Retest variability and diurnal effects in the pattern electroretinogram

Effect of eccentric fixation on the steady-state pattern electroretinogram

PURPOSE

The steady-state pattern electroretinogram (ssPERG) is used to assess retinal ganglion cell function in a variety of research contexts and diagnostic applications. In certain groups of patients or study participants, stable central fixation of the stimulus is not guaranteed. The present study aimed at assessing the effects of misfixation on the ssPERG response to checkerboard reversal stimuli.

METHODS

Using two check sizes (0.8° and 15°), we compared ssPERG responses for several amounts of fixation deviation, ranging from 0° to 19° horizontally and from 0° to 14° diagonally. The stimulus area extended to 15° eccentricity, stimulus reversal rate was 15/s.

RESULTS

Up to around 7° eccentricity, there was no sizable effect of fixation deviation under most conditions. Effects were somewhat larger for nasal than for temporal deviation, in particular for small checks. Diagonal deviation was associated with a response to luminance onset/offset at 7.5 Hz (subharmonic of the reversal rate), most prominently when the interior of a large check was fixated.

CONCLUSION

Generally, moderate inaccuracies of fixation do not have a sizable effect on ssPERG amplitude. However, with large checks, the luminance response has to be considered.

Prognostication in Stargardt Disease Using Fundus Autofluorescence: Improving Patient Care

PURPOSE

To explore fundus autofluorescence (FAF) imaging as an alternative to electroretinography as a noninvasive, quick, and readily interpretable method to predict disease progression in Stargardt disease (STGD).

DESIGN

Retrospective case series of patients who attended Moorfields Eye Hospital (London, United Kingdom).

PARTICIPANTS

Patients with STGD who met the following criteria were included: (1) biallelic disease-causing variants in ABCA4, (2) electroretinography testing performed in house with an unequivocal electroretinography group classification, and (3) ultrawidefield (UWF) FAF imaging performed up to 2 years before or after the electroretinography.

METHODS

Patients were divided into 3 electroretinography groups based on retinal function and 3 FAF groups according to the extent of hypoautofluorescence and retinal background appearance. Fundus autofluorescence images of 30° and 55° were reviewed subsequently.

MAIN OUTCOME MEASURES

Electroretinography and FAF concordance and its association with baseline visual acuity (VA) and genetics.

RESULTS

Two hundred thirty-four patients were included in the cohort. One hundred seventy patients (73%) were in electroretinography and FAF groups of the same severity, 33 (14%) were in a milder FAF than electroretinography group, and 31 (13%) were in a more severe FAF than electroretinography group. Children < 10 years of age (n = 23) showed the lowest electroretinography and FAF concordance at 57% (9 of the 10 with discordant electroretinography and FAF showed milder FAF than electroretinography), and adults with adult onset showed the highest (80%). In 97% and 98% of patients, 30° and 55° FAF imaging, respectively, matched with the group defined by UWF FAF.

CONCLUSIONS

We demonstrated that FAF imaging is an effective method to determine the extent of retinal involvement and thereby inform prognostication by comparing FAF with the current gold standard of electroretinography. In 80% of patients in our large molecularly proven cohort, we were able to predict if the disease was confined to the macula or also affected the peripheral retina. Children assessed at a young age, with at least 1 null variant, early disease onset, poor initial VA, or a combination thereof may have wider retinal involvement than predicted by FAF alone, may progress to a more severe FAF phenotype over time, or both.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Monitoring for glaucoma progression with SAP, electroretinography (PERG and PhNR) and OCT

Purpose

To investigate the value of pattern electroretinography (PERG) and photopic negative response (PhNR) in monitoring glaucoma compared to standard clinical tests (standard automated perimetry (SAP) and clinical optic disc assessment) and structural measurements using spectral-domain OCT.

Methods

A prospective study included 32 subjects (32 eyes) with ocular hypertension, suspect or early glaucoma monitored for progression with clinical examination, SAP, PERG, PhNR and OCT for at least 4 years. Progression was defined clinically by the documented change of the optic disc and/or significant visual field progression (EyeSuite™ trend analysis). One eye per patient was included in the analysis.

