Visual evoked responses and visual symptoms in multiple sclerosis

Measures of visual pathway structure and function in MS: Clinical usefulness and role for MS trials

Over the past decade, the visual pathway in multiple sclerosis (MS) has become an important system for assessing both patient function and disease burden. Abnormalities of low-contrast acuity, long recognized as important correlates of driving, facial recognition, and other activities of daily living, are now noted to be common among patients with MS, even among those with no history of acute optic neuritis (ON). Low-contrast letter acuity scores correlate well with brain MRI lesion burden, visual-evoked potential (VEP) amplitudes, health-related quality of life (QOL), and retinal nerve fiber layer (RNFL) axonal and neuronal loss as measured by optical coherence tomography (OCT). Axonal and neuronal degeneration in MS is likely to be an important cause of visual impairment and disability, particularly among patients with progressive MS subtypes. With the advent of OCT and the use of low-contrast letter acuity charts in MS research and clinical trials, the structure-function correlations afforded by the anterior visual pathway can be assessed and potentially harnessed as a model for testing new therapies. Recent advances in OCT, such as high resolution spectral-domain techniques and computerized algorithms for image analysis, have allowed for measurement of specific retinal layers, including the ganglion cell (GCL) neuronal layer and its intimately associated, thin layer of interneurons, the inner plexiform layer (IPL). Longitudinal collaborative studies of GCL+IPL thinning and RNFL axonal loss are providing an in vivo view into neuroretinal pathology, and are providing new insights into how the visual pathway may reflect overall mechanisms of disease in MS.

Pattern visual evoked potentials in Japanese patients with multiple sclerosis without history of visual pathway involvement

A delay of the peak latency of the pattern visual evoked cortical potentials (pVEP) is accepted as one of the paraclinical evidence for a diagnosis of multiple sclerosis (MS). The purpose of this study was to evaluate the pVEPs in Japanese patients with MS without a history of visual pathway involvement. We studied the medical records of 29 MS patients without any history of visual pathway involvement, and with visual acuity correctable to > or = 20/20. The Goldmann visual fields, pupillary light reflexes, and optic disks were normal in all. pVEPs elicited by 3 rev/s (transient) and 12 rev/s (steady-state) were recorded from the MS patients and compared with those recorded from normal subjects. The latency of the P100 component of the transient pVEPs was significantly prolonged in 9/29 (31%) MS patients. A phase lag in the steady-state pVEPs was found in 6/29 (21%) MS patients, and the mean amplitude was significantly smaller. The incidence of cases with abnormal pVEPs is lower than that reported from Europe and United State. This difference is possibly due to racial differences, and the use of different criteria for diagnosing optic neuritis.

Comparison of psychophysical and evoked potential methods in the detection of visual deficits in multiple sclerosis

We compared the diagnostic sensitivity of traditional visual tests such as the Snellen-test and pattern reversal VEPs with psychophysical and electrophysiological tests involving motion processing and psychophysical tests of contrast processing in detecting visual deficits in a group of MS patients. A total of 30 patients with a definite diagnosis of MS and 22 age-matched controls selected from a pool of healthy volunteers participated in this study. Visual evoked potentials elicited by reversing checkerboards and moving random dot patterns (motion-onset VEPs) were recorded. The recognition of motion-defined forms (motion-defined letter test, MDL-test) and of contrast reduced optotypes was measured psychophysically. Of 30 patients, 29 showed deficits in at least one of the tests applied. The highest detection rate was obtained for a simple psychophysical test, the MDL-test, which revealed abnormalities in 80% of the patients. This is about 12% more than pattern VEPs could detect. Conversely, abnormalities in motion-onset VEPs were found in only 16% of the patients. Our results show that by adding a simple psychophysical test of form-from-motion analysis our capability to demonstrate an involvement of the visual system in MS patients may be promoted considerably.

