Pattern-reversal electroretinograms in squint amblyopia, artificial anisometropia and simulated eccentric fixation

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.

Impact of Amblyopia on the Central Nervous System

Amblyopia is a common perceptual disorder resulting from abnormal visual input during development. The clinical presentation and visual deficits associated with amblyopia are well characterized. Less is known however, about amblyopia's impact on the central nervous system (CNS). While early insights into the neuropathophysiology of amblyopia have been based on findings from animal models and postmortem human studies, recent advances in noninvasive magnetic resonance imaging (MRI) techniques have enabled the study of amblyopia's effects in vivo. We review recent retinal and neuroimaging research documenting amblyopia's structural and functional impact on the CNS. Clinical imaging provides some evidence for retinal and optic nerve abnormalities in amblyopic eyes, although the overall picture remains inconclusive. Neuroimaging studies report clearer changes in both structure and function of the visual pathways. In the optic nerves, optic tracts, and optic radiations of individuals with amblyopia, white-matter integrity is decreased. In the lateral geniculate nuclei, gray matter volume is decreased and neural activity is reduced. Reduced responses are also seen in the amblyopic primary visual cortex and extrastriate areas. Overall, amblyopia impacts structure and function at multiple sites along the visual processing hierarchy. Moreover, there is some evidence that amblyopia's impact on the CNS depends on its etiology, with different patterns of results for strabismic and anisometropic amblyopia. To clarify the impact of amblyopia on the CNS, simultaneous collection of retinal, neural, and perceptual measures should be employed. Such an approach will help (1) distinguish cause and effect of amblyopic impairments, (2) separate the impact of amblyopia from other superimposed conditions, and (3) identify the importance of amblyopic etiology to specific neural and perceptual deficits.

Simultaneous pattern visual evoked potential and pattern electroretinogram in strabismic and anisometropic amblyopia

BACKGROUND

Amblyopia is a relatively common condition in which visual acuity through an eye is subnormal despite no overt pathology. Pattern visual evoked potential (PVEP) can detect any defect from optic nerve to occipital cortex and pattern electroretinogram (PERG) can detect retinal defects specially the ganglion layers. This study was performed to evaluate the cortical and retinal activity in strabismic and anisometropic amblyopia.

METHODS

PVEP and PERG were recorded simultaneously in 40 amblyopes (20 strabismics and 20 anisometropics) and 20 normal control subjects. Normal subjects were age and sex matched with patients.

RESULTS

The P(100) latency in PVEP was increased in both groups of patients but the P(100) amplitude was reduced only in anisometropic group. In PERG, the amplitude of P(50) was reduced in all patients with no significant change in latency.

CONCLUSION

Beside reduced PVEP responses in strabismic and anisometropic amblyopia, the activity of retina reduced too. It is likely that retinal impulses can affect the development of visual system.

Electro-oculographic abnormalities in amblyopia

BACKGROUND

Electrodiagnostic tests have been used to investigate retinal function in amblyopia but previous results have been conflicting.

METHODS

It was decided to investigate whether the electro-oculogram (EOG) showed any abnormalities in 12 adult amblyopes and 12 age and sex matched controls with normal vision. The mean amplitudes of the EOG recordings from each eye during 12 minutes of darkness and 18 minutes of light were compared.

RESULTS

The mean values from the amblyopic eyes were lower than those from the fellow non-amblyopic eyes. At most time points the difference was significant (p < 0.05). After normalisation of the data to minimise intersubject variation, the reduction in EOG amplitudes of the amblyopic eyes at all time points was significant (p < 0.05). There was no significant difference between the mean values obtained from the right and left control eyes at any time point, either before or after normalisation.

CONCLUSIONS

These results provide evidence for a retinal abnormality in amblyopia and implicate the retinal pigment epithelium as being involved. A deficiency in retinal dopaminergic function in amblyopia is proposed as a possible mechanism causing these results.

Pattern electroretinography in senile macular degeneration

The value of pattern electroretinography (PERG) for the detection and differentiation of senile macular degeneration (SMD) was assessed. A group of patients with atrophic SMD and a group with exudative SMD were compared with an age-matched reference group. Differences were only found in visual acuity and in the amplitude of the negative peak N95. No statistical differences between the atrophic and exudative groups were found. Conclusions may be drawn from these results with regard to the use of PERG in senile macular degeneration.