Results

During the follow-up, 13 eyes (40.6%) showed progression, whereas 19 remained stable. In the progressing group, all parameters showed significant worsening over time, except for the PhNR, whereas in the stable group only the OCT parameters showed a significant decrease at the last visit. The trend of change over time using linear regression was steepest for the OCT parameters. At baseline, only the ganglion cell complex (GCC) and peripapillary retinal nerve fibre (pRNFL) thicknesses significantly discriminated between the stable and progressing eyes with the area under the ROC curve of 0.72 and 0.71, respectively. The inter-session variability for the first two visits in the stable group was lower for OCT (% limits of agreement within ± 17.4% of the mean for pRNFL and ± 3.6% for the GCC thicknesses) than for ERG measures (within ± 35.9% of the mean for PERG N95 and ± 59.9% for PhNR). The coefficient of variation for repeated measurements in the stable group was 11.9% for PERG N95 and 23.6% for the PhNR, while it was considerably lower for all OCT measures (5.6% for pRNFL and 1.7% for GCC thicknesses).

Conclusions

Although PERG and PhNR are sensitive for early detection of glaucomatous damage, they have limited usefulness in monitoring glaucoma progression in clinical practice, mainly due to high inter-session variability. On the contrary, OCT measures show low inter-session variability and might have a predicting value for early discrimination of progressing cases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10633-021-09854-8.

Normalization of increased retinal background noise after ADHD treatment: A neuronal correlate

Inattention, distractibility, and problems inhibiting irrelevant information impose a large disease burden in attention deficit hyperactivity disorder (ADHD). Problems with cognitive function are found in many major psychiatric disorders, and our understanding of ADHD might add to our knowledge of other neuropsychiatric disorders. Despite the high impact of ADHD, the pathophysiology and the mechanism of treatment action remains poorly understood. Increasing evidence suggests that elevated neuronal and retinal background noise plays a prominent role in ADHD. However, the effect of treatment (e.g., methylphenidate) on noise remains unclear. For this study, retinal background noise was assessed with the pattern-electroretinogram (PERG) in 20 drug-naïve adults with ADHD before and after treatment with methylphenidate and in 21 control subjects. Background noise was identified using the Fourier magnitude at frequencies adjacent to the stimulus-response frequency of 12.5 Hz. At baseline, we found an elevated retinal background noise in ADHD patients (Mdn = 0.079 μV) compared to controls (Mdn = 0.062 μV; z = -2.79, p = 0.016, r = -0.44). The noise in the ADHD group decreased significantly at follow-up after treatment with methylphenidate (Mdn = 0.069 μV, z = -2.39, p = 0.035, r = -0.39) while there was no change in the control group. PERG-based retinal noise is increased in adult ADHD and normalizes along with clinical symptoms following treatment with methylphenidate. The retinal noise level might be a promising marker of ADHD in clinical and basic research and illustrates the biological match with nonhuman ADHD models.

Assessment of macular function, structure and predictive value of pattern electroretinogram parameters for postoperative visual acuity in patients with idiopathic epimacular membrane

Purpose

To evaluate macular function and structure before and after epimacular membrane surgery and to estimate the usefulness of pattern ERG test parameters in predicting the postoperative visual acuity.

Methods

We evaluated 32 eyes of 32 patients (mean age 70.8 ± 6.7 years) before and 12 months after successful 25G pars plana vitrectomy with epimacular membrane removal and internal limiting membrane peeling. Distance best-corrected visual acuity (DBCVA—logMAR), foveal thickness (optical coherence tomography—OCT) and macular function [pattern electroretinogram—PERG (ISCEV standard): amplitudes (A) of P50- and N95-waves, implicit time (IT) of P50-wave] were assessed. To estimate the differences between the mean values of considered characteristics, the t test or Wilcoxon matched pair test was used. Correlation between preoperative data of PERG and preoperative and final DBCVA were investigated using Pearson correlation analysis. A receiver operating characteristic curve was constructed to obtain a cutoff value allowing prediction of visual prognosis. We tried to obtain the P50 and N95 amplitudes cutoff value in prediction of good visual outcome (DBCVA of 0.3 or less).

