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Latent Nystagmus
Vestibular Nystagmus With a Twist
Michael C. Brodsky, MD;
Ronald J. Tusa, PhD
Arch Ophthalmol. 2004;122:202-209.
ABSTRACT
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Background Latent nystagmus is a horizontal binocular oscillation that is evoked by unequal visual input to the 2 eyes. It develops primarily in humans with congenital esotropia.
Objective To investigate the interrelationship between latent and peripheral vestibular nystagmus and their corollary neuroanatomical pathways.
Methods Examination of subcortical neuroanatomical pathways producing latent nystagmus and review of the neurophysiological mechanisms by which they become activated in congenital esotropia.
Results The vestibular nucleus presides over motion input from the eyes and labyrinths. Latent nystagmus corresponds to the optokinetic component of ocular rotation that is driven monocularly by nasal optic flow during a turning movement of the body in lateral-eyed animals. Congenital esotropia alters visual pathway development from the visual cortex to subcortical centers that project to the vestibular nucleus, allowing this primitive subcortical motion detection system to generate latent nystagmus under conditions of monocular fixation.
Conclusions Latent nystagmus is the ocular counterpart of peripheral vestibular nystagmus. Its clinical expression in humans proclaims the evolutionary function of the eyes as sensory balance organs.
INTRODUCTION
Vestibular disease holds little interest for the ophthalmologist. Although patients with vestibular disease can develop nystagmus, diplopia, and oscillopsia, these symptoms can be treated empirically. Peripheral vestibular disease is caused by injury to the labyrinth rather than to the eye, whereas central vestibular disease is caused by brainstem or cerebellar disorders involving the central vestibular pathways along their course to the ocular motor nuclei.1-2 But the ophthalmologist encounters a unique form of vestibular nystagmus that is caused by unbalanced input from the two eyes rather than from the two labyrinths. This visuo-vestibular nystagmus is known as "latent nystagmus."
Congenital esotropia is associated with a clinical triad of latent nystagmus, inferior oblique muscle overaction, and dissociated vertical divergence.3 These unique eye movements conform to primitive vision-dependent tonus mechanisms that are reactivated by congenital strabismus or early abnormal visual experience.4-7 Evolutionary analogues of primary oblique muscle overaction and dissociated vertical divergence have been identified in lower vertebrates.5-6 In fish, these are physiologic extraocular movements that use weighted binocular visual input to modulate extraocular muscle tonus and to maintain visual orientation during body movements.5-6 The stimulus for bilateral inferior oblique muscle overaction corresponds to a visuo-vestibular imbalance in the sagittal (pitch) plane, while dissociated vertical divergence corresponds to a similar imbalance in the coronal (roll) plane.5-7 We propose that latent nystagmus results from a similar visuo-vestibular tonus imbalance in the horizontal turning (yaw) plane.
WHAT IS LATENT NYSTAGMUS?