Results with multichannel topographical bipolar recordings of the visual evoked potential in normals and amblyopes

The method currently used to record topographical voltage distributions of visual evoked potentials is that of monopolar recording with respect to some arbitrary reference electrode. As an alternative we investigated the method of bipolar topographical recording of the checkerboard reversal evoked potential in four amblyopic patients and ten normal persons. The recordings were performed with a system of 14 bipolar channels, arranged in a rectangular grid with an interelectrode distance of 15% of the inion-nasion distance and centered 3 cm above the inion. The gradient distributions of the visual evoked potential obtained from normal persons showed at the time of component P2 a maximum about 3 cm above the inion. The healthy eyes of the amblyopic patients showed similar distributions and latencies. The amblyopic eyes showed normal or reduced amplitudes and normal or increased latencies with large checks and low amplitudes with smaller checks. The topographical distributions recorded with the bipolar recording system showed shifts of the maxima with the larger patterns in three of four amblyopic eyes. A technical comparison showed that these shifts can be recorded by the bipolar recording method with higher accuracy than with the monopolar method, promising a better understanding and more accurate diagnosis of amblyopia.

Pattern electroretinography in patients with delayed pattern visual evoked potentials due to distal anterior visual pathway dysfunction

Between March 1983 and January 1988 delayed pattern visual evoked potentials (PVEP) were observed in 67 patients with distal visual pathway dysfunction. Many of these patients had been referred for neurophysiological examination because of possible optic nerve dysfunction. These patients also had pattern electroretinography (PERG) performed which in all cases showed an abnormality of the main positive P50 component. None of these patients had an abnormality confined to the negative N95 component, the type of abnormality usually found if the PERG is abnormal in optic nerve disease. It is suggested that PERG recording should now be a routine adjunct to the PVEP in the assessment of anterior visual pathway dysfunction.

Retinal and retinocortical times to pattern stimulation in amblyopic children

In order to determine whether in amblyopes retinal conduction delays contribute to the cortical measureable delays in the visual evoked cortical potential (VECP), peak latencies of the pattern electroretinogram (ERG) are measured in amblyopic children. The results are compared with those of the normal fellow eyes and those of a healthy control group. Simultaneously the latencies in the VECP are recorded and the determination of the retinocortical times is performed. Statistically retinal b-wave (Q) and a-wave (P) of the pattern ERG of amblyopic eyes do not show significant delays of peak latency. In retincortical times, however, there are significant prolongations. During occlusion therapy retinocortical values of normal fellow eyes are also delayed in comparison with the control group. A pathological conduction delay of visual information on the retinal level up to the generators of the pattern ERG can thus be excluded in amblyopia. The total latency delay in the VECP of amblyopes consists solely in a prolongation of retinocortical times.

Abnormality of the pattern electroretinogram and pattern visual evoked cortical response in esotropic cats

In six 3-week-old kittens, the tendon of the lateral rectus muscle of one eye was sectioned under anaesthesia in order to induce an esotropic (nasally-directed) squint. At maturity, the pattern electroretinogram (PERG) and pattern visual evoked response (PVER) were recorded under anaesthesia to phase-reversal at 1.67 Hz of a 0.5c/deg square wave grating pattern of 75% contrast. Refraction was determined by retinoscopy and confirmed by recording the PERG for different trial lenses. The amplitude and time-to-peak of the PERG and PVER were compared between operated and unoperated eyes, and with the responses of one normal and two sham-operated cats. With stimulation of the operated eye, the amplitude of the PVER was consistently reduced by 50% compared with the non-operated eye, confirming that the cats had become amblyopic as a result of the squint. While the appreciable inter-eye variations in the amplitude of the PERG of the normal and sham-operated cats precluded identification of a change in the esotropic cats, there was a consistent prolongation of the time-to-peak of the PERG by 50% with stimulation of the operated eye. This result is consistent with a retinal component to strabismic amblyopia.

A review of the clinical applications of the pattern electroretinogram

The pattern electroretinogram (PERG) has recently been introduced as a clinical procedure. It has been thought by many to represent activity of the retinal ganglion cells, although this is still a matter of contention. The exciting prospect of a selective test of ganglion cell function led to the application of the PERG in a variety of ophthalmological conditions. In the course of these investigations the PERG was found to be diminished in cases of maculopathy, optic atrophy, optic neuritis, toxic optic neuropathy, neurotransmitter disorders, glaucoma and ocular hypertension and in retinal vascular disorders such as diabetes. It was also affected in some cases of amblyopia. This paper briefly describes the techniques used to record the PERG and reviews current literature pertaining to its clinical application.

