Visually evoked potentials: theory, techniques and clinical applications

Evoked potentials in the clinical neurosciences

The use of evoked potentials for the evaluation of disorders of the nervous system has become a most valuable aid to the neurosurgeon and neurologist, often providing information of critical value without recourse to invasive techniques. In order to employ these techniques, it is helpful to understand the principles of evoked potential electrogenesis and the methodology used for analysis of evoked potential clinical data. This article is aimed at providing the clinical neurosurgeon with this type of information and with a review of current clinical applications in this rapidly developing field.

Visual electrophysiology and reading/learning difficulties

The literature relating visual problems, reading and academic difficulties with neurophysiological mechanisms shows positive interaction among these factors. Both visual evoked response and encephalographic studies show parietal or occipitoparietal neurophysiological anomalies which can be used to discriminate between normal and disabled readers to better than an 85% accuracy. Visual training and visual biofeedback have been used to change encephalographic alpha rhythm patterns with concomitant changes in attention and reading skills.

Relationship between visual evoked potentials and intracranial pressure

The relationship between intracranial pressure (ICP) and latency of visual evoked potentials (VEP) was investigated in hydrocephalic patients with severe head trauma. A positive correlation of increase in latency of wave N2 (normal latency 71 +/- 9.2 msec) of the VEP with elevations in ICP was observed. A potential role for VEP in both the assessment of shunt function and the monitoring of patients with severe head injury is suggested by these findings.

Checkerboard reversal pattern and flash VEPs in dialysed and non-dialysed subjects

In forty-three patients suffering from chronic renal failure (CRF; 32 non-dialysed, 11 dialysed) the flash VEP and the pattern VEP to different spatial (7.5', 15', 30', 60' checks) and temporal (2, 10, 20, 40 Hz) frequencies were investigated. Abnormally delayed flash VEPs were obtained in 53.6% of the cases; transient pattern VEPs had a delayed major positive deflection in 46% of the tested eyes and altered steady-state response in 58% with a total of 65% abnormal responses. Chronically dialysed subjects showed higher levels of altered VEPs for both the techniques. The shorter latencies normally observed with coarser checks in healthy people were maintained in CRF patients, resulting in an abnormal prolongation of the shift between high and low spatial frequencies. Higher statistical correlations linked the pattern VEP with blood urea nitrogen (P less than 0.001) and other renal chemistry than the flash VEP. These and other features could be ascribed to a specific involvement of a maculo-calcarine channel devoted to 'detailed' vision and suggest the usefulness of this method to provide a tool for the early detection of an encephalopathic uraemia.

VEP measurement of the amplitude of accommodation

The amplitude of accommodation was measured in 44 subjects by visually evoked potentials (VEP) in response to negative lenses. It was found that about two-thirds of the subjects responded to the lenses by increasing their accommodation in order to minimise blur, while the other one-third accepted the blur and accommodated very little. The amplitude was also determined subjectively (push-up method) and it correlated very well with the VEP measurement (r = 0.91). It is therefore suggested that VEP constitutes a feasible method of assessing objectively the amplitude of accommodation.

Changes in the binocular fixation patterns and the visually evoked potential in the treatment of esotropia with amblyopia

Using the binocular fixation pattern (BFP) and the visually evoked potential (VEP), thirteen amblyopic patients with comitant, non-accommodative esotropia with an angle range up to 30 delta were studied before and during occlusion therapy. A graded BFP with a stronger preferred fixation could be used to diagnose amblyopia. The BFP, however, showed no significant change with therapy despite improvement in acuity. In contrast, the VEP amplitudes, initially reduced in amblyopia, increased significantly as the vision responded to patching. The VEP was useful in diagnosing strabismic amblyopia and giving a predictive range of acuities.

Clinical uses of the visually evoked potential

The visually evoked potential is defined and described. The development of this test as a valuable clinical aid is traced. Its assistance in the diagnosis of Lesions of the Optic Nerve and Visual Pathways is shown, together with its use and assessment of visual acuity and visual fields both in infancy and adults. Future developments of this test are reviewed.

Subjective visual symptoms and electroencephalographic analysis before and after removal of occipital falx meningioma

A 41-year-old patient underwent surgery for the partial removal of a posterior falx meningioma, which invaded deeply into both occipital lobes. After surgery, the patient complained of marked metamorphopsia, colored photopsia, afterimage and loss of stereopsia. The relationships between these symptoms and features of the VEP and EEG power spectra (both under resting and photic driving conditions) were investigated. Marked photopsia on the 4th postoperative day was accompanied by EEG features indicating asynchronous excitation of injured cortical cells, while the presence of a strong afterimage on days 17 through 24 coincided with indications of an apparent refractoriness in cortical activation.