Latent nystagmus is a binocular horizontal oscillation that becomes apparent when 1 eye is covered. First described by Faucon in 1872,8 latent nystagmus develops when congenital esotropia precludes frontal binocular vision early in infancy.9-13 In this setting, a conjugate horizontal jerk nystagmus can be induced by covering 1 eye, blurring 1 eye, or reducing image brightness in 1 eye.1, 10, 14 In latent nystagmus, the slow-phase rotation of the fixating eye is directed toward the nose and the fast-phase rotation of the fixating eye is directed toward the ear.1-2,14 As such, fixation with the right eye generates a right-beating nystagmus, while fixation with the left eye produces a left-beating nystagmus.10 In children with congenital esotropia and alternating fixation, the direction of nystagmus will spontaneously reverse when fixation is switched from one eye to the other.10-15 Even after the eyes have been surgically realigned, occlusion of either eye will continue to induce latent nystagmus. The intensity of latent nystagmus is maximal in abduction and minimal in adduction, causing some patients to maintain a head turn to place the fixating eye in an adducted position. The intensity of latent nystagmus decreases when visual attention declines and increases during attempted fixation.13-22 In fact, some patients can reverse the direction of their latent nystagmus by looking at an imagined target and mentally switching fixation from one eye to the other.16, 22
In children with latent nystagmus, the development of amblyopia or the recurrence of ocular misalignment can disrupt binocular vision and make a latent nystagmus become manifest.23 The magnitude of the resulting manifest latent nystagmus is proportional to the degree of the interocular visual disparity.1 Most patients with clinical latent nystagmus actually have a small spontaneous jerk nystagmus that can be measured with both eyes open using eye movement recording.14 However, successful treatment of amblyopia or strabismus can convert a manifest latent nystagmus to a clinical latent nystagmus.23 Manifest latent nystagmus has also been reported in children with unilaterally reduced vision and sensory esotropia resulting from congenital disorders such as cataract or optic nerve hypoplasia.9-11 In this setting, a child will often maintain a head turn to position the fixating eye in adduction.9-11
Various theories have been advanced to explain latent nystagmus.24 These include a primitive tonus imbalance,1 an egocentric disorder,14 a disorder of the subcortical optokinetic system,21 a subcortical maldevelopment of retinal slip control,22 abnormal cortical motion processing,25-26 a disorder of proprioception,27 and an evolutionary preponderance of the nasal half of the retina.3, 28 These disparate theories can be reconciled by considering the critical evolutionary function of the eyes as sensory balance organs.
NASOTEMPORAL ASYMMETRY AND LATENT NYSTAGMUS
Latent nystagmus is associated with nasotemporal asymmetry of the horizontal optokinetic response during monocular viewing.25-26 However, not all patients with nasotemporal asymmetry have latent nystagmus.29 In patients with nasotemporal asymmetry, the monocular optokinetic responses to nasally moving targets are brisk, while those to temporally moving targets are poor in each eye. This "nasalward" movement bias under monocular viewing conditions corresponds both in direction and in waveform to the nasalward slow-phase drift of the fixating eye in latent nystagmus.21, 29 To our knowledge, Roelofs30 first observed horizontal optokinetic asymmetry in patients with latent nystagmus in 1928. Fifty years later, experiments by van Hof–van Duin31 and Wood et al32 suggested that reduced binocularity in strabismus can lead to nasotemporal asymmetry. In 1977, Kommerell33 suggested that latent nystagmus could be regarded as the consequence of horizontal optokinetic asymmetry. In 1982, Hoffman34 developed a model to explain nasotemporal asymmetry based on combined cortical and subcortical input to the nucleus of the optic tract in the cat. In 1983, Schor21 proposed that latent nystagmus and nasotemporal optokinetic asymmetry are mediated by the nucleus of the optic tract.
Human nasotemporal asymmetry has received considerable attention because it persists throughout life in humans with congenital strabismus.15, 21, 25, 34-36 Even after surgical realignment, nasotemporal asymmetry remains as a "footprint in the snow" of abnormal visual development.36 Nasotemporal asymmetry is seen in rabbits, kittens, monkey infants, and human infants within the first 6 months of life.36 The evolutionary retention of this primitive nasotemporal asymmetry in human infancy shows how ontogeny recapitulates phylogeny during human visual development.37-38
In ordinary life, large parts of the visual field move together during self-motion.39 Optic flow occurs during translation (which is signaled by the otoliths and linear optic flow) and rotation (which is signaled by the semicircular canals and rotational optic flow).39 The low sensitivity to nasal to temporal optic flow in afoveate, lateral-eyed animals is commonly assigned the function of preventing the locomoting animal from responding to the image motion of stationary contours during forward motion, while permitting full compensation for rotational input during turning movements.39-41 The absence of nasotemporal optokinetic responses in lateral-eyed animals assures that during forward movements, ineffective temporalward eye movements do not destabilize images of objects that are directly ahead of the animal.39-41 The optokinetic responses of both eyes are controlled by whichever eye is stimulated by temporal-to-nasal movement of the visual world.40 Latent nystagmus recapitulates this monocularly driven horizontal optokinetic movement.