[Abnormalities of pattern reversal evoked visual potential in patients with defined multiple sclerosis]

The pattern reversal visual evoked potential with checks of 14' and 28' was retrospectivelly studied in 28 patients with definite multiple sclerosis. We observed abnormal responses in 27/28 (96.4%) patients, in 31/36 (86%) of symptomatic eyes, and in 16/20 (80%) of asymptomatic eyes. When we classified the abnormalities in each eye according to the findings obtained with each check, there was a correlation between the pattern of abnormalities and the severity of visual involvement. Occasionally there were isolated abnormalities of N75 or only in P100 obtained with 28' checks. In conclusion the methodology applied was very sensible in detecting abnormalities in visual pathway. We could classify the findings in each eye and correlate them with the severity of visual involvement. The findings showed uneven distribution of lesions in visual pathway, affecting preferentially the central vision afferents.

P 100 latency of the visual evoked response in multiple sclerosis compared with normative data

P 100 latency of the pattern-reversal visual evoked response in 65 patients with definite multiple sclerosis was compared with normative data derived from 65 control subjects. On the basis of the results of distribution analyses it could be concluded that in definite multiple sclerosis there is visual system involvement in all patients, even if their P 100 latencies are within normal limits. Implications with respect to suspected multiple sclerosis were discussed.

Detection of hidden visual loss in multiple sclerosis. A comparison of pattern-reversal visual evoked potentials and contrast sensitivity

The detection of hidden visual loss is important in establishing the diagnosis of multiple sclerosis, especially in patients who have neurologic symptoms of the disease. Both visual evoked potentials and contrast sensitivity have been used for this purpose. We compared the sensitivities of pattern-reversal visual evoked potentials and contrast sensitivity, measured with the Vistech VCTS 6500 chart, in detecting hidden visual loss in 18 patients with multiple sclerosis whose visual acuity was correctable to 20/20 (6/6) or better in the examined eye. Thirteen patients had delayed visual evoked potential latencies. An additional four patients had reduced P100 amplitudes without prolonged latencies. Nine patients had abnormal contrast sensitivity. The visual evoked potential was more sensitive than contrast sensitivity at detecting hidden visual loss in patients with multiple sclerosis (p less than 0.01).

Central fiber contribution to W-shaped visual evoked potentials in patients with optic neuritis

We studied W-shaped waveforms that occurred in full-field responses to pattern large-field stimulation in patients who had optic neuritis. Affected eyes showed no absolute scotomata; visual acuity was normal at the time of recording. To evaluate the contribution of macular- and paramacular-derived components to the development of the W-shaped waveforms in the patients, half-field and central full-field stimulation was used. The responses were compared with those obtained with the use of experimental scotomata in healthy subjects. The W-shaped waveforms recorded in the patients closely resembled the responses observed in healthy subjects after the introduction of experimental scotomata. In all affected eyes, half-field stimulation showed absence of the ipsilateral P100 component or its interaction with the P135 component. Enhanced paramacular N105 and P135 components were seen over the contralateral hemisphere. Responses to central full-field stimulation were an attenuated and prolonged P100 in the majority of affected eyes. Results of our study showed that W-shaped waveforms in response to large full-field stimulation may reflect impaired function of macular fibers. These electrophysiologic findings, however, were not always associated with evidence of a central field defect demonstrated by Friedmann perimetry.

Electrophysiological detection of "silent" plaques in the optic pathways

The incidence of a delayed P100 component of the VEP after checkerboard stimulation in probable or possible multiple sclerosis (MS) without history, signs or symptoms of optic neuritis is not significantly different from that found in other neurological disorders in which the visual system is unaffected. This reduces the diagnostic validity of a delayed P100 as evidence of "silent" plaques in the optic pathway, at least in suspected MS. The use of grating increases the VEP sensitivity in the MS group, but it still leaves more than a 30% chance of error in attributing a delayed P100 to a demyelinating disorder. In this respect the discordant behaviour of checkerboard and grating responses may represent a useful clue.