Trigeminal evoked potentials in humans

Techniques were described for recording trigeminal evoked potentials in humans in response to maxillary gum stimulation. Variations in the responses as a function of stimulating and recording electrode position and stimulus intensity were presented. A standard methodology was proposed in order to obtain the characteristic response repeatedly. The characteristic triphasic wave form consisted, primarily, of an N20, P34, N51 sequence of deflections. Average latencies of components were derived from recordings following stimulation of the asymptomatic side of the face in a patient population. Indirect tests supported the view that muscle potentials did not significantly contribute to the characteristic response. It was concluded that the techniques were suitable for electrophysiological testing of fifth nerve function in routine clinical applications.

Studies on visual evoked cortical response by liquid crystal phase difference haploscope: amblyopia and/or strabismus

Visual evoked cortical potentials induced by alternate stimulation of checkerboard pattern with the use of liquid crystal phase difference haploscope were investigated. The synchronization ratio of brain response was determined both in the group of normal binocularity and in the group with binocularity defect. The synchronization ratio of the group of normal binocularity (N = 30) is 100% at 1 Hz x 2, 98.5% +/- 3.9% at 5 Hz x 2, and 83.7% +/- 15.8% at 10 Hz x 2. On the other hand, in the group with binocularity defect, strabismus without amblyopia shows a reduction of the ratio only at the frequency of 10 Hz x 2. Anisometropic amblyopia shows a significant reduction of the ratio at 10 Hz x 2 and strabismus with amblyopia shows a remarkable reduction at 5 Hz x 2 and 10 Hz x 2. As for strabismus with or without latent binocularity, the latter cases show a significant reduction of the synchronization ratio. The reduction of synchronization ratio of VECP seems to have some relationship to the binocularity.

Foveal versus peripheral retinal responses: a new analysis for early diagnosis of multiple sclerosis

Visual potentials evoked by brief flashes (VEPs) were recorded in 85 patients with MS, 30 healthy controls and 25 neurological patients without demyelinating diseases. In a group of 44 patients, diffuse field stimulation was used, resutling in 15 altered responses (34%). In a second group of 41 patients with superimposable characteristics including age, sex, diagnosis of MS (definite, probable and possible) and score on the Rose's scale, the separate responses of central and peripheral retina were analysed. A total of 30 abnormal VEPs were recognized (73%) with the following distribution: 16 foveal VEPs (78%), 4 peripheral responses (13%) and 26 centreperiphery latency differences (CPLD, 60%). Twenty subjects of Group A with normal or slightly altered full field VEPs were retested with both stimulating methods with the following result: 3 altered full field VEPs (15%) and 15 abnormal CPLD (75%). With diffuse field stimulation, the correlation between the VEP scores and the results of other examinations (visual, pyramidal, etc.) were insignficant; on the contrary, the CPLD was clearly correlated with the involvement of the visual and other systems in the fluctuating clinical course of MS.

Dynamic depth reversal stereograms

A new stimulus was generated to create dynamic stereopsis. Binocular disparity was alternated between crossed and uncrossed conditions, while mean luminance remained constant, to produce a stereoscopic stimulus that appeared to move continuously in depth. These dynamic depth reversal stereograms provided an effective stimulus to elicit the visual evoked potential (VEP). Neural coding of stereopsis was reflected in the cortical response with VEP amplitudes changing as a function of binocular disparity and of image blur in the stereograms.

Visual evoked potentials in Reye's syndrome

Visual evoked potentials (VEPs) induced by flash stimulation in a child who was recovering from Reye's syndrome with complaints of poor left-eye vision were recorded. Ophthalmological examination disclosed intact visual fields and normal visual acuity. Analysis of VEP's, however, showed a grossly abnormal configuration of wave forms, marked prolongation in latency and reduction in amplitude when the left eye was stimulated. Follow-up observation 3 weeks later showed parallel improvements in VEPs and subjective complaints. Thus, from VEP analysis, it may be possible to detect subtle changes in the visual system that have diagnostic and prognostic value.

