Hereditary congenital nystagmus and gaze-holding failure: the role of the neural integrator

Congenital nystagmus (CN) may be due to an instability of the neural integrator responsible for gaze holding. This longitudinal study tests that hypothesis, investigates the saccadic instabilities of relatives, and assesses the effects of afferent stimulation on both the CN and the coexisting gaze-holding failure. We recorded four siblings who had CN and gaze-holding failure while fixating in primary position and lateral gaze. In lateral gaze, the CN waveforms were superimposed on the centripetal drift caused by the gaze-holding failure; the drift time constants ranged from 300 to 1,450 msec. CN waveforms lacked extended foveation periods. Saccadic instabilities were present in the father and two clinically unaffected siblings; the mother's eye movements were normal. We conclude that CN in the subjects of this study, and in others with idiopathic CN, is not due to gaze-holding abnormalities, and we speculate that development of the fixation reflexes that produce CN foveation periods requires some minimal foveation interval during which the target image is in the foveal area with low retinal slip velocity and acceleration.

Effectiveness of botulinum toxin administered to abolish acquired nystagmus

We injected botulinum toxin into the horizontal rectus muscles of the right eyes of 2 patients who had acquired pendular nystagmus with horizontal, vertical, and torsional components. This treatment successfully abolished the horizontal component of the nystagmus in the injected eye in both patients for approximately 2 months. Both patients showed a small but measurable improvement of vision in the injected eye that may have been limited by coexistent disease of the visual pathways. The vertical and torsional components of the nystagmus persisted in both patients. In 1 patient, the horizontal component of nystagmus in the noninjected eye increased; we ascribe this finding to plastic-adaptive changes in response to paresis caused by the botulinum toxin. Such plastic-adaptive changes and direct side effects of the injections--such as diplopia and ptosis--may limit the effectiveness of botulinum toxin in the treatment of acquired nystagmus. Neither patient elected to repeat the botulinum treatment.

Experimental tests of a superposition hypothesis to explain the relationship between the vestibuloocular reflex and smooth pursuit during horizontal combined eye-head tracking in humans

1. We used a modeling approach to test the hypothesis that, in humans, the smooth pursuit (SP) system provides the primary signal for cancelling the vestibuloocular reflex (VOR) during combined eye-head tracking (CEHT) of a target moving smoothly in the horizontal plane. Separate models for SP and the VOR were developed. The optimal values of parameters of the two models were calculated using measured responses of four subjects to trials of SP and the visually enhanced VOR. After optimal parameter values were specified, each model generated waveforms that accurately reflected the subjects' responses to SP and vestibular stimuli. The models were then combined into a CEHT model wherein the final eye movement command signal was generated as the linear summation of the signals from the SP and VOR pathways. 2. The SP-VOR superposition hypothesis was tested using two types of CEHT stimuli, both of which involved passive rotation of subjects in a vestibular chair. The first stimulus consisted of a "chair brake" or sudden stop of the subject's head during CEHT; the visual target continued to move. The second stimulus consisted of a sudden change from the visually enhanced VOR to CEHT ("delayed target onset" paradigm); as the vestibular chair rotated past the angular position of the stationary visual stimulus, the latter started to move in synchrony with the chair. Data collected during experiments that employed these stimuli were compared quantitatively with predictions made by the CEHT model. 3. During CEHT, when the chair was suddenly and unexpectedly stopped, the eye promptly began to move in the orbit to track the moving target. Initially, gaze velocity did not completely match target velocity, however; this finally occurred approximately 100 ms after the brake onset. The model did predict the prompt onset of eye-in-orbit motion after the brake, but it did not predict that gaze velocity would initially be only approximately 70% of target velocity. One possible explanation for this discrepancy is that VOR gain can be dynamically modulated and, during sustained CEHT, it may assume a lower value. Consequently, during CEHT, a smaller-amplitude SP signal would be needed to cancel the lower-gain VOR. This reduction of the SP signal could account for the attenuated tracking response observed immediately after the brake. We found evidence for the dynamic modulation of VOR gain by noting differences in responses to the onset and offset of head rotation in trials of the visually enhanced VOR.(ABSTRACT TRUNCATED AT 400 WORDS)

