The diagnostic significance of the multifocal pattern visual evoked potential in glaucoma

Evaluation of Retinal Ganglion Cell via Visual Evoked Potential

Visual Evoked Potential (VEP) is an electrical signal recorded from the visual cortex in response to light stimulation. It can be used as an in vivo method to objectively access the functional integrity of the retinogeniculocortical pathway. Here we describe the methods to perform flash VEP (FVEP) recording in rodents and goat and pattern VEP (PVEP) recording in rhesus macaque.

Monitoring of optic nerve function in Neurofibromatosis 2 children with optic nerve sheath meningiomas using multifocal visual evoked potentials

Monitoring optic nerve sheath meningiomas (ONSM) in Neurofibromatosis type 2 (NF2) patients remains difficult. Other ocular manifestations of NF2 may obscure ophthalmic assessment of optic nerve function in these patients. Serial magnetic resonance imaging (MRI) used to assess the optic nerve is not without limitations, being expensive and often requiring general anaesthetic in children, with associated risks. This study was undertaken to describe the use of multifocal visual evoked potentials (multifocal VEP, mfVEP) in the regular monitoring of NF2 patients with ONSM. This study involved three NF2 patients with ONSM who undertook mfVEP testing at an academic ophthalmic centre. Same day mfVEP and routine ophthalmic testing were undertaken. Topographical function of the optic nerve was assessed, utilising tools such as asymmetry deviation and accumap severity index. Results were assessed alongside MRI and visual acuity (VA). From the three patients, five eyes had ONSMs, of which two caused unilateral blindness. The remaining three affected eyes had initial VAs 6/6, 6/24, and 6/18. Over follow up, ranging from 5 to 12 years, all tumours progressed, and VA declined for all patients. Multifocal VEP detected optic nerve functional loss corresponding with visual decline. This case series suggests mfVEP is effective in the objective topographic monitoring of optic nerve function in NF2 patients with ONSM. Due also to its safety in a paediatric population, the test may be considered in the routine monitoring of these patients, to be used to assist regular ophthalmic review and MRI scans.

Electroretinogram and visual-evoked potential assessment of retinal and central visual function in a rat ocular hypertension model of glaucoma

PURPOSE/AIM

The aim of the study was to investigate the long-term functional changes that may occur in the retina and visual cortex in a rat ocular hypertension (OHT) model of glaucoma, used in our lab for treatment studies, using electroretinogram (ERG) and visual-evoked potential (VEP) cortical recordings in order to test the hypothesis that experimental glaucoma has differential retinal and central effects.

MATERIALS AND METHODS

Experimental glaucoma was induced unilaterally in Dark Agouti rats using hypertonic saline injection into the episcleral veins. After 3, 8, 16 and 26 weeks, ERGs and VEPs were recorded under scotopic conditions using brief full-field white flashes (10 μcd s m(-2) to 10.4 cd s m(-2)) and under photopic conditions using a rod-adapting background and white light flashes (0.13-10.4 cd s m(-2)).

RESULTS

At 16 and 26 weeks after OHT induction, there was a significant reduction in the amplitudes of the a- (50% and 30% of unoperated eye values, respectively) and b-waves (55% and 40%, respectively) of the scotopic ERG and the b-waves of the photopic ERG (55% and 45%, respectively) in the glaucomatous eyes. However, no significant changes in the VEPs simultaneously recorded over the visual cortex were seen at any of the time points.

CONCLUSIONS

The reductions in ERG amplitudes suggest that this model of glaucoma not only causes retinal ganglion cell (RGC) degeneration but also degeneration of the outer retinal cells, and this was confirmed by histology showing a reduction in the outer retinal layers in the glaucomatous eyes. Cortical VEPs did not show detrimental effects suggesting that the retinal damage in this model was not extensive enough to be detected with the VEP methods used or that there could be central compensation in this model of glaucoma.