Aberrant wave forms to pattern reversal stimulation: clinical significance and electrographic 'solutions'

Aberrant wave forms (AWFs) which are pattern visual evoked potentials (PVEPs) with ambiguous P2 peaks, were found in 13.7% of our patient population and not in our control group. The distribution of AWFs among the clinical entities of chronic myelopathy, questionable early MS, MS without visual lesions, MS with visual lesions and other neurological disorders, were similar to that of normally formed PVEPs with delayed P2 peaks. AWFs therefore appear to have a similar clinical significance to that of delayed, normally formed PVEPs, whether or not their latencies are within the normal range. Electrographic fields of non-aberrant PVEPs with well defined P2 peaks were studied to define the following obligate P2 characteristics: P2 always attained highest amplitude at the midline occipital position (Ox), hemifield stimulation using Fz as a reference always evoked a larger NPN complex over the hemisphere ipsilateral to stimulation than over the contralateral hemisphere. Multiple-channel analysis of both fullfield and hemifield stimulation and bipolar transverse montages reconstructed from referential recordings were employed to identify the P2 peak in AWFs. Bipolar reconstruction identified the P2 peak in 61% of all instances in which it was employed while hemifield stimulation did so in 84% of instances. In the 82 instances in which both were employed, hemifield stimulation identified the P2 in 84% as compared to only 33% for bipolar reconstruction. Therefore, although bipolar reconstruction may often identify the P2, hemifield stimulation is more effective for particularly complex situations.

Power spectral analysis of visual evoked potentials in multiple sclerosis

Power spectral analysis (PSA) was performed on the visual evoked potentials (VEPs) to counterphased checkerboard stimuli from 98 eyes in 49 patients with multiple sclerosis (MS) and 54 eyes of 27 normal volunteers. Attenuation of high frequency components of the transient visual evoked potential was found in 53% of MS patients and 4% of controls. Loss of high frequency components was poorly correlated with prolonged latency (r = 0.346). The consideration of both PSA and latency of the VEP increased the percentage of MS patients exhibiting visual pathway conduction abnormalities from 61% to 86%. The use of PSA in the diagnosis of MS is useful in increasing detection especially in cases where deformed waveforms preclude a reliable estimation of latency.

Steady-state and transient visual evoked potentials in clinical practice

The electrophysiological analysis of visual evoked responses is a powerful tool for the study of visual function. The combined application of pattern-reversal transient visual evoked potentials, critical frequency of photic driving, and visual evoked spectrum array has enhanced the reliability and the yield of these tests for the diagnosis of visual dysfunctions. Prechiasmatic and retrochiasmatic lesions are characterized by different abnormalities. Prechiasmatic lesions often can be further differentiated into retinal and optic nerve lesions by the simultaneous recording of retinal and cortical potentials.

Success and failure of evoked potentials in detecting clinical and subclinical lesions in multiple sclerosis patients

The value of visual, brain stem auditory and somatosensory potentials in detecting clinical and subclinical lesions as compared to the routine neurological, ophthalmological and vestibular examinations was investigated in 100 M.S. patients. It would appear that the VEP and SEP are far superior to the routine techniques in demonstrating lesions. On the other hand, the BAEP is inferior to the clinical and vestibular test as an indicator of brain stem lesions. All clinically manifest posterior column lesions are associated with abnormal SEP. However a substantial proportion of clinically evident lesions in the visual pathways or the midbrain and pons are not detectable by the VEP and BAEP.

[Clinical applications of visual evoked potentials for detection of chiasmal and postchiasmal lesions (author's transl)]

Visual evoked potentials (VEP) were recorded in 18 patients with pathologic processes confirmed by computerized tomography in the chiasmal (n = 9) and parietooccipital region (n = 9). Reactions from the right and left hemisphere could be recorded separately in spite of using a simple one-channel apparatus and electrodes only at Oz and Cz. In 17 cases changes of the VEP provided information concerning the localization and extension of the lesion. In chiasmal processes we found a prolongation of monocular latencies, and a delayed or extinguished reaction to half-field stimulation from temporal retinal areas. However, the VEP was often pathologic for half-field stimulation of the nasal hemiretina. Pathologic VEPs were not always accompanied by visual field defects. In contrary to patients with chiasmal processes no pathologic reaction could be found to full-field stimulation in parieto-occipital lesions. Only when the affected hemisphere was stimulated selectively were diminution of amplitudes, prolongation of latencies, or extinguished responses observed. The VEP changes were uniform despite the cause of the lesion (tumor, ischemia). In chiasmal and parieto-occipital processes the VEP supplements computerized tomography by detecting deficits in function. This method appears suitable for monitoring the course of disease before and after neurosurgery.