Foveation dynamics in congenital nystagmus. II: Smooth pursuit

It has been shown that, during 5 seconds of fixation, an individual with congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (SD = 12.87 minarc) and maintain low retinal slip velocities (SD = 118.36 minarc/sec). Smooth pursuit data from several CN subjects showed that eye velocities during these foveation intervals approximated target velocity. Despite some claims that CN is caused by absent or "reversed" smooth pursuit, those with CN hardly ever experience oscillopsia or exhibit any accompanying symptoms of such deficits in pursuit; they are able to master sports requiring tracking of rapidly moving small objects (e.g. racquetball or handball). We developed and describe several new methods to accurately assess the function of smooth pursuit in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during smooth pursuit to test the hypothesis that eye velocities would match target velocities during these periods. Unity or near-unity instantaneous (beat-to-beat) pursuit gains of both experimenter-moved and subject-moved targets at peak velocities ranging from only a few deg/sec up to 210 degrees/sec were measured. The dynamic neutral zone was found to shift oppositely to target direction by amounts proportional to the increase in target speed. Our methods proved that eye velocity is made to match target velocity during the foveation intervals and support the conclusion that smooth pursuit in individuals with CN is functioning normally in the presence of the CN oscillation. In addition, we hypothesize that the same fixation mechanism that prevents oscillopsia during fixation of stationary targets, also does so during pursuit.

Foveation dynamics in congenital nystagmus. III: Vestibulo-ocular reflex

It has been shown that, during fixation of a stationary target with a fixed head, an individual with congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (within 13 minarc) and maintain low retinal slip velocities (less than 4 degrees/sec). With the head in motion, vestibuloocular reflex (VOR) data showed eye velocities during these foveation periods that approximation head velocity. Despite some claims that the VOR of CN subjects was deficient or absent, individuals with CN hardly ever complain of oscillopsia or exhibit any of the symptoms that would accompany such deficits in the VOR, whether during simple walking and running or while skiing down a mogul field. We developed and describe several different and unrelated methods to accurately assess the function of the VOR in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during head rotation to test the hypothesis that eye velocities would match head velocities during these periods. At about 1 Hz, horizontal VOR instantaneous (beat-to-beat) gains were 0.96 in the light and 0.94 in the dark while imaging a stationary target. Vertical VOR gains were 1.00 and 0.99 for these two conditions at the same frequency; the CN was horizontal. Also, during the VOR there is a CN neutral-zone shift comparable to that found during smooth pursuit. Our methods demonstrated that gaze velocity was held constant during foveation periods and we conclude that the VOR in this subject is functioning normally in the presence of the CN oscillation. Based on our findings in this and previous studies, we hypothesize that CN may be due to a peripheral instability.

Foveation dynamics in congenital nystagmus. I: Fixation

Congenital nystagmus (CN) has been described as a 'fixation' nystagmus implying an inability to fixate a target. However, each cycle of CN contains a target-foveation period during which the eye velocity is at, or near, zero. Prolongation of foveation time, reduction of retinal image velocity and cycle-to-cycle foveation repeatability all contribute to increased visual acuity. We developed several methods to accurately measure the dynamics of foveation in CN; their use is illustrated on an individual with typical idiopathic CN and no afferent defects. During eight 5-second intervals of fixation on a stationary target, the horizontal standard deviation (SD) of the mean foveation position (FPOS) was 12.82 minarc and the SD of foveation velocity was 118.36 minarc/sec. The SD of the means of total eye position and of the non-foveating peak of the CN were 43.17 and 25.32 minarc respectively. The mean foveation-time interval (eye velocity less than or equal to 4 degrees/sec) was 57.27 msec. The SD FPOS for the best 1-second interval (4 successive CN cycles), in a typical 5-second record, was 0.71 minarc. Histograms revealed peaks of eye position at 0 +/- 10 minarc and of eye velocity at 0 +/- 240 minarc/sec. The small vertical component of the CN (16 minarc peak-to-peak) had a SD of 6.56 minarc. A nystagmus foveation function related to visual acuity was derived that was more sensitive than CN intensity. The increased visual acuity resulting from the use of convergence or base-out prisms was due to increased foveation time. Although it might appear that CN is a defect of fixation, this individual with CN had strong fixation reflexes in the sense that he was able to accurately (within 1 minarc) achieve (interbeat) and maintain (intrabeat) target foveation for appreciable periods of time. Our data support the hypothesis that individuals with idiopathic CN do not have a primary disturbance of fixation.