Results

Twelve months after surgery, mean of DBCVA significantly increased in comparison with preoperative value (0.31 ± 0.12 vs. 0.6 ± 0.15; p < 0.001) and 23/32 eyes (72 %) achieved visual improvement of two and more Snellen lines. In OCT test, the significant reduction in foveal thickness mean (313.34 ± 47.01 vs. 509.03 ± 93.88 µm; p < 0.001) was obtained. In PERG test, the significant increase in the mean amplitudes of P50- (AP50) and N95 (AN95)-waves as well as significant decrease in the mean implicit time (IT) of P50-wave were achieved (AP50: 3.41 ± 1.48 vs. 2.38 ± 1.23 µV; p < 0.001; AN95: 5.46 ± 1.72 vs. 3.75 ± 1.48 µV; p < 0.001; IT P50: 55.00 ± 3.60 vs. 56.75 ± 5.78 ms; p < 0.001). Twelve months postoperatively, DBCVA was significantly correlated with preoperative IT P50 (r = 0.39; p = 0.027), AP50 (r = −0.68; p < 0.001) and AN95 (r = −0.73; p < 0.001).

Conclusion

Removal of idiopathic epimacular membranes with internal limiting membrane peeling not only provided increase in visual acuity and reduction in foveal thickness but also caused improvement of innermost retinal layer function in macular region. Pattern ERG test might be a valuable tool in predicting the postoperative visual acuity.

Effect of antidepressive therapy on retinal contrast processing in depressive disorder

BACKGROUND

Recently, we reported a reduced retinal contrast gain in unmedicated and medicated patients with major depression.

AIMS

To analyse whether the contrast gain normalises after successful antidepressive therapy by recording the pattern electroretinogram (PERG) in healthy controls and patients with depression before and after antidepressive therapy.

METHOD

Fourteen patients diagnosed with major depression were repeatedly scanned and the results compared with that from 40 matched controls.

RESULTS

The retinal contrast gain was lower at baseline in patients with depression, was normalised with remission and correlated with the severity of depression. Patients who did not achieve remission retained significantly lower contrast gain at follow-up.

CONCLUSIONS

The study provides evidence for a state-dependent modulation of retinal contrast gain in patients with major depression. Reduced contrast gain normalised after therapy. A PERG-based contrast gain could serve as a state marker of depression.

Simultaneous retinal and cortical visually evoked electrophysiological responses in between migraine attacks

PURPOSE

People with migraine often report aversion to flickering lights and show abnormal results on behavioural tasks that require the processing of temporal visual information. Studies have reported that the cortically evoked electrophysiological response to a flickering visual stimulus is abnormal; however, none have considered whether there is an underlying pre-cortical abnormality. In this cross-sectional study, we consider whether people with migraine have retinal and cortical electrophysiological abnormalities to flickering stimuli.

METHODS

Monocular transient (1 Hz) and steady-state (8.3 Hz) pattern reversal electroretinograms (PERGs) and pattern visual evoked responses (PVERs) were measured simultaneously in 45 people with migraine (26 without aura, 19 with aura) and 30 non-headache controls at a time between migraine attacks.

RESULTS

PERG amplitude and timing did not differ significantly between groups. Transient PVER amplitude was significantly reduced (28%) in the migraine with aura group compared to the controls F(2,72) = 3.6, p = 0.03). Both migraine groups showed significant reductions (32%, 39%) in steady-state PVER amplitude relative to controls (F(2,70) = 4.3, p = 0.02).

CONCLUSIONS

This study finds normal retinal processing of flickering stimuli in the presence of abnormal cortical function between migraine attacks.

Pattern electroretinogram association with spectral domain-OCT structural measurements in glaucoma

PURPOSE

To describe the association between pattern electroretinogram (PERG) amplitude and spectral domain-optical coherence tomography (SD-OCT) macular thickness, retinal nerve fibre layer (RNFL) thickness and optic disc topography measurements.

SUBJECTS AND METHODS

Both eyes (n = 132) of 66 glaucoma patients (mean age = 67.9 years) enrolled in the University of California, San Diego, CA, USA, Diagnostic Innovations in Glaucoma Study (DIGS) were included. Eyes were tested with PERG (Glaid PERGLA, Lace Elettronica, Pisa, Italy), RTVue SD-OCT (Optovue Inc., Fremont, CA, USA) GCC, and NHM4 protocols on the same day. Of the 66 enrolled patients, 43 had glaucoma defined by repeated abnormal standard automated perimetry (SAP) results in at least one eye and 23 were glaucoma suspects defined by a glaucomatous-appearing optic disc by physicians' examination in at least one eye and normal SAP results in both eyes. Associations (R(2)) were determined between PERG amplitude (μV) and SD-OCT macular ganglion cell complex (GCC) thickness (μm), macular thickness (μm), macular outer retinal thickness (macular thickness minus GCC thickness) (μm), RNFL thickness (μm), neuroretinal rim area (mm(2)), and rim volume (mm(3)).