Follow-up studies of visual potentials in multiple sclerosis evoked by checkerboard and foveal stimulation

Visual cortical potentials were evoked by reversal of a checkerboard pattern and a small quadrangular foveal stimulus. Examination of 68 patients with MS showed the highest detection rate for abnormality of VEPs with a combination of both methods. Follow-up studies revealed changes (improvement or impairment) of VEP amplitudes or latencies in nearly half of the patients. Most of the changes were correlated with a history of acute optic neuritis. For the detection of changes the foveal stimulus is preferable to the checkerboard pattern.

Diagnostic techniques in multiple sclerosis

Although multiple sclerosis (MS) has been an area of great interest and effort in numerous scientific disciplines, its etiology, pathogenesis and therapy remain mysterious. Perhaps the most significant advances have resulted from the ability to be more precise in establishing the diagnosis of MS. Among the most important of the new diagnostic tools are electrophysiological techniques for examination of the visual and oculomotor systems. Tests such as the visually evoked potential (VEP) can confirm the existence of clinically suspected lesions and document the presence of asymptomatic ones. Various electrophysiological techniques for detection and evaluation of MS, as well as CSF analysis, psychological and neuroradiological procedures, and immunological observations are described. The relationship between optic neuritis and MS is reviewed, as are therapeutic regimens in use and under study.

Electroretinogram (ERG) and visual evoked response (VER) studies in patients with optic disc drusen

Non-corneal ERGs, recorded from infraorbital skin electrodes to flash stimulation and mid-occipital and parasagittal VERs to both flash stimulation and pattern reversal were performed in 26 patients with optic disc drusen. ERGs were normal in all patients. The mean VER amplitude was lower in the eyes with optic disc drusen than the mean amplitude of VERs in the normals but the interindividual variation was also so great in normals that the difference was not significant. The waveform of the major positive peak was quite often broad or split. VER latencies were usually in normal range although the visual field defects could be rather severe. Some other cause was present when the major positive peak was delayed.

Binocular interaction in the visual evoked response: temporal factors

Binocular visual evoked responses (VERs) were examined in normal subjects and in patients with prolonged monocular latency. Visual evoked responses to binocular stimulation are normally similar to the response to monocular stimulation but are somewhat larger. When conduction from one eye is slowed relative to that from its fellow (as often occurs in demyelinating disease) binocular interaction varies widely. When the interocular difference in latency is substantially elevated the binocular response is often that of the fastest pathway with little or no contribution from the slower pathway. These findings suggest a delayed reciprocal inhibitin between the inputs from the two eyes which may suppress late arriving signals.

Visual evoked potentials in normal subjects and patients with multiple sclerosis

Visual evoked potentials (VEPs) by checkerboard pattern-reversal stimulation were recorded in 70 subjects aged 10--69 years and in 100 patients with definite, probable or possible multiple sclerosis (MS). Longer latencies and smaller amplitudes of the major positive component were found in male subjects, in old subjects and when the amplifier's band-pass was narrowed. Subjects 10--14 years old had longer latencies and higher amplitudes than mature adults. Based on findings in the normal material, the following three criteria were used in evaluating the recordings from patients: the latency, the side difference in latencies and the ratio of amplitudes between the two sides of the major positive component with various limits for the two sexes and different age groups. The incidence of abnormal recordings was 85% for all the patients, 100% in 50 patients with definite, 70% in 50 patients with probable or possible MS, 73% in patients who had a history of spinal symptoms only, 98% if they had and 74% if they had not experienced optic neuritis. The incidence of abnormal findings increased with increasing duration of symptoms. All patients with visual acuity below 0.67 had abnormal VEPs. The high incidence of abnormal recordings confirmed the value of the test in establishing the diagnosis, and suggested that the use of different normative values for sex and age may increase the diagnostic yield without increasing the number of false positive findings.

Clinical applications of evoked responses: a survey

Evoked responses were first described in the late forties. In the late fifties, technology had so far advanced that the method became practically useful in clinical applications. Descriptions of the normal response and the influence of age and the effect of attention were extensively studied. There are marked interindividual and interlaboratory differences, partly explained by differences in test set-up. This is why it is still necessary that every laboratory, using its own test set-up, should determine its own standard values in normal test subjects. The value of ER studies in clinical diagnosis may be summarized as follows: --ophthalmology, organic psychiatry, neuropaediatrics, multiple sclerosis, uraemia: ER studies often supply important diagnostic information; --local lesions of the cerebral hemispheres, peripheral nerves and spinal cord, coma: ER studies can be useful as source of supplemental diagnostic information; --non-organic psychiatry and psychology: ER studies are still of minor practical value in individual clinical diagnosis. Research into applications of spinal evoked potentials and brain stem potentials is still in its initial phase.