Superior oblique myokymia. Quantitative characteristics of the eye movements in three patients

Using the magnetic search coil technique, we measured horizontal, vertical, and torsional rotations of both eyes of two patients with idiopathic superior oblique myokymia, and of the affected eye in a third patient. Superior oblique myokymia was strictly monocular and consisted of an initial intorsion and depression of the affected eye and subsequent oscillations with torsional and vertical components. The peak-to-peak torsional and vertical amplitudes of the oscillations were less than 1 degree, but peak velocities frequently exceeded 4 degrees/sec in both planes. Fourier analysis indicated two features: (1) a broad range of frequencies up to about 50 Hz, indicating irregular oscillations; and (2) a superimposed larger-amplitude oscillation in the range from 1.5 to 6 Hz. Taken with electromyographic data from other studies, these results indicate that superior oblique myokymia reflects spontaneous discharge of trochlear motor neurons that have undergone regenerative changes.

Torsional nystagmus: quantitative features and possible pathogenesis

We measured eye rotations in three planes in a patient with acquired, torsional nystagmus. This nystagmus had linear or increasing-velocity waveforms, was increased after active pitch rotations of the head, and was suppressed by convergence. Magnetic resonance imaging demonstrated a midpontine lesion that was probably a venous angioma. We postulate that torsional nystagmus in this patient was due to disruption of central vestibular connections.

Oscillopsia, retinal image stabilization and congenital nystagmus

Most individuals with congenital nystagmus (CN) do not complain of oscillopsia (visual inconstancy) even though the amount of retinal image slip varies considerably according to gaze angle and CN waveform. We induced oscillopsia in four subjects with CN by artificially stabilizing images upon the retina under several conditions. Every subject reported oscillopsia during retinal image stabilization, but the condition of stabilization varied from one individual to another. Our results indicate that a variety of mechanisms operate to maintain spatial constancy in congenital nystagmus; some individuals appear to use one mechanism more than another. Possible mechanisms include use of extra-retinal signals, elevated threshold for motion detection and "suppression" of visual input except during foveation periods.

Latent/manifest latent nystagmus reversal using an ocular prosthesis. Implications for vision and ocular dominance

Latent/manifest latent nystagmus (LMLN) is a jerk nystagmus with a decreasing-velocity or linear slow phase whose fast phase is in the direction of the fixating eye. Change of the fixating eye by alternating tropias or cover will cause reversal of the LMLN to preserve this relationship. In the dark, where no fixation is possible, the fast phases of LMLN are in the direction of the intended fixating eye; actual visual input will override this effect of intention. A patient with persistent hyperplastic primary vitreous of the right eye, which was enucleated at age 9 due to glaucoma, had LMLN with fast phases to the left. His congenitally blind right eye was replaced with an ocular prosthesis whose movements were conjugate with his seeing eye. In the dark, we found that his LMLN spontaneously reversed as the normally fixating left eye became esotropic. Furthermore, as is the case with LMLN patients who have sight in both eyes, he was able to willfully reverse his LMLN in the dark by alternating his "fixating" eye. We conclude from these observations that the direction of LMLN is determined at a cortical level, is intimately related to the intended fixating eye and that eye "dominance" is predetermined and not altered by visual abnormalities, including blindness.

