Ocular motor myotonic phenomenon in myotonic dystrophy

OBJECTIVE

To detect disconjugate ocular motor abnormalities and a possible extraocular muscle myotonic phenomenon in patients with myotonic dystrophy (MyD).

METHODS

The magnetic scleral search coil technique was used to record monocularly the small (25 degrees ) and large (50 degrees ) saccades, which were paced to two interstimulus intervals (ISIs), one short (1 s), the other long (5 s). The case study comprised 20 patients with MyD, 10 patients with multiple sclerosis (MS), and 10 controls. The amplitude, duration, peak velocity, and skewness of the velocity profile (ratio between the acceleration and the deceleration periods) of each saccade were measured. The disconjugate parameters (difference between the two eyes of the same measure), and the myotonic parameter (the maximal (as absolute value) short-long ISI difference between the same measures) were considered.

RESULTS

The disconjugate parameters were the same in all three groups. The mean values of myotonic parameters found in patients with MyD for duration (for both small and large target displacements) and skewness (for small target displacements only) differed from those found for both the MS and the control groups. Additionally, the occurrence of individual patients presenting with abnormal duration and skewness parameters was higher in the MyD than in the MS group. In patients with MyD, the saccade duration was longer for long than for short ISI; the effect derived from a prolongation of the acceleration period, which manifested as an increase in skewness.

CONCLUSION

The results can be explained by a combination of the myotonic and the warm up phenomena. A delay in the relaxation (myotonia) of the extraocular muscle may be more evident after a long fixation period (long ISI) and it may improve by increasing saccade pacing (short ISI-warm up). This phenomenon is slight, and is unlikely to affect saccade performance significantly, but it may provide some insight into the nature of the disorder affecting extraocular and skeletal muscles in myotonic dystrophy.

Vestibular evoked myogenic potentials: test-retest reliability

Vestibular evoked myogenic potentials (VEMPs) are myogenic responses induced by stimulation of the saccular macula by intense sound stimuli. The responses are recordable from the sternocleidomastoid (SCM) muscles. We recorded VEMPs from normal subjects (up to three times in each subject) to identify: i) the best recording procedures, ii) the reliability, and iii) the normal limits for both individual point and test-retest evaluation. We adopted a recording setting in which the subjects were asked to simultaneously activate both SCM muscles by pushing their forehead against a load cell during a bilateral acoustic stimulation. This system enabled subjects to monitor their intensity of SCM activation and to keep intensity constant; us to record VEMPs from both sides simultaneously, and thus to minimize the duration of the recording session. For each subject we considered the mean and the difference (divided by the mean) of the values derived from the two SCM muscles of the latency of the P13 and N23 components and of the P13-N23 peak-to-peak amplitude. Reliability was evaluated by estimate of the intraclass correlation coefficient, and was good or excellent for all parameters, with the exception of the P13-N23 amplitude side-difference. To take advantage of all the data available, we computed the normal limits for both individual point and test-retest evaluation by means of the variability indices used for the evaluation of reliability. In this system, VEMP recording is simple, inexpensive and rapid. It is well tolerated by subjects, and easily implemented in laboratories equipped for evoked potential recording.

Visual evoked potential abnormalities in dyslexic children

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.

Circadian and hypothermia-induced effects on visual and auditory evoked potentials in multiple sclerosis

OBJECTIVES

Body cooling has been proposed as a symptomatic treatment for multiple sclerosis. This study aimed to assess the effects of body cooling and of circadian variations on clinical parameters and on visual and auditory evoked potential measures in multiple sclerosis patients.

METHODS

Clinical status was assessed and VEPs, BAEPs and MLAEPs (all with two stimulus frequencies) were recorded a total of 4 times on two separate days (two times per day at 08:30 and 15:00 h each day) in 10 multiple sclerosis patients and 10 controls. On one of these days, the subjects were submitted to body cooling before the afternoon session.