Pattern reversal visually evoked potentials in general anesthesia

Various authors have tried to determine visual acuity both electrophysiologically (Sokol, 1976 and 1978; Marg et al., 1976; Maffei and Fiorentini, 1977) and with psychophysical methods (Teller et al., 1974; Banks and Salapatek, 1976; Atkinson et al., 1976) in awake children. Our experiments try to verify the possibility of assessing visual acuity in children under Ethrane anesthesia. We conclude that under Ethrane anesthesia VEPs produced by phase-alternating square-wave gratings do not allow a fine assessment of visual acuity. Gross indications of visual acuity can however be obtained.

Visual-evoked response differentiation of ischemic optic neuritis from the optic neuritis of multiple sclerosis

Fifteen patients with ischemic optic neuritis studied electrophysiologically had a characteristic change of marked reduction in the amplitude of the visual-evoked response even when loss of vision was moderate. The optic neuritis of multiple sclerosis rarely produced this change. Occasionally, small increases in the latent period of the visual-evoked response were recorded from the patients with ischemic optic neuritis. The optic neuritis of multiple sclerosis usually produced significant increases in the latent period. When the normal nerve was tested in patients with ischemic optic neuritis, the visual evoked response was normal. In patients with optic neuritis of multiple sclerosis, stimulation of the "normal" nerve usually produced an increase in the latent period similar to that seen when the involved nerve was stimulated.

Application of basic research data to clinical amblyopia

The rhesus monkey has evolved as a useful model for the study of amblyopia caused by neonatal lid closure and experimental strabismus and anisometropia. In view of the behavioral and anatomic similarities of the visual system in humans and rhesus monkeys, there is reason to believe that the neurophysiologic and anatomic anomalies identified in animals as part of the "visual deprivation syndrome" occur in humans as well. Animal research has led to better understanding of the basic mechanisms responsible for the development of amblyopia and the manner in which form vision deprivation and abnormal binocular interaction contribute to the various forms of unilateral and bilateral amblyopia. The period of susceptibility to abnormal visual stimulation in humans needs to be better defined, and the clinician must take precautions to avoid uncontrolled occlusion or prolonged unilateral cycloplegia in infants. Even though the factors leading to amblyopia in patients with unilateral or bilateral congenital cataracts are now better understood, the visual results are still disappointing. It is possible that the residual aniseikonia after contact lens correction may be amblyopiogenic in these patients.

Steady-state evoked potentials

The advantages of steady-state EP recording include (1) speed in assessing sensory function in normal and sick infants (e.g., in amblyopia) and in sick adults (e.g., in multiple sclerosis); (2) monitoring certain activities of sensory pathways that do not intrude into conscious perception; (3) rapidly assessing sensory function when a large number of subjects must be tested (e.g., in refraction); (4) objective measurement at very high suprathreshold levels where psychophysical methods are difficult or ineffective; (5) rapidly assessing sensory function in normal subjects when EP variability and nonstationarity preculde lengthy experiments; and (6) proving a speedy objective equivalent to behavioral test in animals.

Neuro-ophthalmology, for the pediatrician

The pediatrician needs to develop some skills in evaluating afferent visual functions and ocular motor abnormalities. He must know some fundamental neuro-ophthalmologic facts to prevent his patients from undergoing unnecessary diagnostic and surgical procedures. In addition, he needs to understand the fundamentals of strabismus and amblyopia, which are briefly considered in this article but are explored thoroughly elsewhere in this issue of PEDIATRIC ANNALS. First, the most common type of nystagmus in children is congenital nystagmus. These children often have a head turn or tilt. Also, it should be remembered that numerous drugs may cause nystagmus. Second, any child with a head turn or tilt must be considered to have some ocular motor abnormality until a complete ophthalmologic evaluation has eliminated this possibility. In addition, before a child is considered to have an isolated sixth-nerve palsy, Duane's retraction syndrome should be looked for. Also, before an inferior oblique palsy is diagnosed, a Brown's tendon sheath syndrome should be considered. Thus, any ocular muscle abnormality deserves an ophthalmologic evaluation. Third, when bilateral swollen optic nerves are noted as an incidental finding, drusen of the optic nerve head should be suspected. A unilateral swollen disk with decreased visual acuity and a Marcus Gunn pupil should bring to mind a papillitis, which is a local inflammatory condition of the optic nerve head. Remember, papilledema is a bilateral condition secondary to increased intracranial pressure with normal vision. Children with papilledema usually have other signs of increased intracranial pressure. If the article has convinced you of only two facts -- that head tilts and turns are usually ocular in origin, and that bilaterally swollen optic nerves in an otherwise asymptomatic child may be optic nerve drusen -- it has accomplished its purpose.