Spasmus nutans. A quantitative prospective study

Spasmus nutans includes ocular oscillations, head nodding, and anomalous head positions. No quantitative longitudinal study verifying the natural history of this self-limited condition has appeared in the literature. Using infrared oculography, we prospectively examined the eye movements of otherwise neurologically normal infants in whom a diagnosis of spasmus nutans had been made. At this writing the ocular oscillations and head movements in two of the children were clinically absent, thereby confirming the diagnosis. The ocular movements are characterized by a phase difference between the oscillations of each eye that varies both during one recording session and during development. The dissociated, pendular nystagmus consists of high-frequency oscillations that may be disconjugate, conjugate, or purely uniocular.

Hyperactive vestibulo-ocular reflex in cerebellar degeneration: pathogenesis and treatment

We studied a patient with a cerebellar degeneration and hyperactive vestibulo-ocular reflex (VOR). He complained of oscillopsia and blurred vision with head movement. A twofold increase in VOR gain (peak eye velocity/peak head velocity) at high frequencies was associated with a VOR time constant of 6 seconds (low normal). Visual cancellation ("suppression") of the VOR and smooth pursuit were also abnormal. We hypothesized that his high VOR gain was due to dysfunction of olivocerebellar projections. Physostigmine reduced his VOR gain, consistent with the hypothesis that these projections are cholinergic.

Slow saccades and hypometria in anticonvulsant toxicity

We report a patient with abnormal saccades in association with anticonvulsant toxicity (phenytoin 27.5 micrograms/ml, phenobarbital 18.8 micrograms/ml). The patient looked toward visual targets either with multiple, small, hypometric saccades or with single slow saccades. These abnormalities resolved when anticonvulsant levels returned to therapeutic range. Thus, slow saccades may be clinical evidence of anticonvulsant toxicity.

Audio-ocular response: saccadic programming

The eye movements elicited by auditory stimuli--the audio-ocular response (AOR)--differ from those made in response to a visual target. The movements consist of both monosaccadic and multiple saccadic refixations (MSR). In visual refixation, monosaccadic refixations are always accurate; in AOR, they rarely are. In MSR, many strategies were used in the attempt to find the target but they were not always successful. However, final amplitudes of the total refixation were quite accurate in both MSR and monosaccadic refixations. Velocity profiles of the AOR showed such anomalies as discrete decelerations and multiple, closely-spaced saccades. These data suggest that, without visual feedback, the location of acoustic targets is difficult. In the absence of visual afference, when vigilance may be decreased by the lack of arousal, the velocity profiles also became abnormal, even at small amplitudes. Thus, for cockpit warning devices, a combination of auditory and visual indicators should be used.

The nystagmus blockage syndrome. Congenital nystagmus, manifest latent nystagmus, or both?

We have carefully studied, by quantitative oculography, a patient with the nystagmus blockage syndrome (NBS), and two patients with a similar disorder of eye movements that might be mistaken clinically for NBS. Our recordings revealed two distinctly different abnormalities present in a single patient with NBS. Our NBS patient exhibited congenital nystagmus (CN) waveforms when viewing at distance; the CN did not damp with convergence on a near target. When the patient allowed one eye to become esotropic, however, the nystagmus damped considerably and abruptly changed from CN to manifest latent nystagmus (MLN). This peculiar transition from CN to MLN has not been described previously. The appearance of MLN in a case with ongoing CN suggests that two different mechanisms may underlie NBS, since the only other case documented with eye movement recordings showed no transition to MLN. Because the diagnosis of NBS usually is made on evidence of clinical signs alone, it is probable that these two types have been combined indiscriminately and presented as one syndrome. In addition, our discovery of two mechanisms discernable only by quantitative recording suggests that NBS has been diagnosed inappropriately in patients with clinically similar but oculographically different eye signs. Further quantitative studies are required to fully define NBS and to determine if these are the only two mechanisms found in this syndrome.