Comparison of the reliability of multifocal visual evoked cortical potentials generated by pattern reversal and pattern pulse stimulation

This study compared the effectiveness of the multifocal visual evoked cortical potentials (mfVEP) elicited by pattern pulse stimulation with that of pattern reversal in producing reliable responses (signal-to-noise ratio >1.359). Participants were 14 healthy subjects. Visual stimulation was obtained using a 60-sector dartboard display consisting of 6 concentric rings presented in either pulse or reversal mode. Each sector, consisting of 16 checks at 99% Michelson contrast and 80 cd/m² mean luminance, was controlled by a binary m-sequence in the time domain. The signal-to-noise ratio was generally larger in the pattern reversal than in the pattern pulse mode. The number of reliable responses was similar in the central sectors for the two stimulation modes. At the periphery, pattern reversal showed a larger number of reliable responses. Pattern pulse stimuli performed similarly to pattern reversal stimuli to generate reliable waveforms in R1 and R2. The advantage of using both protocols to study mfVEP responses is their complementarity: in some patients, reliable waveforms in specific sectors may be obtained with only one of the two methods. The joint analysis of pattern reversal and pattern pulse stimuli increased the rate of reliability for central sectors by 7.14% in R1, 5.35% in R2, 4.76% in R3, 3.57% in R4, 2.97% in R5, and 1.78% in R6. From R1 to R4 the reliability to generate mfVEPs was above 70% when using both protocols. Thus, for a very high reliability and thorough examination of visual performance, it is recommended to use both stimulation protocols.

Remyelination of optic nerve lesions: spatial and temporal factors

Optic neuritis provides an in vivo model to study demyelination. The effects of myelin loss and recovery can be measured by the latency of the multifocal visual evoked potentials. We investigated whether the extent of initial inflammatory demyelination in optic neuritis correlates with the remyelinating capacity of the optic nerve. Forty subjects with acute unilateral optic neuritis and good visual recovery underwent multifocal visual evoked potentials testing at 1, 3, 6 and 12 months. Average latency changes were analyzed. Extensive latency delay at baseline significantly improved over time with rate of recovery slowed down after 6 months. Magnitude of latency recovery was independent of initial latency delay. Latency recovery ranged from 7 to 17 ms across the whole patient cohort (average = 11.3 (3.1) ms) despite the fact that in a number of cases the baseline latency delay was more than 35—40 ms. Optic nerve lesions tend to remyelinate at a particular rate irrespective of the size of the initial demyelinated zone with smaller lesions accomplishing recovery more completely. The extent of the initial inflammatory demyelination is probably the single most important factor determining completeness of remyelination. The time period favorable to remyelination is likely to be within the first 6 months after the attack.

Multi-channel visual evoked potential as an ancillary tool to diagnose intraorbital optic nerve lesions

Multi-channel visual evoked potential (MVEP) recording method was used to assist in diagnosing a 4-month-old Chinese Albino rabbit with an intraorbital mass. Subcutaneous MVEP of its both eyes were recorded simultaneously using 16 electrodes (4 x 4) multi-channel array. Analysis of the cortical potential landscapes (CPL) showed that the conduction function of right eye was remarkably impaired in terms of decreased amplitudes and prolonged latencies. Specific side-dominant distribution asymmetry of the decreased MVEP amplitudes indicated that the temporal side of the optic nerve (ON) was severely involved. Overall prolonged latencies of the CPL without side differences suggested that the functional impairment could have been caused by the mechanical compression exerted by an intraorbital mass. Surgical removal procedures confirmed that the mass was located temporally to the ON. Pathological examination provided a final diagnosis of a giant polycystic mucocele. Beyond its significance as a standard tool to assess functional changes of the visual pathway, MVEP recordings might assist locating intraorbital lesions that involve the ON by careful analysis of abnormal CPLs.

Fellow eye changes in optic neuritis correlate with the risk of multiple sclerosis

BACKGROUND

Recent studies demonstrate early diffuse central nervous system (CNS) inflammation in patients with multiple sclerosis (MS). The clinically unaffected (fellow) eye of patients with unilateral optic neuritis (ON) may reflect the status of normal-appearing white matter in the CNS, which can be assessed electrophysiologically.