Pattern reversal evoked cortical responses in normals. A study of different methods of stimulation and potential reproducibility

Using a commercially available television set for stimulation of 78 healthy subjects, the upper limit of normal for the latency of the major positive wave (p 100) of checkerboard pattern reversal evoked potentials is practically the same as that obtained by employing slide projector combined with a rotating mirror. Potentials evoked by a small pattern, for purely foveal stimulation, were often difficult to evaluate exactly. Fixation of a large pattern at the upper border of the stimulus field brought no advantages as compared to the usual fixation in the center. Continuous prolonged recordings, with successive averaging of 64 pattern reversals, confirmed statements that only the major positive wave and its latency are constantly reproducible without being influenced by fatigue or inattention. Repeated examinations in the same subjects at intervals of 2 weeks showed a considerable spread of the latencies from one session to the other but the absolute values always ranged within the normal. More or less large latency differences (up to 12ms with large pattern stimulation) were found in every subject at least once. A longer latency found at a control examination, or a newly appearing latency difference, therefore do not prove a fresh optic nerve lesion as long as the absolute values are still within the limits of normal. From repeated examinations it could also be seen that foveal stimulation with a small pattern is not suitable for routine examinations because of high variation in the results.

[Event-related cerebral potentials. Classification and clinical use]

Event-related potentials (ERP) constitute an heterogeneous group of brain electrical signals time-locked to sensory stimulation, motor responses, and associative processes, recorded using computer averaging. Several kinds of brain electric activity are encompassed by the generic term "event related potentials": sensory evoked potentials, time-coherent cerebral events related to motor acta, long-latency potentials associated with psychological variables, slow potential shifts linked to psychological constructs, and "emitted" cerebral events of endogenous origin. Shortcomings of this classification are pointed out and a simpler one is proposed, distinguishing between sensory, motor, and associative time-locked potentials. Their clinical use is illustrated under six major headings: testing of sensory function, localization of lesions, maturation of the central nervous system, genetic markers, evaluation of pharmacologic agents, and indicators of cognitive processes. Some methodological issues relevant to the clinical use of ERP are discussed.

Pattern and flash visual evoked responses in multiple sclerosis

In multiple sclerosis (MS), increase of delay may be important, but the earlier change is the behavior of the wave shape; amplitude is always lower even in MS patients without visual subjective symptomatology, provided not only flash but also pattern stimulation is performed. Normal amplitude with pattern stimulation does not necessarily mean normal with flash. Combining both stimulation methods, we obtained lowering of amplitude and change of wave shape in 100% of cases while latency was increased in only 77% of cases. Sectorial ischemic neuropathy, and open angle glaucoma patients show the same findings, which emphasizes that the method is not specific for demyelinating diseases, nor selective for papillomacular bundle involvement, but that VER, considering only its first components, reflects axonal damage somewhere in the visual pathway. Flash and pattern VER might be even more pathognomonic in MS than oligoclonal gammaglobulin fractionation on agar, positive in 75% of cases. It is suggested that subclinical MS cases should be submitted to full electro-ophthalmographic investigation.

Practical application of patterned visual evoked responses in multiple sclerosis

Pattern reversal visual evoked potentials (VEP) were studied in a control group of 72 subjects and in 60 patients with multiple sclerosis. The recording system consisted of a visual stimulator for pattern production on a commercial TV connected to an EMG machine for displaying and averaging of the response. The normal values of latency, the upper limit of normality and right-left differences in our control group were compared with those given by different authors. Likewise a comparison of the positive VEP results in MS was carried out. The reasons for the variability of results were briefly discussed and the importance of control studies for each laboratory starting VEP recording was stressed. This test can easily be carried out in every clinical neurophysiological laboratory and it provides an excellent and rapid technique for the investigation of patients with suspected multiple sclerosis.