RESULTS

PERG amplitude was significantly associated with GCC thickness (R(2) = 0.179, P < 0.001), RNFL thickness (R(2) = 0.174, P < 0.001), and macular thickness (R(2) = 0.095, P<0.001). R(2) associations with other parameters were not significant (all P > 0.624). Significant associations remained for GCC and average RNFL thickness when age and intraocular pressure at the time of testing were included in multivariate models (both P ≤ 0.030).

CONCLUSIONS

PERG amplitude is significantly (but weakly) associated with macular GCC thickness, RNFL thickness, and macular thickness. The lack of association between PERG amplitude and macular outer retinal thickness supports previous results, possibly suggesting that that the PERG is driven primarily by retinal ganglion cell (inner retinal) responses.

Repeatability of pattern electroretinogram measurements using a new paradigm optimized for glaucoma detection

PURPOSE

To determine the within-trial and between-trial repeatability of pattern electroretinogram (PERG) measurements in healthy and patient eyes, using a new clinical instrument, the PERGLA.

STUDY DESIGN

In all, 70 eyes of 35 healthy individuals (intraocular pressure <22 mm Hg, healthy optic disc by stereophotograph assessment, standard visual fields within normal limits) and 90 eyes of 45 clinic patients (ocular hypertensive, glaucomatous optic neuropathy by stereophotograph assessment and/or repeatable abnormal visual fields) enrolled in the University of California, San Diego Diagnostic Innovations in Glaucoma Study (DIGS) were evaluated. Average mean deviation of patient eyes on standard automated perimetry was -1.81 dB (SD=2.61).

METHODS

The PERG was recorded using the PERGLA paradigm from both eyes simultaneously twice (ie, 2 trials) by a single operator with electrodes being removed and reattached between recordings. Repeatability of PERG amplitude (microV) and phase (pi rad) between 2 runs within a single trial (within-trial condition) was compared with repeatability between 2 trials (ie, after electrode replacement, between-trial condition) by calculating the coefficients of variability (CVs) and the intraclass correlation coefficients (ICCs) and displaying Bland-Altman plots.

RESULTS

For healthy eyes, amplitude CVs (SD) were 11.5% (11.5) and 9.9% (0.79) for within-trial and between-trial conditions, respectively. ICCs were 0.91 and 0.85. Phase CVs were 1.3% (1.5) (within-trials) and 1.5% (1.4) (between-trials) and ICCs were 0.85 and 0.88. For patient eyes, amplitude CVs (SD) were 12.2% (10.1) and 11.2% (7.5) for within-trial and between-trial conditions, respectively. ICCs were 0.92 and 0.89. Phase CVs were 2.2% (2.2) (within-trials) and 2.4% (2.2) (between-trials) and ICCs were 0.82 and 0.83. Bland-Altman plots indicated good agreement between the repeated recordings and were similar within-trials and between-trials for healthy and patient eyes.

CONCLUSIONS

Repeatability of PERGLA recordings is good and is similar within-trials and between-trials for both healthy and patient eyes suggesting this technique is promising for monitoring change over time.

Update on the pattern electroretinogram in glaucoma

PURPOSE

To review the efficacy of the pattern electroretinogram (PERG) in early diagnosis of glaucoma.

METHODS

Stimulation parameters of check size and temporal frequency are considered. Analyses of various peaks (P50, N95, the N95/P50) and Fourier steady-state are considered. The relation to visual field defects is explored.

RESULTS

The PERG is markedly alterated in glaucoma. It shows amplitude reductions in (still) normal areas of the visual field. Optical imaging on the retina needs to be optimal. Higher temporal frequency (>10 reversals/s) improves the sensitivity to detect glaucoma compared with transient stimulation. The ratio between the amplitudes to 0.8 degrees checks and to 16 degrees checks, "PERG ratio," exploits a check size-specific reduction in early glaucoma and reduces variability. Longitudinal studies suggest that the PERG can indicate incipient glaucoma damage before evidence from the visual field.

CONCLUSIONS

The PERG is a demanding electrophysiological technique that can serve as a sensitive biomarker for retinal ganglion cell function. With appropriate paradigms, PERG assists in identifying those patients with elevated interocular pressure in whom glaucoma damage is incipient before visual field changes occur.