OBJECTIVE

To study the relationship between electrophysiological parameters in the fellow eye of ON patients, and risk of conversion to MS.

METHODS

Forty-eight consecutive patients with acute unilateral ON were examined 12 months after ON of which 14 had MS, 19 remained high risk (HR) for MS, and 15 had low risk (LR) for MS according to McDonald's criteria. Twenty-five age-matched controls were also tested. Amplitude and latency of multifocal visual evoked potential (mfVEP) in the fellow eyes of patients at 12 months were analyzed and compared with controls.

RESULTS

Average mfVEP amplitude was 240 +/- 35, 232 +/- 36, 181 +/- 38, and 169 +/- 48 nV for controls, LR, HR, and MS groups respectively. Average mfVEP latency for controls, LR, HR, and MS patients was 139.7 +/- 5.5, 141.7 +/- 3.6, 145.9 +/- 8.9, and 152.0 +/- 9.9 ms respectively.

CONCLUSIONS

The magnitude of latency prolongation and amplitude decline 12 months after the initial episode was proportional to the risk of MS. The prognostic significance of these changes as predictors of subsequent MS should be investigated longitudinally.

In vivo functional imaging of intrinsic scattering changes in the human retina with high-speed ultrahigh resolution OCT

Non-invasive methods of probing retinal function are of interest for the early detection of retinal disease. While retinal function is traditionally directly measured with the electroretinogram (ERG), recently functional optical imaging of the retina has been demonstrated. In this manuscript, stimulus-induced, intrinsic optical scattering changes in the human retina are measured in vivo with high-speed, ultrahigh resolution optical coherence tomography (OCT) operating at 50,000 axial scans per second and ~3.3 micron axial resolution. A stimulus and measurement protocol that enables measurement of functional OCT retinal signals is described. OCT signal changes in the photoreceptors are demonstrated. Two distinct responses having different temporal and spatial properties are reported. These results are discussed in the context of optical intrinsic signals measured previously in the retina by fundus imaging and scanning laser ophthalmoscopy. Finally, challenges associated with in vivo functional retinal imaging in human subjects are discussed.

Axonal loss and myelin in early ON loss in postacute optic neuritis

OBJECTIVE

To investigate the relation between retinal nerve fiber layer (RNFL) thickness and latency and amplitude of multifocal visual-evoked potentials (mfVEPs) in the postacute stage of optic neuritis in patients with early or possible multiple sclerosis.

METHOD

Thirty-two patients with clinical diagnosis of unilateral optic neuritis and magnetic resonance imaging lesions typical of demyelination and 25 control subjects underwent mfVEP and optical coherence tomography imaging.

RESULTS

Although there was significant reduction of RNFL thickness in the affected eyes (18.7%), a considerably larger decrease was observed for the amplitude of the mfVEPs (39.8%). Latency of the mfVEPs was also significantly delayed in optic neuritis eyes. In fellow eyes, the amplitude of mfVEPs was significantly reduced and the latency prolonged, but RNFL thickness remained unaltered. RNFL thickness correlated highly with the mfVEP amplitude (r = 0.90). There was also strong correlation between optical coherence tomography measure of axonal loss and mfVEP latency (r = -0.66).

INTERPRETATION

Although our findings demonstrate strong associations between structural and functional measures of optic nerve integrity, the functional loss was more marked. This fact, together with amplitude and latency changes of the mfVEPs observed in clinically normal fellow eyes, may indicate greater sensitivity of mfVEPs in detecting optic nerve abnormality or the presence of widespread inflammation in the central nervous system, or both. The significant correlation of the mfVEP latency with RNFL thickness suggests a role for demyelination in promoting axonal loss.

Multifocal VEP and OCT in optic neuritis: a topographical study of the structure-function relationship

PURPOSE

To investigate topographical relationship between amplitude of multifocal visual evoked potentials (mfVEP) and retinal nerve fibre layer (RNFL) thickness following acute optic neuritis (ON).