IS LATENT NYSTAGMUS A VESTIBULAR NYSTAGMUS?
Vestibular eye movements are reflex contraversive rotations of the eyes that occur during involuntary head movements, acting to stabilize the position of the eyes in space and thereby maintain visual orientation.1-2,42-43 According to Walls, " . . . vestibularly-controlled reflex eye movements are historically the oldest of all, with all other kinds of eye-muscle controls and operations accreted to them above the primitive fish level of evolution."44(p71) During head movements, input to the semicircular canals within the 2 labyrinths provides the afferent stimulus for the vestibulo-ocular reflex.1-2,43, 45 The semicircular canals respond to angular acceleration and produce dynamic vestibulo-ocular eye movements. Damage to a horizontal semicircular canal pathway produces a nystagmus in the plane of the injured canal.43, 45 In lateral- and frontal-eyed animals, the geometry of the semicircular canals conforms closely with the orientation of the extraocular muscles.46 When the head is rotated in a particular plane, a semicircular canal within the labyrinth detects acceleration and sends excitatory innervation to the corresponding extraocular muscles. Within the brainstem and cerebellum, peripheral vestibular input is summated to produce innervation to the appropriate extraocular muscle subnuclei and to maintain the position of the eyes in space. Each horizontal semicircular canal provides excitatory input to the ipsilateral medial rectus muscle and the contralateral lateral rectus muscle.1-2,43, 45
Visual stabilization mechanisms act in concert with labyrinthine reflexes.43 In normal life, optokinetic responses are elicited mainly by head movements, which also stimulate the vestibular system.39 Because vestibular neurons receive such prominent visual and vestibular inputs, disrupting either input reduces the tonic activation of these neurons, with the effect of disturbing the responses to the other sensory modality.39 Thus, labyrinthectomy eliminates optokinetic nystagmus in rabbits,47-48 whereas blocking optic nerve activity with tetrodotoxin reduces the gain of the vestibulo-ocular reflex.39, 49 According to Miles, each sensory modality "has played such a major role in the evolution of the other that it is impossible to understand the operation of either one in isolation."41(p393) A confluence of neuroanatomical, clinical, evolutionary, and experimental evidence has led us to conclude that latent nystagmus is a vestibular nystagmus that is brought about by unequal visual input from the 2 eyes rather than from the 2 ears (ie, a visuo-vestibular nystagmus). The evidence that latent nystagmus arises when the 2 eyes revert to their primitive function as balance organs can be summarized as follows.
Neuroanatomy of Latent Nystagmus
Studies in subhuman primates have shown that latent nystagmus arises as a result of incomplete development of visual input from occipitotemporal cortex to subcortical vestibular pathways.50-51 In monkeys with latent nystagmus, there is a loss of binocularity in the nucleus of the optic tract (NOT), the subcortical structure that feeds into the vestibular system, with most cells driven by the contralateral eye.50-51 The areas that normally provide binocular input to the NOT are the middle temporal (MT) visual area and the medial superior temporal (MST) visual area in occipitotemporal cortex. When strabismus is surgically induced in infant monkeys during the first 2 weeks of life, these monkeys also develop latent nystagmus and visual area MT/MST loses binocularity. If either eye is covered during infancy, visual area MT/MST and NOT develop normal binocularity, but the striate cortex still shows loss of binocularity and these monkeys do not develop latent nystagmus.52 This finding suggests that the initial cause of latent nystagmus is loss of binocularity in visual area MT/MST from the misaligned eye during the first few weeks of life.52