Reproducibility of pattern electroretinogram in glaucoma patients with a range of severity of disease with the new glaucoma paradigm

PURPOSE

To determine the reproducibility of the pattern electroretinogram with the new Pattern Electroretinogram for Glaucoma (PERGLA) recording paradigm in glaucoma patients with a range of severity.

DESIGN

Experimental study.

PARTICIPANTS

Fifty-three glaucoma patients were recruited for the study (mean age +/- standard deviation [SD], 69+/-11 years). Their mean deviation (MD) global indices on static automatic perimetry ranged from 2.16 to -31.36 decibels (mean MD, -9.05).

INTERVENTION

All patients had pattern electroretinogram recordings done 5 times by the same operator, on 5 different days with the standardized PERGLA paradigm.

MAIN OUTCOME MEASURES

Pattern electroretinogram amplitude (microvolts), phase (pi radians), response variability (coefficient of variation [CV] = SD/mean x 100) of amplitude and phase of 2 partial averages that build up the pattern electroretinogram waveform, interocular asymmetry in amplitude and phase (in terms of the CV generated by the pattern electroretinogram software), signal-to-noise (S/N) ratio, SDs, CV, and intraclass correlation coefficient (ICC). All analyses were done on one eye of each subject, except when interocular asymmetry was studied.

RESULTS

The CVs of intrasession variabilities in amplitude and phase were 12.08% and 2.20%, respectively, and those of intersession variabilities were 20.82% and 4.17%. The pattern electroretinogram produced intersession ICCs in amplitude and phase of 0.791 and 0.765, respectively. These ICCs were significantly higher than the ICCs for pattern electroretinogram interocular asymmetry in amplitude and phase (0.659 [P<0.05] and 0.571 [P<0.05], respectively). On average, the pattern electroretinogram S/N ratio in glaucomatous patients was about 5:1.

CONCLUSIONS

The reproducibility of PERGLA in glaucomatous patients is sufficiently good for it to be considered a useful complementary clinical tool. Being more reproducible, direct measures of amplitude and phase should be more useful in monitoring progression than interocular asymmetry comparisons.

Intra-sessional and inter-sessional variability of multifocal electroretinogram

The aim of this study was to investigate the intra-sessional and inter-sessional variability of the multifocal electroretinogram (mfERG). Right eyes of 20 healthy adult subjects with uncorrected visual acuity of 1.0 were studied. mfERG recordings were obtained in two sessions by using jet corneal electrodes. The responses were averaged over five retinal regions, the central hexagon (CH: 0.0-2.3 degrees ) and four concentric rings [ring 1 (R1: 2.3-7.4 degrees ), ring 2 (R2: 7.4-12.0 degrees ), ring 3 (R3: 12.0-19.4 degrees ), and ring 4 (R4: 19.4-30.0 degrees )]. In the first session, recordings were repeated while the electrodes stayed in place (the first and the second recordings). Second sessions (third recordings) were performed 1 week later. Individual coefficient of variation (CV; standard deviation/mean) for P1 and N1 amplitudes and implicit times were calculated for the first to second and first to third recordings to assess intra-sessional and inter-sessional variability, respectively, and were evaluated by Wilcoxon-paired samples test. Regarding implicit time and amplitude of mfERG recordings, there was no significant difference between the first to second and first to third recordings (P > 0.05, Bonferroni-corrected Wilcoxon test). CV values significantly decreased with eccentricity in both intra-sessional and inter-sessional comparisons (P < 0.05, Wilcoxon test). However, partial ring averaging showed no significant difference (P > 0.05) between CV values for single hexagons in each ring. Intra-sessional CV values were significantly lower than corresponding inter-sessional values (P < 0.05, Wilcoxon test). Our results support the importance of electrode replacement on variability. The increased number of hexagons that are averaged is the main factor that explains the lower variability in the more peripheral rings.

A new pattern electroretinogram paradigm evaluated in terms of user friendliness and agreement with perimetry

PURPOSE

To assess ease of use of the pattern electroretinogram optimized for glaucoma screening (PERGLA) paradigm by a novice operator; to study test-retest variability of the PERGLA parameters; and to compare results from the PERGLA to those from perimetry.

DESIGN

Cohort study.

PARTICIPANTS

Twelve healthy control subjects and 16 patients with moderate to advanced glaucoma in at least 1 eye.