RESULTS

Tympanic temperature was significantly higher in the afternoon. Cooling lowered the temperature by 1.4 degrees C. No clinical effects were observed. Circadian effects were detected on VEP amplitude, which increased both in controls and in patients at low stimulus frequency (P<0.01), and increased in controls and decreased in patients at high stimulus frequency (interaction: P<0.01). Cooling determined an increase in BAEP I-V peak-to-peak time in controls, and a reduction in patients at high stimulus frequency (interaction: P<0.01). In patients, cooling also determined a great increase in MLAEP amplitude (interaction: P<0.001). We did not find cooling effects on VEP measures.

CONCLUSIONS

Visual and auditory evoked potentials showed differences in circadian and cooling effects between controls and multiple sclerosis patients. These differences are consistent with the hypothesis of temperature-dependent conduction blocks in demyelinated fibers. Cooling may have a clinical effect in selected patients only.

Periodic alternating nystagmus and vestibulo-spinal system facilitating activity

OBJECTIVE

Periodic alternating nystagmus has been associated with the instability of the velocity storage mechanism, which is known to play an important role in both the vestibulo-oculomotor and the optokinetic systems. In the present study we looked for a possible spinal equivalent to PAN.

METHODS AND RESULTS

In 3 PAN patients, the H-reflex amplitude proved to be slightly but significantly influenced by nystagmus direction, in that it was greater when the nystagmus was beating toward the stimulation side.

CONCLUSIONS

This finding suggests that projections from velocity storage may play a role not only in the ocular motor but also in assisting postural stability through the vestibulo-spinal system.

Auditory saccade impairment after central thalamus lesions

Visual and auditory saccades were studied in three patients with an isolated lesion located in the central thalamus. Visual saccades proved to be normal, whereas for auditory stimuli, the amplitude of the first saccade was asymmetric: saccades ipsilateral to the lesion were significantly smaller than those directed to the contralateral side. The patients were able to make a corrective saccade and hence to improve gain and to decrease gain asymmetry. It is suggested that patients were able to localise auditory targets correctly, but did not correctly take into account eye position during the saccade, probably as a consequence of an inaccurate efference copy (corollary discharge) signal. The findings are in keeping with the hypothesis that the central thalamus deals with saccades that are based on extraretinal signals.

Opsoclonus in a patient with cerebellar dysfunction

After two days of malaise, headache, nausea, and vomiting, a 26-year-old man suddenly developed opsoclonus and stance and gait ataxia, without myoclonus. Having excluded a paraneoplastic etiology, we assumed that the disorder was probably related to a viral infection. Spontaneous resolution occurred in about two months. Opsoclonus became flutter dysmetria and then resolved. Saccadic eye movement recording disclosed the occurrence of hypermetria, increased velocity, and delayed latency, which also resolved. In this patient, the correspondence between clinical and ocular motor abnormality courses suggests a transient cerebellar dysfunction as the possible pathophysiologic mechanism for opsoclonus.

Reversibility of vigabratin-induced visual-field defect

We report a girl with epilepsy aged 10 years receiving vigabatrin and complaining of bumping into objects and presenting visual-field constriction, which disappeared after vigabatrin withdrawal.

Epileptic skew deviation

We describe a 43-year-old neurologically intact patient who reported episodes of diplopia and oscillopsia associated with a right-beating nystagmus and a skew deviation. These symptoms and signs were related to a left posterior epileptic EEG discharge. We suggest that these ocular motor signs derived from an ictal activation of the vestibular cortex, which in turn activated descending projections to the vestibular nuclei, leading to both a dynamic (right-beating nystagmus) and a static (skew deviation) vestibular imbalance.

Eye movement abnormalities in myotonic dystrophy

We studied saccade and smooth pursuit eye movements in 31 patients suffering from myotonic dystrophy (MD). On the basis of mean value comparisons, saccades were slower and hypometric and smooth pursuit eye movements performed worse in MD patients than in controls. On an individual basis, saccade duration was prolonged in 67.7%, saccades were hypometric in 19.4%, saccade latency was delayed in 9.7%, and the smooth pursuit performance index was decreased in 9.7% of patients. Eye movement abnormalities did not correlate with those detectable by visual, brain-stem auditory and somatosensory evoked potentials. We attempted to classify eye movement abnormalities as myogenic or neurogenic on the basis of differences in combination of eye movement abnormalities and the occurrence of D5/D35 dissociation; the latter consists of a prolonged duration for large (35 degrees) but not for small (5 degrees) saccades. Since D5/D35 dissociation occurred in 26/33 multiple sclerosis patients with increased saccade duration, we considered it to be a neurogenic pattern attributable to a central nervous system (CNS) dysfunction. A prolonged duration without dissociation especially in combination with saccade hypometria, is interpreted as a myogenic pattern, although the lack of dissociation may also occur with CNS impairment in case of a marked increase in saccade duration. Accordingly we classified the oculomotor abnormalities detected as neurogenic in 11 MD patients and as myogenic in another 10, but in some subjects belonging to the second group concomitant CNS impairment is not to be excluded.