PATIENTS AND METHODS

Fifty patients with a clinical diagnosis of acute unilateral ON between 6 and 36 months prior to the study and 25 age-matched controls underwent mfVEP testing (Accumap V 2.1, ObjectiVision Pty Ltd, Sydney, Australia) and OCT imaging (fast RNFL protocol, Stratus, software version 3.0, Carl Zeiss Meditec, Inc., Dublin, CA). RNFL thickness and mfVEP amplitude were measured for upper, temporal and lower retinal sectors and corresponding areas of the visual field in affected eyes of ON patients and control eyes. Inter-eye asymmetry coefficients for both RNFL thickness and mfVEP amplitude were calculated for each zone, and corresponding coefficients were correlated between each other.

RESULTS

There was highly significant reduction of RNFL thickness and mean mfVEP amplitude in all three retinal sectors of the affected eye. Largest reduction of RNFL thickness was noticed in temporal sector and of mfVEP amplitude in corresponding central part of the visual field. RNFL thickness correlated highly with amplitude of the mfVEP derived from corresponding areas of the visual field in all three zones.

CONCLUSIONS

We demonstrated strong topographical associations between structural and functional measures of optic nerve integrity in patients with ON.

What is the link between vascular dysregulation and glaucoma?

The need of blood flow to different organs varies rapidly over time which is why there is sophisticated local regulation of blood flow. The term dysregulation simply means that blood flow is not properly adapted to this need. Dysregulative mechanisms can lead to an over- or underperfusion. A steady overperfusion may be less critical for long-term damage. A constant underperfusion, however, can lead to some tissue atrophy or in extreme situations to infarction. Unstable perfusion (underperfusion followed by reperfusion) leads to oxidative stress. There are a number of causes that lead to local or systemic vascular dysregulation. Systemic dysregulation can be primary or secondary of nature. A secondary dysregulation is due to other autoimmune diseases such as rheumatoid arthritis, giant cell arteritis, systemic lupus erythematodes, multiple sclerosis, colitis ulcerosa, or Crohns disease. Patients with a secondary vascular dysregulation normally have a high level of circulating endothelin-1 (ET-1). This increased level of ET-1 leads to a reduction of blood flow both in the choroid and the optic nerve head but has little influence on autoregulation. In contrast, primary vascular dysregulation has little influence on baseline ocular blood flow but interferes with autoregulation. This, in turn, leads to unstable oxygen supply, which seems to be a relevant component in the pathogenesis of glaucomatous optic neuropathy.

Correlation between full-field and multifocal VEPs in optic neuritis

AIM

To compare performance of multi-focal and full-field Visual Evoked Potentials (VEP) in patients with optic neuritis (ON).

METHOD

26 patients with unilateral ON were enrolled. Multi-focal VEP (MF VEP) was recorded using AccuMaptrade mark system. Four bipolar channels were analysed. Full-field VEP (FF VEP) was performed according to ISCEV standard using ESPION with frontal-occipital electrode placement. Pattern-reversal protocol was implemented with check size of 60' and field of view of 30 degrees .

RESULT

For both tests amplitude and latency of affected eye were statistically different from non-affected eye. The asymmetry of amplitude and latency between two eyes was also very similar for both tests. Averaged Relative Asymmetry Coefficient of amplitude (RAC) for the FF VEP was 0.10 +/- 0.15 and for the MF VEP was 0.12 +/- 0.12 (P = 0.21, paired t-test). Averaged latency difference between affected and non-affected eyes was 13.0 +/- 12 ms for FF and 14.1 +/- 11.1 ms for MF VEPs (P = 0.14, paired t-test). Coefficient of correlation (r) of p100 component of the FF VEP and averaged MF VEP was 0.60 (P < 0.0001) for amplitude and 0.79 (P < 0.0001) for latency. Correlation improved when amplitude and latency asymmetry between two eyes was analysed (r = 0.81 and r = 0.92 respectively). Overall 73% of affected eyes were identified as abnormal by amplitude and/or latency of the FF VEP and 89% was considered abnormal when MF VEP was used. Analysis of individual cases revealed superior performance of MF VEP in detecting small or peripheral defects.