METHODS

Pattern electroretinograms were recorded simultaneously from both eyes using a commercially available testing station. Each participant underwent PERGLA procedures in 2 sessions. One eye of each subject was tested on contrast sensitivity perimetry (CSP) in which a 0.4 cycles/degree Gabor patch served as a stimulus. Central visual fields results from conventional automated perimetry (CAP) were obtained from patients' records. Bland-Altman analysis was performed on PERGLA results to assess normal test-retest variability. Differences from mean normal (in decibels [dB]) were compared for PERGLA versus CSP and CAP.

MAIN OUTCOME MEASURES

Pattern electroretinogram amplitude, noise, phase, and test-retest variability (coefficient of variation); contrast sensitivity from CSP; perimetric sensitivity from CAP; and differences from mean normal for PERGLA, CSP, and CAP.

RESULTS

The mean log amplitude (0.08+/-0.12 log muV) and the mean phase (1.92+/-0.07 pi rad) for the control group were consistent with published PERGLA norms, as was test-retest variability for both amplitude (coefficient of variation [CV] = 8.2+/-7.0%) and phase (CV = 1.1+/-0.9%). The mean signal-to-noise ratio (8.7+/-4.5) was lower than published norms. The test-retest variability increased as PERGLA log amplitude decreased (R2>0.12, P<0.05). On average, differences from mean normal were similar for PERGLA versus CSP and for PERGLA versus CAP (mean differences<0.5 dB) with 95% confidence intervals near +/-4 dB in both comparisons.

CONCLUSIONS

A novice operator successfully replicated published PERGLA norms in a young control group for amplitude, phase, and repeatability. Higher test-retest variability was found in eyes with smaller signals. On average, PERGLA results were in reasonable agreement with those from perimetry, although there existed large individual differences which may limit the usage of PERGLA in screening or in following progression of glaucoma.

Acute effects of cigarette smoking on pattern electroretinogram

In this study, acute effects of cigarette smoking on the pattern electroretinogram (PERG) were investigated. First, variability of the PERG was studied in a group of young male smokers (26 right eyes of 26 subjects). Then PERGs were investigated in a group of habitual smokers (17 right eyes of 17 subjects) in separate real smoking and sham smoking sessions. On each session PERGs were recorded pre-smoking (PS), immediately after smoking (IAS) and 5 min after smoking (5th) conditions. Real smoking significantly increased P50 amplitudes and decreased N95 latencies. Regarding P50 amplitudes in the real smoking sessions, the differences were significant between PS and IAS (PS: 3.3 +/- 0.5 muV, IAS: 3.7 +/- 0.7microV, P = 0.015) and between PS-5th (PS: 3.3 +/- 0.5microV, 5th: 4.1 +/- 0.9microV, P = 0.039). There was significant difference (P = 0.024) between N95 latencies of PS (98.5 +/- 6.9 ms) and IAS (94.7 +/- 5.1 ms) in the real smoking sessions. No statistically significant difference was observed in sham smoking sessions. Our results indicated, for the first time, that cigarette smoking may influence PERG amplitude and latency significantly in habitual smokers.

Retinal ganglion cell activity from the multifocal electroretinogram in pig: optic nerve section, anaesthesia and intravitreal tetrodotoxin

Non-invasive recordings of the retinal activity have an important role to play in the diagnosis of retinal pathologies. The detection of diseases that involve retinal ganglion cells (RGCs), such as optic atrophy and glaucoma, may be improved by isolating the RGC contribution from the multifocal electroretinogram (mfERG). In this study, mfERGs were performed on 20 pigs, 1-6 weeks following unilateral retrobulbar optic nerve section (ONS). The stimuli were 103 non-scaled high-contrast hexagons from which summed and individual mfERG responses were obtained in experimental and control fellow eyes under conditions of ketamine (n = 11) or isoflurane anaesthesia (n = 9). The effect of intravitreal injection of tetrodotoxin (TTX; n = 6) was also investigated. The summed mfERG responses showed a first positive peak (P1) with a short latency (21 ms) followed by two smaller peaks (P2 and P3) of longer latency (46 and 65 ms, respectively). While P2 and P3 amplitude were highly correlated with the time post-optic nerve section (ONS) (P2: r(2) = 0.669; P = 0.007; P3: r(2) = 0.651; P = 0.005), P1 was not (r(2) = 0.193; P = 0.38). P1 and P2 showed no implicit time variation as a function of retinal location, while P3 implicit time varied along the axis of the visual streak, generating a naso-temporal asymmetry. However, the P3 implicit time did not vary consistently with distance away from the optic nerve head. Intravitreal injections of TTX reduced P2 and P3 in the control eyes, consistent with the effect of ONS, and also induced a series of regular oscillations lasting up to 200 ms post stimulus. Under isoflurane anaesthesia, all components of the mfERG ifn experimental and control eyes were, at all time points post-ONS, of similar amplitude and without naso-temporal asymmetry, suggesting a reduced participation of RGCs under these anaesthesic conditions. These data clearly demonstrate that it is possible to isolate the RGC contribution from non-invasive multifocal electroretinography.