Visual evoked cortical potentials and pattern electroretinograms in Parkinson's disease and control subjects

Parkinson's disease patients have been shown to have abnormal visually evoked cortical potentials (VEPs) to pattern stimulation. Whereas dopamine is not an important neurotransmitter in the central visual pathways, the retina is rich in dopamine and, together with previous animal and human studies, this suggests that the abnormal VEPs in Parkinson's disease patients may be due to a biochemical and electrophysiological disorder in the retina. This hypothesis has been examined by studying the VEPs and pattern electroretinogram (PERG) of Parkinson's disease patients and matched control subjects. The amplitudes of the cortical and retinal evoked potentials were significantly reduced in Parkinson's disease patients compared with the control subjects and this could not be attributed to any particular feature of the disease or its treatment. There was a significant relationship between the VEP P100 latency and the PERG amplitude. Moreover for those subjects in whom there was an interocular difference in both cortical and retinal evoked potentials, the abnormality was more commonly found in the potentials from the same eye. These findings suggest that the abnormality of the VEP in Parkinson's disease patients is, at least in part, secondary to an abnormality of the retina itself.

Early diagnosis of retinal changes in diabetes: a comparison between electroretinography and retinal biomicroscopy

The present study was undertaken in order to find out whether electroretinographic examinations could reveal signs of functional abnormalities before morphological changes are detected in the diabetic retina. Pattern-reversal and flash electroretinograms (ERG) and oscillatory potentials (OP) were recorded in 24 diabetics and 10 age-matched normal controls. The diabetic group consisted of 11 patients without retinopathy and 13 patients with background retinopathy. No significant changes in pattern-reversal or flash ERG or OP amplitudes were observed in the diabetic group with normal fundus or with background retinopathy. The findings imply that ERG examination with the described techniques does not reveal retinal dysfunction in diabetics before retinopathy can be detected by means of retinal biomicroscopy.

Pattern-reversal electroretinograms in ocular hypertension

Pattern-reversal electroretinograms (ERGs) were recorded from seven patients, aged 50 to 69 years, with clinical diagnosis of unilateral ocular hypertension. In four of the seven patients the pattern ERG amplitude from the hypertensive eye was reduced below the normal level of variability, when compared with the amplitude from the opposite normotensive eye (ratio less than 0.8). In three of these four cases excavation of the optic disk in the hypertensive eye was observed 6 to 15 months after the recording of reduced pattern ERG. The finding suggests that pattern ERG may be an useful objective method for early detection of functional damage in eyes with increased intraocular pressure.

The luminance origin of the pattern electroretinogram in man

Electroretinograms (e.r.g.s) and visually evoked potentials (v.e.p.s) to pattern stimuli were recorded simultaneously from healthy subjects. The stimuli were produced by a configuration in which the luminance of two sets of spatial elements (checks) could be modulated independently. Experiments were designed to distinguish between contrast responses and non-linear luminance responses. In the first of two basic experiments, the luminance of only one set of checks was modulated, at a constant level in every trial. The other set was not modulated, but its luminance was set at various levels. Under these conditions the local luminance stimulation was kept equal for every trial whereas the contrast stimulation varied. Therefore, local luminance responses in these experiments were expected to be constant and contrast responses were expected to vary. The e.r.g.s were identical for all luminance settings of the unmodulated checks, suggesting that luminance rather than contrast determines the response. The v.e.p.s showed, on the contrary, the behaviour expected for contrast responses. In the second basic experiment the local luminance stimulation was also kept constant, but the phase difference between the modulations of the two sets of checks was varied between 0 deg (pure luminance stimulation) and 180 deg (pattern reversal). In this type of experiment the second harmonic responses to local luminance modulation are expected to decrease to a minimum as phase difference goes from 0 to 90 deg and increase again as phase difference goes to 180 deg. Contrast responses are expected to increase monotonically from zero to maximal at phase difference shifts from 0 deg (no contrast stimulation) to 180 deg (contrast reversal). The e.r.g.s decreased to a minimum at 90 deg phase difference and increase again with phase difference going to 180 deg. At 0 and 180 deg the same value was recorded. Consequently, the e.r.g. behaviour suggests a luminance origin of the responses. The v.e.p.s monotonically increased as phase difference went from 0 to 180 deg, thus suggesting a contrast origin of the responses. Two additional control experiments were performed. The first experiment compared the responses to homogeneous field red/green exchange and pattern red/green exchange, with the luminances of the red and green sources matched by heterochromatic flicker photometry. The exchange of luminance-matched red and green checks (pattern reversal) did not produce different e.r.g. responses from those recorded in response to homogeneous field red/green exchange. The v.e.p. showed a significant increase for the pattern stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

