements, often preceded by a blink; each movement is followed by a slow return toward the midline.105

Convergence-retraction nystagmusshould be differentiated from convergenceinduced nystagmus. Voluntarynystagmus, a form of saccadic oscillation in normal individuals, can often be produced only in association with convergence (see VIDEO: "Voluntary nystagmus"; see Chap. 10). Although convergence usually clamps or stops congenital nystagmus, sometimes the opposite occurs. Rarely, acquired con- vergent-divergent pendular nystagmus may be induced by convergence (for example, in patients with multiple sclerosis).219 Lid nystagmus may be affected by convergence tone. Thus, a patient with lateral medullary infarction (Wallenberg's syndrome) showed synchronous lid and ocular nystagmus that was suppressed by convergence.35 Pure lid nystagmus, which is induced by convergence, has also been described.198 Downbeat nystagmus characteristically is brought out or accentuated by convergence, but in some cases, convergence lessens or changes the direction of downbeat or upbeat nystagmus (Chap. 10).

Vergence Oscillations

Divergence nystagmus (nystagmus with divergent quick phases) may occur in patients with hindbrain anomalies (e.g., Arnold-Chiari malformation) who also have downbeat nystagmus.6 Upbeat nystagmus may also have a divergent component (see Fig. 10-6, Chap. 10). These patients have slow phases of nystagmus that are directed upward or downward, and inward. Divergent nystagmus may be an inappropriate manifestation of an otolithic response; normal individuals may show it during forward linear acceleration of the head.224 In the rabbit, the projections from the flocculus to the vestibular nuclei inhibit only the adduction component of the horizontal vestibulo-ocular reflex.95 A flocculus lesion might then lead to excessive adduction and divergence nystagmus.

Repetitive divergence has been reported in a patient comatose because of hepatic encephalopathy.164 The eyes slowly diverged to extreme bilateral abduction and then rapidly returned to the primary

position. A similar abnormality was reported in a neonate in association with abnormalities of the electroencephalogram that were perhaps related to seizures.162

Another type of vergence oscillation occurs in association with contractions of the masticatory muscles. This is called oculomasticatory myorhythmia.211 The ocular oscillations (see VIDEO: "Whipple's disease") are characterized by smooth, pendular, convergent-divergent movements occurring at a frequency of 0.8 to 1.2 Hz. This abnormality has only been reported in patients with Whipple's disease involving the central nervous system; it may be pathognomonic for this disease. Such patients also have a vertical saccade palsy. Somnolence and intellectual deterioration are associated features.

Convergence Spasm

Spasm of convergence (or spasm of the near triad) may be a sign of an organic lesion or of a functional disorder. Cogan20 has described convergence spasm, elicited by extending the neck, in a patient who had downbeat nystagmus. Other causes of increased or sustained convergence include disease at the diencephalic-mesencephalic junction (thalamic esotropia,67'80 occurring with thalamic hemorrhage or pineal tumors), encephalitis, Wernicke-Korsakoff syndrome,79 occipitoatlantal instability with vertebrobasilar ischemia,29 Chiari malformations and other posterior fossa lesions,33 multiple sclerosis,181 metabolic disturbances,157 and phenytoin intoxication.713 Spasm of the near triad has been confused with myasthenia gravis.190 The mechanism underlying organic convergence spasm is not known. One possibility is that it reflects an instability in the neurons that create tonic vergence premotor commands (i.e., the vergence integrator). Convergence spasm must also be distinguished from substitution of vergence for versional movements in patients with horizontal gaze palsies (see Chap. 9).197

Organic causes of spasm of convergence are rare and must be differentiated from the more common, functional spasm of convergence. Functional convergence

spasm consists of voluntary convergence accompanied by pupillary constriction and accommodation; its features are illustrated in the following case history.

CASE HISTORY: Functional

Convergence Spasm

A 20-year-old woman presented to the emergency room complaining of headache and diplopia. Her headache had come on suddenly the previous evening. It had been getting worse, and on direct questioning, she agreed that it was the worst headache of her life.

Despite her pain, she remained alert and oriented. Her vital signs were normal. In the emergency room, she developed a "noticeable esotropia. . . . her eye movements [were] full, but not conjugate." The patient's neck wassupple, and her neurologic examination was otherwise normal.

She was thought to have had a subarachnoid hemorrhage, and so computed tomography and a spinal tap were performed; both test results were normal.

Her headache persisted and the nursing staff noted that she was "unable to focus her eyes well."

When seen in consultation, she was emotionally upset. Her corrected near visual acuitywas 20/30 when each eye was tested separately. Ocular ductions (movements with one eye viewing) were full. With both eyes viewing (versions), there was a characteristic limitation in movement of the abducting eye: as it crossed the midline there were shimmering, small to- and-fro movements associated with varying constriction of the pupils.

It eventually emerged that the patient had been summarily dismissed from her job the afternoon before admission.

Comment: This case history illustrates features typical of spasm of the near triad.66 It is frequently misdiagnosed as bilateral sixth nerve palsy (leading to inappropriate tests and procedures).20'69'199 Careful examination of the eye movements allows the diagnosis to be made. There is often a full range of movements and less pupillary constriction with only one eye viewing.163 With both eyes viewing, the patient limits abduction by imposing a strong convergence command (voluntary vergence) that causes accommodation and, most impor-

Vergence EyeMovements 309

tantly, miosis. On lateral gaze, there may be dissociated nystagmus that is greater in the abducting eye.117 Convergence spasms typically come and go, but some patients can sustain them for long periods. They may cause ocular pain. Rapid, passive head-turns (the doll's- head maneuver) elicit a full range of eye movements. Treatment is best directed toward the underlying psychological factors,199'212 although cycloplegic eye drops and refractive

measures (positive or negative lenses) may be effective.199'223

Divergence Weakness

Abnormalities of divergence (divergence insufficiency and divergence paralysis)should be differentiated from bilateral sixth nerve palsy. Bielschowsky9 defined the diagnosti criteria for divergence paralysis: an esotropia with uncrossed diplopia during fixation of a distant object; single vision during fixation of objects located at about 10 to 20 inches; crossed diplopia with fixation closer than about 10 to 20 inches (due to associated convergence insufficiency); horizontal motion of the eyes that may be normal; and diplopia that is unchanged or may even disappear on lateral gaze. To these should be added another criterion: normal amplitude and speed of horizontal saccades.

Divergence paralysis has been reported with a variety of neurologic diseases, including conditions raising the intracranial pressure (such as tumor, pseudotumor, intracranial hematoma, or head trauma),119 and with tumors in the midbrain.123 It may also occur as the initial sign of the Miller Fisher syndrome,56 in association with diazepam,4 and with intracranial hypotension (the low-pressure syndrome).83'154 Some patients with divergence paralysis have developed frank abducens palsies.32 These patients may show markedly decreased saccadic velocities of the abducting eye, even though the range of motion is full.115 They commonly have increased intracranial pressure. Divergence paralysis, with esotropia greater at distance, has been associated with cerebellar lesions,

including craniocervical-junction anomalies.1'89'126'243 When there are no associ-