The Diurnal Variation of Corneal Topography and Aberrations

PURPOSE

To investigate if diurnal variation occurs in corneal topography.

METHODS

Seventeen young subjects had corneal topography measured 3 times a day (around 9 AM, 1 PM, and 5 PM) on 3 days of the week (Monday, Tuesday, and Friday). Corneal tangential and refractive power were analyzed. To investigate diurnal variation in corneal aberrations, the corneal height data were converted into corneal wavefront error and expressed as Zernike wavefront coefficients.

RESULTS

A significant diurnal change in corneal tangential power maps was found in 15 of the 17 subjects. This change typically consisted of a horizontal band of distortion in the superior and, to a lesser extent, inferior cornea, increasing throughout the day. Corneal refractive power showed small but significant diurnal changes occurring in the best fit sphere. Corneal wavefront error analysis revealed significant diurnal changes occurring in astigmatism 90/180 degrees and astigmatism 45/135 degrees. Two higher-order aberrations, primary vertical coma and trefoil along 30 degrees, also showed significant diurnal change. The combination of these 2 aberrations represents a "wavelike" distortion increasing throughout the day and returning to baseline the next morning.

CONCLUSION

We have shown that significant diurnal change occurs in certain corneal aberrations. The changes appear to be related to forces from the eyelids on the anterior cornea. These findings may have important implications for customized refractive corrections and refractive error development.

Inter-ocular and inter-session reliability of the electroretinogram photopic negative response (PhNR) in non-human primates

PURPOSE

To assess the inter-ocular and inter-session reliability for a range of parameters derived from the photopic electroretinogram (ERG) in a group of normal non-human primates.

METHODS

Inter-ocular differences for photopic ERGs were assessed in a group of normal anesthetized adult rhesus monkeys (Macaca mulatta, n=29); inter-session reliability was assessed for 23 eyes of 23 animals tested 3 months later. Signals were acquired using Burian-Allen contact lens electrodes, whereby the contralateral cornea served as a reference. Photopic ERGs were elicited using red Ganzfeld flashes (-0.5-0.67 log photopic cd.sm(-2)) on a rod suppressing blue-background (30 scotopic cdm(-2)). Measurement reliability was established for a-wave, b-wave, photopic negative response (PhNR) and oscillatory potential (OP) amplitudes, as well as for their implicit times, by calculation of the 95% limits-of-agreement (LOA) and the coefficient-of-variation (COV) for each parameter.

RESULTS

OP and a-wave amplitudes increased with intensity up to 0.67 log photopic cd.sm(-2), following a typical saturating function, whereas b-wave and PhNR amplitudes both declined above 0.42 log photopic cd.sm(-2). Inter-session variability was greater than inter-ocular variability. The inter-session COVs for PhNR amplitude (10-20%) were similar to the other photopic ERG components (a-wave: 12-17%, b-wave: 12-17%, OPs: 13-19%). Inter-session LOAs were also similar across components, but on average, were smallest for responses to moderate intensities (0.0-0.42 log photopic cd.sm(-2)).

CONCLUSION

In non-human primates, the 95% LOA for inter-session measurements of the photopic ERG a-wave, b-wave, OPs and PhNR are all similar. Inner-retinal damage may best be measured using the PhNR amplitude for moderately bright stimulus intensities. B-wave and PhNR amplitudes for brighter flashes are smaller and more variable. The ratio of PhNR:b-wave amplitudes manifests smaller variability and may therefore be useful for detection of selective PhNR loss.