Clinical aspects of fatigue in multiple sclerosis

We studied 100 multiple sclerosis outpatients in order to assess the frequency of fatigue and to detect its relationships with other clinical findings. Fatigue was quantified by fatigue severity scale (FSS; range 1-7); current clinical neurological status and disability level were respectively scored by Kurtzke's functional systems and expanded disability status scale (EDSS). FSS mean score was 4.12; fatigue was totally absent in 3% of the patients. Fatigue started before onset of the disease in 7 patients, at onset in 12 and within the first year in 7. Fatigue intensity and frequency were related to each other. FSS mean scores were significantly higher in the patients with fatigue worsened by heat (p < 0.01), with chronic progressive disease (p < 0.0001), with motor symptoms (p < 0.0001) and with EDSS > or = 3.5 (p < 0.0001). Multiple regression analysis showed a significant effect of EDSS on fatigue (p < 0.0001), not attributable to differences in duration of the disease.

Eye-Head Coordination toward Auditory and Visual Targets in Humans

Eye–head coordination during gaze orientation toward auditory targets in total darkness has been examined in human subjects. The findings have been compared, for the same subjects, with those obtained by using visual targets. The use of auditory targets when investigating eye–head coordination has some advantages with respect to the more common use of visual targets: (i) more eccentric target positions can be presented to the subject; (ii) visual feedback is excluded during the execution of gaze displacement; (iii) complex patterns of saccadic responses can be elicited. This last aspect is particularly interesting for examining the coupling between the eyes and the head displacements. The experimental findings indicate that during gaze orientation toward a visual or an auditory target the central nervous system adopts the same strategy of using both the saccadic mechanism and the head motor plant. In spite of a common strategy, qualitative and quantitative parameters of the resulting eye–head coordination are slightly different, depending on the nature of the target. The findings relating to patterns of eye–head coordination seem to indicate a dissociation between the eyes and the head, which receive different motor commands independently generated from the gaze error signal. The experimental findings reported in this paper have been summarized in a model of the gaze control system that makes use of a gaze feedback hypothesis through the central reconstruction of the eye and head positions.

Eye-head coordination toward auditory and visual targets in humans

Eye-head coordination during gaze orientation toward auditory targets in total darkness has been examined in human subjects. The findings have been compared, for the same subjects, with those obtained by using visual targets. The use of auditory targets when investigating eye-head coordination has some advantages with respect to the more common use of visual targets: (i) more eccentric target positions can be presented to the subject; (ii) visual feedback is excluded during the execution of gaze displacement; (iii) complex patterns of saccadic responses can be elicited. This last aspect is particularly interesting for examining the coupling between the eyes and the head displacements. The experimental findings indicate that during gaze orientation toward a visual or an auditory target the central nervous system adopts the same strategy of using both the saccadic mechanism and the head motor plant. In spite of a common strategy, qualitative and quantitative parameters of the resulting eye-head coordination are slightly different, depending on the nature of the target. The findings relating to patterns of eye-head coordination seem to indicate a dissociation between the eyes and the head, which receive different motor commands independently generated from the gaze error signal. The experimental findings reported in this paper have been summarized in a model of the gaze control system that makes use of a gaze feedback hypothesis through the central reconstruction of the eye and head positions.

MRI and brainstem auditory evoked potential evidence of eighth cranial nerve involvement in multiple sclerosis

An MS patient experienced sudden hearing loss. Brainstem auditory evoked potentials, previously normal, showed substantial abnormalities that suggested the impairment of the distal part of the acoustic nerve. MRI detected a small hyperintense lesion along the acoustic nerve; the lesion decreased in size and then disappeared after steroid treatment. This demonstrates that a demyelinating lesion in the distal tract of the eighth cranial nerve may cause an acute hearing loss in MS.

