Repeat reliability of the multifocal visual evoked potential in normal and glaucomatous eyes.

PURPOSE: To investigate the repeat reliability of the multifocal visual evoked potential (mfVEP). PATIENTS AND METHODS: Fifteen subjects with no known abnormalities of the visual system and 10 patients with glaucoma participated in the study. Monocular mfVEPs were recorded on two separate days, using a 60-sector, pattern-reversal dart board array. Within a single session, two 7-minute. recordings were obtained for each eye. The amplitude of each mfVEP response was obtained using a root mean square measure (RMS). An mfVEP ratio [10*log (RMS day 1 / RMS day 2)] provided a measure of the reproducibility of an individual response. The same calculations were performed for Run 1 compared with Run 2 within a day and Run 1 (Run 2) compared with Run 1 (Run 2) across days. RESULTS: For all 1800 mfVEP responses (60 sectors x 15 subjects x 2 eyes), the correlation between the amplitude on day 2 and the amplitude on day 1 was good (r = 0.85). The mean standard deviation (SD) of the 60 mfVEP ratios for the individual subjects was 1.63 dB for the 14-minute records (the combination of the two 7-minute recordings). On average for the 7-minute records, the mean SD across days was 1.77 dB while the mean SD within a day was 1.53 dB. The correlation within a day (r = 0.87) also was slightly larger than across days (r = 0.80). The mean SD decreased as the RMS amplitude increased. The patients' mean SD was 1.75 dB with r equal to 0.82. CONCLUSIONS: The repeat reliability of the mfVEP was good (approximately 1.6dB); in fact, it was better than that typically obtained with static automated perimetry (approximately 2.7dB). Repeat testing on separate days added surprisingly little to the variability seen with repeat testing within the same session.

The multifocal electroretinogram.

The multifocal electroretinogram (mfERG) technique allows local ERG responses to be recorded simultaneously from many regions of the retina. As in the case of the full-field ERG, the ganglion cells contribute relatively little to the response, which originates largely from the outer retina. The mfERG is particularly valuable in cases in which the fundus appears normal, and it is difficult to distinguish between diseases of the outer retina and diseases of the ganglion cells and/or optic nerve. The mfERG can also help to differentiate among outer retinal diseases, to follow the progression of retinal diseases, and, with the addition of the mfVEP, to differentiate between organic and nonorganic causes of visual loss. However, because the difficulties encountered in recording and analyzing mfERG responses are greater than those involved in full-field ERG testing, mfERG testing is best left to centers with an electrophysiologist familiar with the mfERG test. Although this technique is relatively new and standards are still being developed, centers capable of recording reliable mfERG responses can be found in hundreds of locations around the world.

Multifocal visual evoked potential responses in glaucoma patients with unilateral hemifield defects.

PURPOSE: To determine whether the multifocal visual evoked potential (mfVEP) technique can detect damage to the visual system in the unaffected hemifields of patients with glaucoma and unilateral hemifield defects. DESIGN: Experimental study. METHODS: Monocular mfVEPs and achromatic automated perimetry (AAP) were obtained in both eyes of 16 patients with open-angle glaucoma and unilateral hemifield defects. The mfVEPs were obtained using a pattern-reversal dartboard array with 60 sectors; the entire display was 44.5 degrees in diameter. For each pair of mfVEP responses an interocular ratio of root-mean-square amplitude was calculated. These values were compared with the mean values obtained from 30 control subjects. Probability plots for MfVEP were derived. A cluster analysis was used to determine whether an mfVEP hemifield was normal or abnormal. RESULTS: Three of 60 (5.0%) mfVEP hemifields from control subjects had significant mfVEP deficits based upon a cluster of abnormal points. Significant mfVEP deficits were detected in the affected AAP hemifield in 15 of 16 (93.8%) glaucoma patients and in 6 of 16 patients in hemifields with apparently normal AAP. The percentage of hemifields with abnormal mfVEPs, but normal AAP, was significantly higher for the glaucoma patients than for the controls (37.5% vs 5.0%, P <.001, chi square).In glaucomatous eyes with achromatic visual fields defects limited to one hemifield, the mfVEP technique can detect evidence of glaucomatous damage in the unaffected hemifield.

Visual field defects and multifocal visual evoked potentials: evidence of a linear relationship.