Baseline characteristics of the transient pattern electroretinogram in non-human primates: inter-ocular and inter-session variability

This study assessed the inter-ocular and inter-session variability of the transient pattern electroretinogram (PERG) in a group of non-human primates. The transient PERG was measured both eyes of 29 non-human primates, and again after three months in 23 eyes of 23 of these animals. Signals were elicited using a contrast (90%, 75 cdm(-2)) reversing (5 reversals sec(-1)) checkerboard pattern (0.56 cpd). PERGs were also measured for stimuli of varied spatial frequency (n=8, 0.07-2.22 cpd), contrast (n=4, 20-100%), mean luminance (n=4, 4.7-75 cdm(-2)) and defocus (n=5, +1, +2, +3 diopters). The inter-eye and inter-session limits-of-agreement (LOA; 95%) were determined for each PERG parameter. Variability was also compared with previous studies using the coefficient-of-variability (COV). Pharmacological blockade of the inner retinal contributions to the PERG measured under these conditions was conducted in one animal using intravitreal injection of tetrodotoxin (approximately 6 microM) and N-methyl-D-aspartic acid (approximately 6 microM). The N95 component of the primate transient PERG showed spatial tuning, with a peak between 0.14 and 0.28cpd. This spatial tuning was not as apparent for the P50 component. A linear relationship between P50 and N95 amplitude was found with contrast and mean luminance. Both components were attenuated with the introduction of +2 diopters or more of defocus. The inter-session COV for the P50 and N95 components were 23.8 and 19.2%, respectively, while the LOA were 58 and 46%, respectively. The N95:P50 ratio had smaller inter-session variability, was robust to changes in contrast, mean luminance and defocus, and was effective for characterization of inner-retinal dysfunction after pharmacologic block.

Electrophysiological approaches for early detection of glaucoma

PURPOSE

While elevated intraocular pressure (IOP) is a major risk factor for glaucoma, only about 1% of patients with 25 mmHg develop the condition each year. Since a sizeable proportion of the ganglion cells are already lost when the visual field losses are apparent, the aim is to identify patients with elevated IOP in whom glaucoma damage is incipient before visual field changes occur.

METHODS

This report concerns early diagnosis of glaucoma with electrophysiological techniques, rather than with monitoring the disease using various available psychophysical and morphological methods. Visual electrophysiology offers a wide range of tools to assess function layer-by-layer along the visual pathway. Their clinical value for early detection of glaucoma will be discussed. The pattern electroretinogram (PERG), a direct functional indicator of retinal ganglion cell function, is markedly affected by glaucoma, and in longitudinal studies the PERG correctly indicated eyes at risk before manifest glaucoma occurred.

CONCLUSIONS

Consequently, this report will concentrate on the PERG. Less proven, but promising measures like the "photopic negative response", the motion visually evoked potential (VEP) and the multifocal VEP will also be touched upon.

The effect of time of day and repeat reliability on the fast flicker multifocal ERG

PURPOSE

To investigate the possible influence of different times of the day on multifocal ERG recordings in normals.

METHODS

10 healthy volunteers underwent multifocal ERG recordings (VERIS 3.1.1.) 3 times a day, at 10:30 am, 1:30 pm and 4:30 pm using a fast flicker stimulus of 103 hexagons. Additional night-time recordings were performed in 2 subjects. The first-order and dominant part of the second order response component were analyzed.

RESULTS

Neither peak latencies nor peak-to-peak amplitudes showed significant changes during the day (p >0.05). Comparison between night and day time recordings did also not reveal an influence of a circadian rhythm on the MF-ERG.

CONCLUSIONS

No significant influence of the time of the day suggestive of a circadian rhythm could be observed. Light adaptation due to preadaptation and the flicker sequence used could be a possible cause for these results.

Little correlation of the pattern electroretinogram (PERG) and visual field measures in early glaucoma

Pattern-electroretinograms (PERG) to checkerboard reversal at 16/s. 0.8 degrees and 15 degrees check size and visual fields (Octopus G1) were retrospectively analyzed in 40 eyes of 30 patients with early glaucoma. The mean visual field defect was calculated separately for the central 26 degrees x 34 degrees covered by the PERG stimulus (MDc) and the more peripheral area (MDp) surrounding the stimulus. Deeper field loss was correlated with a reduced pattern electroretinogram amplitude (p < 0.01 for both MDp and MDc), indicating that the pattern electroretinogram deteriorates as glaucoma advances. If the analysis was confined to those 18 eyes (16 patients) that had no field defect within the area covered by the PERG stimulus (normal MDc but abnormal MDp), 13 of these had an abnormal PERG amplitude (p < 0.001). The results suggest that the PERG can reveal impairment of ganglion cell function that is not detected by conventional perimetry.