Effect of fixation tasks on multifocal visual evoked potentials

PURPOSE

This study investigated the effects of cognitive influence on the multifocal visual evoked potential (mVEP) at different levels of eccentricity. Three different foveal fixation conditions were utilized involving varying levels of task complexity. A more complex visual fixation task has been known to suppress peripheral signals in subjective testing.

METHODS

Twenty normal subjects had monocular mVEPs recorded using the AccuMap objective perimeter. This allowed simultaneous stimulation of 58 segments of the visual field to an eccentricity of 24 degrees. The mVEP was recorded using three different fixation conditions in random order. During task 1 the subject passively viewed the central fixation area. For task 2 alternating numbers were displayed within the fixation area; the subject on viewing the number '3' in the central fixation area indicated recognition by pressing a button. Throughout task 3, numbers were displayed as in task 2. The subject had the cognitive task of summating all the numbers.

RESULTS

Analysis revealed that the increased attention and concentration demanded by tasks 2 and 3 in comparison with task 1 resulted in significantly enhanced central amplitudes of 9.41% (Mann-Whitney P = 0.0002) and 13.45% (P = 0.0002), respectively. These amplitudes became reduced in the periphery and approached those of task 1, resulting in no significant difference between the three tasks. Latencies demonstrated no significant difference between each task nor at any eccentricity (P > 0.05). As the complexity of each task increased the amount of alpha rhythm was significantly reduced.

CONCLUSIONS

Our findings indicate that task 1 required a minimal demand of cognition and was associated with the greatest amount of alpha rhythm. It was also the most difficult to perform because of loss of interest. The other two tasks required a greater demand of higher order cognitive skills resulting in significantly enhanced amplitudes centrally and the attenuation of alpha rhythm. Therefore, amplitudes are increased around the area of attention.

A Child with Epilepsy in Whom Multifocal VEPs Facilitated the Objective Measurement of the Visual Field

PURPOSE

We investigated whether visual field defects can be objectively evaluated by using multifocal visual evoked potential (multifocal VEP) in children with epilepsy in whom it is difficult to measure the visual field.

METHODS

First, to determine normal waves in multifocal VEP, recording was performed by using a VERIS Junior Science (Mayo, Aichi, Japan) in 21 healthy children (21 eyes) consisting of 12 boys and nine girls aged 6 to 15 years (mean age, 11.4 years). Responses from eight sites in each subject were divided into four quadrants (superior and inferior temporal quadrants and superior and inferior nasal quadrants). In each quadrant, two response waves were grouped and averaged; peak latency and amplitude were used for assessment. In a child with epilepsy in whom kinetic or static perimetry was impossible, multifocal VEPs were recorded and compared with the peak latency and amplitude in the healthy subjects.

RESULTS

In the 21 healthy children, no significant differences were observed in the peak latency or amplitude among four quadrants by one-way analysis of variance. Multifocal VEPs in the child with epilepsy showed abnormal waves in the right hemivisual field, which corresponded to the lesion observed with magnetic resonance imaging.

CONCLUSIONS

The objective evaluation of visual field defects by using multifocal VEP may be useful in children with epilepsy in whom kinetic/static perimetry as a subjective examination is difficult.

Effect of check size and stimulation rate on blue-yellow multifocal visual evoked potentials

BACKGROUND

To determine the effect of different stimulus frame rates and check sizes on blue-yellow multifocal visual evoked potentials (mVEP).

