Visual Evoked Potentials in Neurological Disorders

Electrophysiological Evaluation of Cranial Neuropathies

BACKGROUND

Cranial nerve abnormalities occur frequently in both focal and diffuse neurologic disorders and can be evaluated by electrophysiological techniques available in most clinical neurophysiology laboratories.

REVIEW SUMMARY

The optic nerve is evaluated by visual evoked potentials. Measurements of latency, amplitude, and waveform morphology are especially useful in detecting demyelinating lesions. Brain stem auditory evoked potentials evaluate the auditory portion of the eighth cranial nerve. Using an auditory stimulus, a number of waveforms are generated, and changes in the normal patterns of response can detect abnormalities. Assessment of the trigeminal and facial nerves is done using a series of electrical stimulation techniques including the blink, masseter, and masseter inhibitory reflexes and facial motor nerve conduction studies. The blink reflex detects lesions of the first division of the trigeminal nerve and the facial nerve. The masseter reflex evaluates the third division of the trigeminal nerve. Changes in responses are measured and, using a combination of these techniques, localization of lesions at specific sites can be made. Accessory motor nerve conduction is useful not only in focal nerve injury, but repetitive stimulation on the accessory and facial nerves is used in diagnosing neuromuscular junction disorders. In addition, many of the voluntary muscles innervated by the cranial nerves are accessible to needle electrode examination, and evaluation can aid in identification of focal nerve lesions, as well as diagnosis in diffuse nerve and muscle disorders.

CONCLUSION

Electrophysiological techniques offer reliable means of measuring the integrity of the cranial nerves and their central pathways.

Near ultraviolet radiation elicits visual evoked potentials in children

OBJECTIVE

Ultraviolet radiation can be transmitted through the ocular media, as well as stimulate the retina, in some invertebrate, vertebrate and mammalian species. This study sought to determine if near ultraviolet radiation (UV-A) can elicit visual evoked potentials (VEPs) in young humans.

METHODS

VEP responses to 10 nm half-peak bandwidths of 340, 360 and 500 nm stimuli were measured in 8 children aged 7-10 years. Each VEP was based on an averaged response to 200 flashes and was recorded using a sensitivity of 250 microV (full scale) with the International 10-20 electrode placements Fz, O1, Oz, O2, and A1. Peak latencies (ms) were measured for the second negative peak, N2, third positive peak, P3, and third negative peak, N3. The amplitude (microV) between N2 and P3 was also measured.

RESULTS

Each child demonstrated a VEP response to both visible and UV-A stimuli. Most VEP parameters relative to the 340 and 360 nm stimuli (P < 0.05 to P < 0.001) were significantly different from the VEP responses to the 500 nm stimulus.

CONCLUSION

The results indicate that the near ultraviolet stimuli were indeed visible to the young human eye.

The effects of ultraviolet-A radiation on visual evoked potentials in the young human eye

A recent study from this laboratory using visual evoked potentials (VEPs) demonstrated that children's eyes are capable of detecting ultraviolet radiation. The aim of this study was to compare dose-response relationships in two age groups, 6-10 years (n = 10) and 20-25 years (n = 10). Under photopic viewing conditions (550 lux), exposures of monochromatic UV-A (339 nm) and visible radiation (502 nm) were correlated to VEPs. The results demonstrate that monochromatic UV-A can elicit age and dose dependent responses in the human visual system, suggesting that the eyes of children are more responsive to UV stimuli than the eyes of young adults.

Colour vision abnormalities in multiple sclerosis

A battery of colour vision tests was employed to evaluate visual function in patients with multiple sclerosis (M.S.). Colour deficits were found in 45% of patients tested with the Ishihara plates and 42.5% of patients tested with the FM 100-Hue test. 65% of M.S. patients failed at least one of the tests. The colour vision deficits were not restricted to patients with optic neuritis or with visual evoked potential (VEP) abnormalities and there was no significant correlation between an abnormal VEP latency and a colour vision deficit. Colour vision testing may be a useful option to consider in the investigation of M.S. patients, even if there is no other evidence of visual system involvement.

Comparison of fiber optical and video monitor stimulators in normals and multiple sclerosis patients

A comparison of VEP findings using a fiber optical stimulator with three color combinations (black/red, black/green and red/green) and a conventional video monitor stimulator in black/white was performed in 3 groups of subjects: a group with definite lesions in the visual pathway, a group with suspected lesions and a control group. No significant correlations of P100 latency were found in the normative group, probably because of the small range of their values. All correlations were significant in the two patient groups, except for the red/green stimuli in the definite group. In general, the red/green combination was inferior to other color combinations in eliciting VEPs. The patient groups with definite, as well as suspected, lesions indicated no benefit from the color stimuli, as compared with black/white. The major advantage of the fiber optical stimulator is its simplicity, the lack of stimulus artifacts and the accessibility to the patient's bed side, thanks to its small size. Stimulators with higher illuminance and improved pattern element shape should narrow the still unacceptable normative variability of the wave forms recorded.