Neuroanatomical experiments have confirmed the Schor hypothesis21 that the NOT is the generator of latent nystagmus.21, 34, 50-52 A latent nystagmus occurs in monkeys following artificial induction of esotropia within the first 2 weeks of life.53 Unilateral electrical stimulation of the NOT in binocularly deprived monkeys induces a conjugate nystagmus with the slow phases directed toward the side of stimulation.54-55 Latent nystagmus can be abolished by direct injection of muscimol, a potent  -aminobutyric acid A agonist into the NOT in monkeys.50 Simultaneous bilateral blockage of the NOT virtually abolishes latent nystagmus for the duration of the blockade.50
Subcortical optokinetic responses are also mediated by the pretectal NOT.15, 21, 34-36 The monocular pathways subserving nasotemporal asymmetry and its neutralization by binocularly driven pathways from the visual cortex were first elucidated by Hoffman in the cat.34-35 The cat NOT is a diffuse cell aggregation in the pretectum that is optimally located to integrate direct retinal and diffuse cortical projections.34 These nuclei have high levels of spontaneous activity and operate in a push-pull fashion such that the sum of their opponent innervation determines the optokinetic response.21, 34
The NOT contains neurons that are sensitive to visual motion.54 Many units in the primate NOT have large receptive fields that are appropriate for encoding full-field visual motion to support optokinetic eye movements.54 Stimulation of the right and left NOT results in optokinetic nystagmus with slow phases to the right and left, respectively.21 Output from the NOT is maximal for horizontal movements but 0 for vertical movements.34
This phylogenetically ancient subcortical system is depicted in Figure 1. Crossed connections from each eye to the contralateral NOT transmit horizontal visual motion information to the vestibular nucleus before impinging on the ocular motor nuclei.40, 56 Pretectal neurons in the left NOT receive only crossed input from the right eye and respond only to leftward motion, while those in the right NOT receive only crossed input from the left eye and respond only to rightward motion.15, 21, 36 In the first 6 months of infancy, this subcortical system predominates in humans, so that temporally directed monocular optokinetic responses are poor in early infancy compared with nasally directed optokinetic responses.36 By 6 months of age, cortical binocular pathways, which are responsive to temporally directed motion, provide a route whereby the NOT, with its specialized directional responses, can be accessed from either eye.37-38 In animals with well-developed foveae and frontal, stereoscopic vision, the visual inputs feeding directly to the pretectum are supplemented by inputs routed through the visual cortex that selectively respond to moving images with no positional disparity in the 2 eyes.57 This coupling between optokinetic nystagmus and stereopsis allows frontal-eyed animals to selectively stabilize the moving images of those parts of the scene within a selected depth plane, while disregarding induced image motion of the visual world at other distances.40, 57 In humans with congenital strabismus, binocularly driven cortico-pretectal pathways never become established, allowing the primitive monocular nasotemporal asymmetry to predominate.
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Figure 1. Schematic diagram depicting cortical and optokinetic pathways. Cortical input to temporally directed movement, which is present only in frontal-eyed animals, requires the establishment of normal binocular cortical connections. This input is absent in humans with congenital strabismus. Direct crossed pathways from the eye to the nucleus of the optic tract provide nasalward subcortical optokinetic responses even when binocular cortical connections are absent (R and L represent monocular cortical cells corresponding to the right and left eyes, respectively). Note that the nucleus of the optic tract (NOT) relays horizontal visuo-vestibular information to the vestibular nucleus (VN), where it is integrated with horizontal vestibular input from the labyrinths to establish horizontal extraocular muscle tonus. LGN indicates lateral geniculate nucleus; CC, corpus callosum; V1, abducens nucleus; III, oculomotor nucleus; LR, lateral rectus muscle; MR, medial rectus muscle; AC, anterior canal; PC, posterior canal; and HC, horizontal canal.