Disorders of binocular control of eye movements in patients with cerebellar dysfunction

Recent research has implicated the cerebellum in conjugate ocular motor control, including steady gaze-holding and accuracy of pursuit and saccades. Whether the cerebellum also has a role in the control of the alignment of the eyes during fixation and of the yoking of the eyes during movement i. less certain. We have studied binocular (disconjugate) ocular motor control in nine patients with cerebellar dysfunction and compared the results with those of normal subjects. Eye alignment during fixation and the yoking of the eyes during and immediately after saccades were quantified by recording the movements of both eyes using scleral search coils. Patients had disturbances of ocular alignment. All had an esophoria during monocular viewing and many an esotropia during binocular viewing, implying an increase in convergence tone. Most had a vertical misalignment that varied with horizontal eye position ('alternating skew deviation'). Patients showed conjugate dysmetria (saccade under- or overshoot and postsaccade drift) and disconjugate dysmetria (the eyes were poorly yoked during and immediately after saccades). Both the conjugate and disconjugate abnormalities were incommitant, i.e. they varied with orbital eye position. Correlations amongst the various abnormalities suggested that one part of the cerebellum, perhaps the dorsal vermis and the underlying posterior fastigial nucleus, controls the conjugate size of saccades and that another part of the cerebellum, perhaps the flocculus/paraflocculus, controls the yoking of the eyes during saccades and both the disconjugate and conjugate components of postsaccade drift.

Estimating reliability of evoked potential measures from residual scores: an example using tibial SSEPs

A normative study of tibial SSEPs was performed in 74 healthy subjects, and the effects of the variables sex, age and height on SSEP parameters were assessed. In a subgroup of 20 subjects a test-retest study was also performed, which allowed us to estimate the reliability of the different parameters by means of the intraclass correlation coefficient. We demonstrated that the intraclass correlation coefficient may be biased by predictable effects of subject-related variables (such as age, height and sex), if it is computed from raw original values. This bias can be eliminated by estimating reliability indices on residual scores calculated as the differences between observed values and those predicted by subject's age, height and sex.

Saccadic and smooth pursuit eye movements in memory-impaired elderly people

The aim of the study was to compare eye movements recorded in 14 non-demented memory impaired elderly subjects with those of 36 sex- and education-matched controls. Eye movements were recorded with the bitemporal electrooculographic technique, and analyzed with a personal computer. Saccades were elicited in accordance with reflexive, predictive and antisaccade paradigms. Smooth pursuit eye movements were elicited with a triangular ramp paradigm. The memory-impaired subjects showed a higher prevalence of increased reflexive saccade latency and a lower smooth pursuit performance index. In addition, we detected a correlation between antisaccade and reflexive saccade latencies. Our results showed very slight differences in eye movement parameters. However, our findings are in keeping with the hypothesis of a subtle involvement of differing cortical areas in memory impaired subjects.

Saccade latency toward auditory targets depends on the relative position of the sound source with respect to the eyes

The latency of saccadic eye movements evoked by the presentation of auditory and visual targets was studied while starting eye position was either 0 or 20 deg right, or 20 deg left. The results show that for any starting position the latency of visually elicited saccades increases with target eccentricity with respect to the eyes. For auditory elicited saccades and for any starting position the latency decreases with target eccentricity with respect to the eyes. Therefore auditory latency depends on a retinotopic motor error, as in the case of visual target presentation.

Recovery functions of early cortical median nerve SSEP components: normative data

The recovery functions of parietal P14-N20, N20-P27 and frontal P22-N30 amplitudes were assessed in 17 healthy controls aged 20-50 years by means of the paired stimulus technique. One unpaired and 4 paired stimuli with interstimulus intervals (ISIs) of 25, 50, 75 and 100 msec were cyclically presented in a single run. Responses to the unpaired stimulus were subtracted off-line from paired stimulus responses. The highest suppression was reached at shorter ISIs for components with shorter latencies. The mean suppression of P22-N30 was influenced by the subject's age, being greater in younger subjects. Normative data are reported.