METHODS

Subjects were examined at the Save Sight Institute at the University Sydney. Experiment 1 involved five adult subjects who underwent binocular stimulation by the Accumap multifocal objective perimeter. The eyes were stimulated with a cortically scaled dartboard pattern consisting of isoluminant blue and yellow checks. These were arranged in three concentric rings extending to an eccentricity of 26 degrees in the visual field. The stimulus pattern was driven by binary sequences resulting in pseudorandom binary exchange of two opposite checkerboard patterns at each of the 32 sites in the visual field. The mVEP were recorded at two different rates of display of the pattern stimulus. In experiment 2, mVEP were tested on 10 normal subjects. Each of the 36 stimulation sites contained a checkerboard pattern of 20, 30, 42 or 56 checks/site, the stimulation pattern was displayed at the optimum rate found in experiment 1. The size of the checks was inversely proportional to the number of checks per site.

RESULTS

In experiment 1, the slow frame rate significantly increased the average amplitude throughout the field tested by 50 +/- 10.1% (P = 0.001). Latency was significantly shortened by 6.3% (P < 0.01). In experiment 2, the average amplitude peaked at 30 checks per segment; however, this was only calculated to be significantly different from the smallest check size (F(crit range 4,27) = 0.09 P < 0.05, anova, Tukey's T method). A similar difference was found in ring 1 (F(crit range 4,27) = 0.09, P < 0.05, anova, Tukey's T method). In ring 2, however, there was also a significant difference between 56 checks and 20, 30 and 42 (F(crit range 4,27) = 0.09, anova, P < 0.05). Altering the check sizes did not significantly affect the amplitudes in ring 3. The latencies were not significantly modified by altering check size at any eccentricity.

CONCLUSIONS

These findings suggest that slowing the stimulation rate and displaying 30 checks per stimulation segment optimizes the blue-yellow mVEP stimulus.

Effect of pupil size on multifocal pattern visual evoked potentials

The purpose of this study was to investigate the influence of pupil diameter on the amplitude and latency of multifocal visual evoked potentials (mfVEP). The multifocal objective perimeter (Accumap; Objectivision) was used to stimulate the visual field at 56 sites extending to 32 degrees using a pseudo-random pattern stimulus. The mfVEP were recorded using bipolar occipital electrodes, 7 min/eye. Ten normal subjects were recruited from the community and one eye was randomly selected for testing. The mfVEP were recorded at four different pupil diameters (2 mm, 4 mm, 6 mm, 8 mm), obtained by applying tropicamide (0.5%) or pilocarpine (2%) in different dilutions. Appropriate refractive correction was provided to overcome cycloplegia and achieve a visual acuity of 6/7.5 or better. Analysis revealed that at most pupil diameters the normalized full field amplitude did not show significant variation, except at the most miotic pupil diameter (2 mm), where the amplitude became reduced, based on 2-way anova and Tukey's T method. There was, however, significant correlation between latency and pupil area (correlation coefficient: upper field -0.63, lower field -0.76). The results suggest that even in the presence of mydriatics or miotics, the mfVEP test can be used to assess diseases that affect amplitude, provided near correction is used. The interpretation of latency, however, must be made with caution, as a borderline conduction defect with a dilated pupil may appear normal.

Multifocal objective perimetry in the detection of glaucomatous field loss

PURPOSE

To test the ability of a new type of multifocal objective perimetry to identify glaucomatous visual field defects.

METHODS

A multichannel visual evoked potential was recorded using the ObjectiVision Accumap perimeter. One hundred patients (age, 62.2 +/- 9.8 years, mean MD -6.5 +/- 4.17 dB) with open-angle glaucoma and confirmed glaucomatous visual field defects were tested and compared with the normal database of 100 normal subjects (age, 58.9 +/- 10.7 years). Both eyes were tested, but for determining sensitivity the eye with the lesser field defect was chosen if both qualified. The amplitude and intereye asymmetry coefficient for each zone of the field were calculated. A mean amplitude and multifocal objective perimetry severity index was calculated for each subject.