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Clinical Signs of Vestibular Origin
Bilateral positioning of the eyes and ears promotes survival by enabling the organism to crosslink input from different sense organs to impart balance. Each eye and its ipsilateral semicircular canals share the same directional bias to movement. For example, the right horizontal semicircular canal is activated by head rotation to the right (which induces a rotation of the visual world to the left) and inhibited by head rotation to the left (which induces a rotation of the visual world to the right).2, 43, 45, 58-59 The monaural and monocular directional biases summate, so that activation of the right horizontal semicircular canal during rightward head rotation is reinforced by the physiologic activation of the right eye by the induced nasal rotation of the visual world. The close geometrical relationship between the semicircular canals and the extraocular muscles presumably facilitates the integration of head motion and visual movement and their orderly summation to produce transformation to an appropriate ocular motor response.46, 60
Latent nystagmus usually conforms to Alexander's law, which states that the intensity of a peripheral vestibular nystagmus increases when the eyes are moved in the direction of the fast phase and decreases when the eyes are moved in the direction of the slow phase.2, 21, 60-63 Latent nystagmus damps when the fixating eye is turned toward the nose (which is also the direction of the slow phase) and increases in intensity when the fixating eye is turned toward the ipsilateral ear (which is in the direction of the fast phase).14-15,62-63 A similar damping of horizontal nystagmus is seen in peripheral horizontal vestibular nystagmus after disease or injury to 1 horizontal semicircular canal. By contrast, Alexander's law does not apply to congenital nystagmus, which reverses direction in different positions of gaze. The contraversive head turn in latent nystagmus (ie, a head turn opposite in direction to the deviation of the fixating eye) also characterizes vestibular eye movements.2
Additional evidence for the duality of optic and vestibular innervation can be elicited by occluding 1 eye in the patient with latent nystagmus, spinning the patient, suddenly stopping the spin, then immediately observing the effect of the postrotational nystagmus on the latent nystagmus when either eye is occluded. A horizontal nystagmus induced by body spinning nullifies or accentuates latent nystagmus depending on the direction of spin relative to the fixating eye (Figure 2). For example, spinning the patient to the right excites the right horizontal canal and inhibits the left horizontal semicircular canal to induce a nystagmus with a slow-phase rotation to the left and a fast-phase rotation to the right. If the spin is suddenly stopped (after approximately 10 rotations), a shift in endolymph deflects the cupula in the opposite direction, causing transient excitation of the left horizontal semicircular canal and transient inhibition of the right horizontal semicircular canal and inducing a left-beating nystagmus (termed "postrotational nystagmus"). If the left eye is occluded to induce latent nystagmus prior to this maneuver, the latent nystagmus will diminish or disappear immediately following cessation of the spin. If the occluder is quickly moved to cover the right eye, the intensity of the latent nystagmus with the left eye viewing will be correspondingly increased relative to that observed with the left eye fixating before the spin. In this way, the clinician can observe how visual input is summated with vestibular input to establish central vestibular tone in the horizontal plane.
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Figure 2. Visual and vestibular interaction in latent nystagmus. Latent nystagmus decreases with spinning toward the fixating eye and increases with spinning toward the occluded eye. O represents direction of ocular (visuo-vestibular) tonus; V, direction of horizontal vestibular tonus. Both O and V correspond to the slow phase of the induced nystagmus. ++ Indicates stimulated horizontal semicircular canal; --, inhibited horizontal semicircular canal; A, Occlusion of the left eye increases visuo-vestibular tonus to the left. B, The patient with latent nystagmus is spun to the right to stimulate the right horizontal semicircular canal, which increases leftward horizontal vestibular tonus and causes a slow conjugate drift of both eyes to the left. At this point, the latent nystagmus would be enhanced by vestibular input (if the examiner could observe it). C, When the spinning is suddenly stopped, the opposite vestibular stimulus is exerted, causing the left semicircular canal to drive the eyes to the right. This rightward vestibular tonus imbalance nullifies the leftward visual tonus imbalance induced by monocular fixation with the right eye, thereby reducing the intensity of the latent nystagmus. D, When the occluder is quickly switched to the right eye, the visual tonus imbalance is augmented by an ipsidirectional visual tonus imbalance, increasing the intensity of the latent nystagmus.