Pattern-reversal electroretinograms and visual evoked cortical potentials in multiple sclerosis

Pattern-reversal and flash electroretinograms (ERG) and visual evoked cortical potentials (VECP) were recorded from 15 patients with definite multiple sclerosis (MS). All patients had prolonged VECP latency, indicating demyelination of one or both optic nerves. The pattern-reversal ERG amplitude was reduced below the level of normal variation (mean -2 SD) in 11 of the 22 eyes with prolonged VECP latency and in one of the eight eyes with normal VECP latency. The mean pattern-reversal ERG amplitude from eyes with prolonged VECP latencies was significantly lower than the mean amplitude from the normal controls. No abnormalities were observed in the flash ERGs. Degeneration of retinal ganglion cell axons has been demonstrated in MS patients. The amplitude reduction in the pattern-reversal ERG, observed in some 50% of the eyes with prolonged VECP latencies, is supposed to reflect retinal ganglion cell dysfunction or degeneration secondary to demyelination of the optic nerve.

Pattern electroretinogram and visually evoked cortical potentials in glaucoma

Electroretinograms (P-ERG) and cortical potentials (P-VECP) evoked by checkerboard patterns were examined in patients with defects of the ganglion cell and nerve fiber layers due to glaucoma. Only a few patients exhibited a prolonged latency in the P-VECP, whereas in the P-ERG all patients with papillary and visual field defects revealed a significantly attenuated amplitude. Since there is a substantial fluctuation in the assessment of papillary excavation and visual field, the P-ERG offers a further means of evaluating and follow-up of retinal function in glaucoma patients.

Oscillatory potentials, flash and pattern-reversal electroretinograms in amblyopia

Oscillatory potentials (OP), flash and pattern-reversal electroretinograms (ERG) were recorded in 8 adult patients with amblyopia (visual acuity 0.3 or less) and 10 age-matched normal controls. No significant differences were observed in OPs between amblyopic and opposite eyes and normal controls. The pattern-reversal ERGs were significantly reduced in all amblyopic eyes. No significant side difference was found in the flash ERGs. The observed amplitude reduction in the pattern-reversal ERG might reflect dysfunction of the retinal ganglion cells, which have been supposed to be the main source of this type of ERG. No electrophysiological evidence could be demonstrated for a disturbed retinal function prior to the ganglion cell level.

Assessment of retinal function in severely amblyopic individuals

A quantitative psychophysical and electroretinographic assessment of severely amblyopic human eyes revealed no electrophysiological retinal analogue of the psychophysical deficit, when objectively refracted. Electroretinograms to pattern and uniform field flicker were normal in all but one amblyope. In this amblyope, although the electroretinographic deficit was uncorrelated with the psychophysical deficit, the reduced amplitude of the pattern-evoked response was also seen in the second harmonic component to uniform field stimulation. The deficit for each stimulus was only present at high signal levels. The similarity of the pattern evoked response and the second harmonic response to uniform field stimulation in normal and amblyopic eyes suggests a similar underlying process.

Comparison of the spatial response properties of the human retina and cortex as measured by simultaneously recorded pattern ERGs and VEPs

Electroretinograms and visual evoked potentials were simultaneously recorded from adult subjects using a checkerboard pattern stimulus reversing at 0.94, 3.75 and 7.5 Hz. Two contrast levels were used: 30 and 85%. The data obtained from the cortex (VEPs) show spatial tuning properties for all temporal frequencies at both contrast levels, with the peak of the amplitude-check size function occurring between 15 and 30 min. Tuning properties were found at the retina but only at the high contrast level and for the faster (3.75 and 7.5 Hz) temporal frequencies. The results demonstrate that spatial tuning is present in the human retina but not under as wide a range of conditions as found at the cortex.