RESULTS

In 95 of 100 (95%) patients with glaucoma Humphrey field defects were correlated with visual evoked potential amplitude reductions identifying a cluster of three or more abnormal zones. In two of five remaining patients with glaucoma the defect was detected on the intereye asymmetry analysis. Topographic location was well correlated with Humphrey fields. Mean amplitude was significantly reduced in 86 of the glaucoma cases (86%). The glaucoma severity index was abnormal in 93 glaucoma cases and showed a correlation with Humphrey MD (r = 0.67 right eyes, 0.69 left eyes). In 37 glaucoma cases with no scotoma by definition in the fellow eye, 22 (59.4%) had an abnormal multifocal objective perimetry, whereas only eight had some other aspect of their Humphrey visual field flagged as abnormal.

CONCLUSIONS

Multifocal objective perimetry can assess the visual field and identify glaucomatous visual field defects. It may have the potential for identifying defects earlier than conventional perimetry.

Multifocal pattern electroretinogram does not demonstrate localised field defects in glaucoma

PURPOSE

To determine if a multifocal PERG could be recorded in normals, and to examine changes in the multifocal PERG in glaucoma patients. To compare the ability of multifocal PERG and multifocal VEP responses in the same individuals to identify localised field defects in glaucoma.

METHODS

Using the VERIS Scientific system multifocal PERGs were recorded from 19 sites of the visual field according to pseudo-random binary m-sequence. Twenty normals and 15 glaucoma subjects were tested. Multifocal pattern VEPs were also recorded in the glaucoma cases using a cortically scaled stimulus.

RESULTS

The second order kernel of the PERG shows a consistent signal. The overall PERG amplitude decreases with age in normals. In glaucoma the PERG amplitude was reduced across the field, but reductions did not correspond to the area of the scotoma. The VEP showed localised signal reductions in all 15 cases of glaucoma.

CONCLUSION

A multifocal PERG can be recorded in normals. However it did not reflect localised ganglion cell losses, whereas the multifocal pattern VEP recorded to a very similar stimulus in the same individual did show losses in the scotoma area.

Objective perimetry in glaucoma

PURPOSE

Objective perimetry in glaucoma is described using the multifocal pattern visually evoked potential (VEP). A multichannel recording technique was used to improve signal detection in healthy volunteers and assess its ability to detect glaucoma and early changes in patients with suspected glaucoma.

DESIGN

Prospective, case-control study.

PARTICIPANTS

Thirty healthy volunteers, 30 patients with suspected glaucoma, and 30 patients with glaucomatous visual field defects were tested.

METHOD

The VEP was recorded using cortically scaled, multifocal, pseudorandomly alternated pattern stimuli with the VERIS system (Electro-Diagnostic Imaging, Inc., San Francisco, CA). An array of four bipolar occipital electrodes provided four differently oriented channels for simultaneous recording. Signals were compared for different locations within the field up to 26 degrees of eccentricity. Healthy volunteers, patients with suspected glaucoma, and glaucoma patients with established visual field defects were tested, and results were compared with Humphrey visual fields (Humphrey Systems, Dublin, CA) performed on the same day. For reproducibility, five healthy volunteers were each tested on four separate days. The patients with suspected glaucoma and the established glaucoma patients were analyzed for intereye asymmetry of signals, and these data were compared with the asymmetry values of the healthy volunteers.

RESULTS

Multiple recording channels significantly enhanced the recording of signals from parts of the visual field not reliably sampled with a single channel technique in all healthy volunteers, particularly along the horizontal meridian (P: < 0.001). Signal amplitude did not decline with age in healthy volunteers. Recordings showed good reproducibility within individuals. In all 30 glaucoma patients, the Humphrey visual field defects were well demonstrated by the VEP, and topographic location was strongly correlated (r(s) = 0.79). Despite large interindividual variations in amplitude, scotomas were well demonstrated when compared with normal values. In the patients with suspected glaucoma, smaller changes in signal amplitude could be identified in parts of the field still normal on perimetry using intereye asymmetry analysis.

CONCLUSIONS

The multifocal, multichannel VEP can objectively detect glaucomatous visual field defects. The nasal step region can be more reliably tested using multiple channels. Asymmetry analysis has the potential to detect early defects. This technique represents a significant step toward the clinical application of objective perimetry in glaucoma.