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The more visual input is dominated by 1 eye in latent nystagmus, the higher the velocity of the slow-phase rotations in the direction toward the opposite eye.22 Simonsz and Kommerell63 performed eye movement recordings before and after occlusion therapy for amblyopia in patients with latent nystagmus. After prolonged occlusion, the slow-phase velocity of the nystagmus in the amblyopic eye decreased to the same extent that the slow-phase velocity of the nystagmus in the preferred eye increased. The sum of the 2 slow-phase velocities remained the same in straight-ahead gaze, demonstrating that visual input to the 2 eyes (just like rotational input to the 2 horizontal canals) maintains a push-pull relationship.21, 63 This observation lends further support to a vestibular underpinning for latent nystagmus. The clinical similarities between latent nystagmus and peripheral vestibular nystagmus are summarized in Table 1.
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Table 1. Peripheral Vestibular Nystagmus vs Latent Nystagmus
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Evolutionary Underpinnings of Latent Nystagmus
The notion of latent nystagmus as a horizontal visuo-vestibular tonus imbalance provides conceptual unification with its associated inferior oblique overaction and dissociated vertical divergence in patients with congenital esotropia. The evolutionary progenitors of all of these visuo-vestibular movements use binocular input to establish physical orientation in space. These primitive reflexes rely on a dissociated form of binocular vision between the 2 laterally placed eyes, which has been superseded by normal cortical binocular vision in humans.5 In congenital esotropia, however, these primitive subcortical reflexes are not erased by binocular cortical input. Eye movement recordings have demonstrated that dissociated vertical divergence incorporates a vertical latent nystagmus, suggesting a shared common origin for these movements.64
Given that visual and labyrinthine input are pooled together within the central vestibular system of lower animals,65-67 a visual counterpart to peripheral vestibular nystagmus would seem necessary on evolutionary grounds. Many authors have attributed latent nystagmus as a tonus imbalance of the horizontal extraocular muscles.1, 4, 7, 14, 19, 68 Latent nystagmus corresponds to a tropotactic vision-induced tonus imbalance (ie, one that functions to reestablish binocular equilibrium rather than to directionally orient an eye toward incoming light).69 Ohm recognized the physiologic coaptation of visual and vestibular innervation and its role in the generation of latent nystagmus long before others did (as was also the case with dissociated vertical divergence and primary oblique muscle overaction).70-73 In a monograph written near the end of his life, he stated, "The impulses that originate from both eyes keep both vestibular nuclei in equilibrium. The equilibrium becomes unbalanced when one eye is being occluded. Then, a nystagmus beating towards the side of the open eye appears."72 Kestenbaum10 emphasized that latent nystagmus could not be attributed to luminance per se, since shining a bright light in the right eye worked like occlusion of the right eye and caused a left-beating nystagmus. He noted that the presence of a sharper visual image on the retina of one eye than the other appeared to be the decisive stimulus for inducing latent nystagmus.
Predominance of a primitive visuo-vestibular imbalance provides an evolutionary basis for the shift in egocenter that has been invoked to explain latent nystagmus.14 According to this hypothesis, the egocenter is localized to the median body plane under normal binocular conditions, but shifts to the side of the fixating eye under monocular conditions. Dell'Osso et al14 hypothesized that humans with latent nystagmus retain an abnormal egocenter in the median plane even under monocular conditions, causing the fixating eye to drift toward midline. In the lateral-eyed animal, fixation with the right eye would instantaneously shift the egocenter to the left of the object of regard, necessitating a body turn to frontalize the object and a contraversive eye rotation to maintain fixation.14 As neatly summarized by Dichgans and Brandt:
. . . the results of visual and vestibular stimulation on egocentric localization indicate the close similarity in the perceptual consequences of stimulation of the two organs. The assumption of a unitary central representation of egocentric space, based on visual and vestibular (as well as acoustic and somatosensory) afferents is perceptually obvious.42(pp763-764)
It remains to be determined whether a higher order egocentric shift could cause the visuo-vestibular imbalance which generates the linear slow phase of latent nystagmus.