The refractory period of the visual evoked potential produced by pattern reversal in multiple sclerosis

Refractoriness of the visual evoked potential to pattern-reversal stimulation has been studied using a light-emitting diode stimulator, the effect on a test P100 of a conditioning pattern reversal being determined for conditioning-test intervals of 20 ms, 50 ms and 100 ms. In 20 normal subjects, the latency of the test P100 was increased by a mean of 9.8% (SD 5.0%) with the 20-ms interval, 5.8% (SD 6.6%) with the 50-ms interval, and 2.8% (SD 4.7%) with the 100-ms interval. Patients with clinically definite multiple sclerosis were divided into 3 groups based on the latency of P100 in the conventional VEP: within 2 SD of the normal mean (7 patients), plus 3-6 SD (7 patients), and greater than 10 SD (7 patients), representing normal, mildly abnormal and grossly abnormal latencies, respectively. Only 4 patients had "abnormal" results in the conditioning-test paradigm, 2 of whom were in the "grossly abnormal" group and 1 in each of the other two groups. The prolongation in latency tended to be greater the longer the control P100. The conditioning-test paradigm did not reveal evidence of a significantly prolonged refractory period of demyelinated axons in the optic nerve, and did not assist in the identification of changes "typical of demyelination" in patients with normal or mildly abnormal control P100s. It is concluded that testing the refractoriness of the VEP may not enhance the diagnostic yield of the VEP in multiple sclerosis.

Some comments on the clinical use of evoked potentials

In this survey we describe the uses of somatosensory, visual, and auditory evoked potentials (EPs), adding some critical comments on the values and pitfalls of these methods. It must be stressed that the application of EPs is most valuable when combined with a thorough neurological examination. There is general agreement that EP measurement is one of the best techniques for objective, noninvasive study of brain function in humans.

Detection and measurement of steady-state evoked potentials in real-time using a lock-in amplifier. Technical note

A system for recording evoked potentials, in which a lock-in amplifier replaces the averaging computer, is described. With this system, the set-up is much the same as in the conventional apparatus, with the following exceptions: 1) the amplifier gain is set at only a few thousands; 2) analogue-to-digital conversion and computer averaging is replaced by a lock-in amplifier; and 3) the output of the system is an analogue signal representing the amplitude of the recorded steady-state evoked potentials and the phase of baseline crossings. This set-up is much less costly than the conventional apparatus, much easier to operate, and, with its real-time output, may provide an ideal technique for screening and monitoring purposes.

A pattern reversal stimulator using optical fibers

A combination of light emitting diodes, as light sources, and optical fibers, as channels for the emitted light to form patterns, has resulted in a pattern reversal stimulator that is miniature in size, has pattern color options, is low in power requirements, and hence in possible stimulus artifacts. In addition, the use of optical fibers enables better separation of pattern elements and easier focusing on the pattern. The stimulator incorporates all the assets of previous versions of miniature pattern stimulators, is comparable or even less in price, and can interface with commercial averagers.

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.

Effects of spatially structured stimulus fields on pattern reversal visual evoked potentials

Pattern-reversal visual evoked potentials (PRVEPs) were tested in 11 healthy individuals. A photic stimulator for pattern production on a TV monitor, an electroencephalograph for EEG signal amplification and a computer for averaging the potentials and displaying the traces were used in the experiments. The peak latencies and both trough-to-peak and baseline-to-peak amplitudes of the main PRVEP components were found to be closely related to both the check size of checkerboard pattern and the bar width of barred pattern. The latencies decreased exponentially and amplitudes varied non-monotonically with check size or bar width. Most of the amplitudes of these components were greatest when checkerboard patterns with check sizes subtending visual angles of 60--70 min of arc, or barred patterns with bar widths subtending angles of 7--15 min of arc, were presented.

Auditory averaged evoked potentials and aging: factors of stimulus, task and topography

The averaged evoked potential to brief tones was compared for 10 young and 10 elderly female subjects. The amplitudes of the sensory components (P1, N1 and P2) were not affected by an infrequent change in pitch of the tones or instructing subjects to count or ignore them; but overall the elderly had a larger P1 and smaller P2 amplitude and a difference in the scalp distribution of P2. Repetition of the tones produced a decrement in these sensory components and a differential one for young and old subjects A slow potential complex consisting of components N2, P3 and SW appeared to changes in tone pitch and became more pronounced when attention was directed to the tones. Overall smaller SPs for oder subjects were interpreted as evidence for a change with age in the cortical representation of the orienting response. Topographical analysis of SPs indicated diminished activity in frontal (Fz) electrodes for elderly persons, suggestive of an enhanced aging process in the frontal cortices.

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.

Pattern shift visual evoked response in carotid occlusion

Pattern shift visual evoked response (PSVER) latency has been shown to be an extremely sensitive but nonspecific measure of optic nerve dysfunction. The test has been most useful in detecting unsuspected demyelinating lesions of the optic nerve, but abnormal results have been reported in a variety of other conditions affecting the optic nerve, including ischemic optic neuropathy. We report a patient with left internal carotid artery occlusion, possibly secondary to neck trauma, with normal ophthalmological examination and abnormal PSVER. The suggested mechanism is subclinical optic nerve ischemia due to decreased blood flow in the ophthalmic artery. PSVER may have value as a sensitive indicator of internal carotid artery disease.

True checkerboard pattern reversal with light-emitting diodes

The use of rectangular red light-emitting diodes permits construction of a true checkerboard pattern-reversal stimulator, which given evoked responses comparable in amplitude and wave form to those obtainable by a TV pattern generator. The device interfaces easily with commercial avergers and allows the incorporation of several features that may increase the sensitivity of visual evoked responses in screening for multiple sclerosis.

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.