OBJECTIVE: To determine the relationship between spatially localized multifocal visual evoked potentials (mfVEPs) and Humphrey visual fields (HVFs) in patients with unilateral field defects. METHODS: Humphrey visual fields and mfVEPs were obtained from 20 patients with unilateral field losses due to either ischemic optic neuropathy or glaucoma. Monocular mfVEPs were obtained for each eye. The amplitude of the mfVEP responses was calculated using root-mean-square and signal-noise ratio measures. Estimates of the HVF loss in the same regions of the field used for the mfVEP were obtained by interpolating the 24-2 HVF data. RESULTS: Monocular mfVEP amplitude decreased with HVF loss, although small mfVEP signals were not uniquely associated with poor fields. On average, the monocular mfVEP was indistinguishable from noise for field losses between -5 and -10 dB, and good monocular mfVEP amplitudes were never associated with extensive visual field loss. The interocular ratio of the mfVEP amplitudes correlated well with the difference between the HVF values of the 2 eyes, and this correlation improved with increased signal-noise ratio. CONCLUSIONS: The monocular and interocular results were consistent with a linear relationship between the amplitude of the signal portion of the mfVEP response and linear HVF loss. One way to produce this relationship would be if both the signal in the mfVEP and linear HVF loss were linearly related to the percentage of local ganglion cells lost. The clinical limitations of the mfVEP technique can be understood by taking the signal-noise ratio, and the linear model proposed herein, into consideration.

Quantifying the benefits of additional channels of multifocal VEP recording.

For some individuals and for some locations, multifocal visual evoked potentials (mfVEP) may be too small or appear 'too noisy' to be reliably measured. By adding electrodes, especially electrodes placed lateral to the midline, and by recording with multiple channels, the amplitude of the signal can be increased in some field locations. However, the addition of electrodes involves certain costs; the set-up time is longer and the data analysis more time consuming and complex. The objective of this study was to assess the benefits of adding electrodes by quantifying these benefits using a signal-to-noise measure. In addition to the typical midline placement of electrodes, two electrodes were placed 1 cm above and 4 cm lateral to the inion on each side. This allowed for 3 channels of recording and 3 additional, derived channels. The mfVEPs were recorded with a 60 sector, pattern-reversing display presented to one eye. Two 7 min records were obtained from 14 individuals with no known visual problems. The two records were averaged and a signal-to-noise (SNR) measure was obtained for every response from all 6 channels. For each sector of the display and each subject, the benefits of additional electrodes were quantified by comparing the SNR from the traditional midline channel to the best SNR from amongst the 6 channels. The number of responses exceeding any given criterion SNR value was increased with the additional channels. For example, 79% of the responses for the typical midline channel exceeded a SNR of 0.6 (a false positive rate of about 2.5%) and this increased to 93% when the best SNR value was used. As expected, summing the mfVEP responses from contiguous sectors also increased the SNR values. Additional electrodes and multiple channels of recording substantially improve the quality of the mfVEP records and the SNR measure provides a useful metric for assessing these benefits.

A signal-to-noise analysis of multifocal VEP responses: an objective definition for poor records.

Sixty local VEP records, called the multifocal VEP (mfVEP), can be obtained over a wide retinal area. From subject-to-subject, from day-to-day, and from location-to-location, these records can vary in quality presenting a challenge to quantitative analyses. Here three procedures are described for specifying the quality of mfVEP recordings in terms of signal-to-noise ratios. Monocular mfVEPs were recorded in two, 7-min runs. A '2-run signal-to-noise ratio' (2rSNR) was obtained as [RMS(RunA+RunB)]/[RMS(RunA-RunB)]-1, where RMS is the root-mean-square amplitude of the response over the period from 45 to 150 ms (signal window). Two 'noise-window signal-to-noise ratios' were obtained with the same numerator as the 2rSNR but with the denominators based upon the RMS of a signal-free window from 325 to 430 ms. In one case, inSNR, the denominator was the RMS of the record's noise window and in the other case, mnSNR, the denominator was the mean of the RMS amplitudes of all the signal-free noise windows for the subject. The SNRs were related to false-positive rates (i.e., detecting a signal when none was present) by recording mfVEPs with some of the sectors of the display occluded. In particular, the outer three rings (36 sectors) of the display were occluded so that only noise was recorded; false-positive rates for different values of SNR were calculated. The 2rSNR had the highest false-positive rate largely due to alpha in the records of some subjects. The mnSNR had a lower false-positive rate than did the inSNR because there was little correlation between the RMS of the noise in the signal-free window and the RMS of the noise within the signal window. Use of the mnSNR is recommended over the 2rSNR, especially where alpha contamination can not be eliminated. Ways to improve the SNR of the records are discussed.