Experimental Evidence That Latent Nystagmus Is Vestibular in Origin
Optokinetic responses are fundamentally intertwined with vestibular responses, and a major site of this commingling is the vestibular complex.39 Waespe and Henn58 and Henn et al59 performed single-cell recordings from the medial vestibular nucleus in monkeys and found that single neurons can be activated either by body rotation or optokinetic stimulation. Units that were excited by head acceleration to the left were also exited by motion of optokinetic stripes to the right. Most cells responded to both the whole-field visual motion, as well as to the vestibular indications of head rotation, and the responses of vestibular neurons followed approximately the same time course as the delayed component of optokinetic nystagmus.58-59 As summarized by Dichgans and Brandt:
All of the recent studies performed in awake animals show a tonic modulation of resting discharge of vestibular units in response to exclusive constant velocity motion of the visual surround. The modulation, although to a variable degree, seems to occur in the great majority of horizontal semicircular canal-dependent units of all vertebrate species tested. A unit which is excited by a head acceleration, say, to the left is also excited by motion of the surround to the right, which represents the optokinetic stimulus that in man would cause the sensation of turning to the left.42(pp781,783)
This underlying vestibular response to both visual motion and body rotational stimuli may explain the overlapping nystagmus response that characterizes latent, optokinetic, and peripheral vestibular nystagmus.2, 19, 21, 62, 71 This overlap may reflect the fact that all 3 movements subserve a similar physiologic function (ie, detection of rotation of the body and visual environment).
Latent nystagmus, optokinetic nystagmus, and the vestibuloocular reflex also show velocity storage, a phenomenon in which constant vestibular input or visual flow in the same direction is stored for up to 20 seconds in the brainstem, even when the stimulus is terminated (Table 2).50-51 The presence of velocity storage serves to enhance the slow-tracking eye movements to vestibular stimulation and optic flow response at low frequencies of rotation.2, 74 Although latent nystagmus has variously been attributed to anomalous cortical motion processing,25-26 or a cortical pursuit asymmetry,53, 75 the absence of velocity storage mechanism within the pursuit system implicates the vestibular system as the generator of latent nystagmus.
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Table 2. Experimental Evidence That Latent Nystagmus Is Vestibular in Origin
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CONCLUSIONS
Latent nystagmus is a unique form of vestibular nystagmus that is evoked by unbalanced visual input from the 2 eyes rather than unequal rotational input from the 2 labyrinths. The neurophysiological substrate for latent nystagmus is operative in lateral-eyed animals and in human infants with undeveloped binocular corticopretectal pathways. When congenital esotropia disrupts the establishment of these binocular visual connections, visual input from the fixating eye to the contralateral NOT evokes a visuo-vestibular counterrotation of the eyes that corresponds to a turning or twisting movement of the body toward the object of regard ("vestibular nystagmus with a twist"). In this setting, unbalanced binocular visual input can induce a motion bias in the vestibular nucleus to generate the visual counterpart of horizontal labyrinthine nystagmus, namely, latent nystagmus. As the eyes rotate frontally during evolution, this visuo-vestibular function is sacrificed, but the central nervous system retains these latent subcortical visual pathways. Latent nystagmus is nature's proclamation that our 2 eyes, when dissociated from birth, can revert to their ancestral function as sensory balance organs.
AUTHOR INFORMATION
Corresponding author and reprints: Michael C. Brodsky, MD, Departments of Ophthalmology and Pediatrics, Arkansas Children's Hospital, 800 Marshall St, Little Rock, AR 72202.
Submitted for publication October 8, 2002; final revision received September 15, 2003; accepted October 14, 2003.
This study was supported in part by a grant from Research to Prevent Blindness Inc, New York, NY.
We thank Guntram Kommerell, MD, for his valuable suggestions during the preparation of the manuscript.
From the Departments of Ophthalmology and Pediatrics, University of Arkansas for Medical Sciences, Little Rock (Dr Brodsky), and the Departments of Neurology and Otolaryngology, Emory University, Atlanta, Georgia (Dr Tusa). The authors have no relevant financial interest in this article.
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