Saccadic latency measurements in dementia

We measured saccadic latencies in patients with Alzheimer's dementia (AD) and other types of dementia. The saccadic latencies for both groups were considerably longer than those for age-matched controls. The prolongation was as extensive in patients with other types of dementia as it was in those with AD. There was no correlation between latency and the severity of the dementia.

Edrophonium test in Eaton-Lambert syndrome: quantitative oculography

Accurate ocular motility recordings were made of the saccadic responses of five patients with Eaton-Lambert syndrome (ELS). It was found that, contrary to common belief, the ocular motor system is affected. The saccades of ELS patients mimicked those of patients with myasthenia gravis (MG). Both groups exhibited hypometria and multiple, closely spaced saccades. Two patients demonstrated both saccadic facilitation and positive edrophonium tests. The ELS patients had slow or normal saccadic velocities, not the "super-fast" velocities found in patients with ocular MG.

Downbeat nystagmus with a pseudocycloid waveform: improvement with base-out prisms

Downbeat nystagmus in primary position and oscillopsia resulted from nutritional deficiency during prolonged intravenous therapy of a patient with hyperemesis gravidarum. Wide bandwidth infrared oculography demonstrated a pseudocycloid nystagmus waveform with an increasing-velocity exponential slow phase. Because the oscillopsia decreased and the nystagmus was damped with convergence, visual acuity improved with the addition of base-out prisms to each spectacle lens.

The effects of age on normal saccadic characteristics and their variability

Age effects on human saccadic eye movements were tested with infrared reflectance oculography in 34 subjects. In contrast to a prior report, only a slight non-significant change was observed in saccadic velocity and duration. An increase in saccadic latency comparable to that found in several previous reports was observed, however. All parameters showed considerable intersubject variability for both age groups. Decreased velocities or increased durations outside of these normal, broad ranges should be regarded as pathological for all subjects; they are not physiological effects of the aging process.

Alexander's law: a model and resulting study

Recently we developed an analog model to simulate Alexander's law in nystagmus secondary to dysfunction of a semicircular canal. Alexander's law is based on the observation that the amplitude of the nystagmus grows with increasing gaze in the direction of the fast phase and diminishes with gaze in the opposite direction. To investigate the assumptions made in the model, we conducted quantitative experimental studies on the effect of gaze on caloric-induced nystagmus in human subjects. A weak stimulus (water at 26.5 degrees C and 240 ml/min) was administered for several minutes which caused the development of jerk nystagmus. Both the average slow phase velocity and frequency reached a steady state at about three minutes after the start of irrigation and remained stable until the flow of water was stopped. To investigate the effect of gaze, each subject was asked to hold gaze at various positions from center, to the right, to the left, and to repeat the cycle. Results indicated that the slow phase velocity of the nystagmus was greatest in the direction of the fast phase and decreased approximately linearly with gaze in the other direction in accordance with Alexander's law. Frequency was not a function of gaze. We speculate as to the biological advantages of the brainstem neural circuitry responsible for Alexander's law.

Internuclear ophthalmoplegia: recovery and plasticity

We studied refixational eye movements of a patient during the gradual resolution of an internuclear ophthalmoplegia (secondary to head trauma) in an attempt to determine the relative contributions of both medial longitudinal fasciculus (MLF) recovery and secondary central plastic changes. Adduction-refixational eye movements in the affected eye consisted of an initial fast (saccadic) portion followed by a slow drift toward the new intended eye position. The fast and slow components of the movements reflected, respectively, the pulse and step increases in neural innervation. Shortly after the traumatic insult, the affected eye exhibited low adduction gain (pulse gain 0.34; step gain 0.37) and slow saccades with peak velocities of 55% and durations of 278%, normalized for achieved, rather than intended, amplitudes. Several months later the pulse and step gains, peak velocities, and durations of the saccades improved to 0.81, 0.92, 87%, and 145%, respectively. The increased gains and faster velocity were accomplished by increases in the firing frequency of the pulse and step, reflecting recovery of MLF axons, rather than saccadic system plasticity, which would have resulted in increased duration of the saccadic pulse.