ERGs, cone-isolating VEPs and analytical techniques in children with cone dysfunction syndromes

Chromatic visual evoked potentials: A review of physiology, methods and clinical applications

Objective assessment of the visual system can be performed electrophysiologically using the visual evoked potential (VEP). In many clinical circumstances, this is performed using high contrast achromatic patterns or diffuse flash stimuli. These methods are clinically valuable but they may only assess a subset of possible physiological circuitries within the visual system, particularly those involved in achromatic (luminance) processing. The use of chromatic VEPs (cVEPs) in addition to standard VEPs can inform us of the function or dysfunction of chromatic pathways. The chromatic VEP has been well studied in human health and disease. Yet, to date our knowledge of their underlying mechanisms and applications remains limited. This likely reflects a heterogeneity in the methodology, analysis and conclusions of different works, which leads to ambiguity in their clinical use. This review sought to identify the primary methodologies employed for recording cVEPs. Furthermore cVEP maturation and application in understanding the function of the chromatic system under healthy and diseased conditions are reviewed. We first briefly describe the physiology of normal colour vision, before describing the methodologies and historical developments which have led to our understanding of cVEPs. We thereafter describe the expected maturation of the cVEP, followed by reviewing their application in several disorders: congenital colour vision deficiencies, retinal disease, glaucoma, optic nerve and neurological disorders, diabetes, amblyopia and dyslexia. We finalise the review with recommendations for testing and future directions.

Dyschromatopsia: a comprehensive analysis of mechanisms and cutting-edge treatments for color vision deficiency

Color blindness is a retinal disease that mainly manifests as a color vision disorder, characterized by achromatopsia, red-green color blindness, and blue-yellow color blindness. With the development of technology and progress in theory, extensive research has been conducted on the genetic basis of color blindness, and various approaches have been explored for its treatment. This article aims to provide a comprehensive review of recent advances in understanding the pathological mechanism, clinical symptoms, and treatment options for color blindness. Additionally, we discuss the various treatment approaches that have been developed to address color blindness, including gene therapy, pharmacological interventions, and visual aids. Furthermore, we highlight the promising results from clinical trials of these treatments, as well as the ongoing challenges that must be addressed to achieve effective and long-lasting therapeutic outcomes. Overall, this review provides valuable insights into the current state of research on color blindness, with the intention of informing further investigation and development of effective treatments for this disease.

A reduced visual pathway response in infantile nystagmus syndrome

PURPOSE

To investigate visual cortical responses in children with infantile nystagmus syndrome (INS) and the potential contribution of foveation periods.

METHODS

The medical records of children with INS who had visual evoked potential (VEP) recordings to reversing checkerboards and onset of horizontal gratings were reviewed retrospectively. VEP recordings underwent objective selective averaging for extraction of brief periods having consistent amplitude and timing with the stimulus presentation. VEP amplitude, latency, and signal-to-noise ratios (SNR) were compared to results from published age-matched controls under the same conditions. Relative foveation in INS subjects was determined from the proportion of time a video-oculography recording met eye position and velocity criteria.

RESULTS

A total of 26 children met inclusion criteria. Selective averaging increased VEP amplitude and SNR in INS by 270%-420% compared to standard averaging (P < 0.0001). The INS change in VEP response was greater for reversing checkerboard stimulation than horizontal-grating onset and was significantly greater than that in controls (P < 0.001). Latency was not changed by selective averaging. Relative foveation was correlated with increasing VEP amplitude (P = 0.02) and number of trials chosen for selective averaging (P < 0.01). After selective averaging, relative foveation correlated with VEP amplitude to reversing checkerboards only (P = 0.007).

CONCLUSIONS

Nystagmus likely causes a reduced visual cortical response in children with INS. A significantly larger response can be extracted from brief periods during nystagmus eye movements, supporting the hypothesis that the INS visual system generates a larger cortical signal during brief foveation periods.

Retinal and visual function in infants with non-accidental trauma and retinal hemorrhages

PURPOSE

To investigate retinal function and visual outcomes in infants with retinal hemorrhages due to non-accidental trauma (NAT).

METHODS

This is a retrospective review of full-field or multifocal electroretinogram (ERG) recordings, visual acuity in log minimum angle of resolution (logMAR), clinical status, and neuroimaging. Multifocal ERGs from the central 40° were compared to corresponding fundus imaging. Visual acuity was measured by Teller cards at follow-up. ERGs were compared to controls recorded under anesthesia.

RESULTS

Sixteen children met inclusion criteria (14 recorded during the acute phase and 2 during long-term follow-up). During the acute phase, ERGs (n = 4 full field; n = 10 multifocal ERG) showed abnormal amplitude, latency, or both in at least one eye. Ten subjects had significantly reduced responses in both eyes, 3 of which had an ERG dominated by a negative waveform (absent b-wave or P1). The remaining six subjects had responses in one eye that were near normal (≥ 50% of controls). ERGs were sometimes abnormal in local areas without hemorrhage. ERGs could be preserved in local areas adjacent to traumatic retinoschisis. Two subjects with reduced visual acuity had belated ERGs: One had an abnormal macular ERG and the other had a normal macular ERG implying cortical visual impairment. At follow-up, 10 of 14 subjects had significant visual acuity loss (≥ 0.7 age-corrected logMAR); four subjects had mild vision loss (≤ 0.5 age-corrected logMAR). Visual acuity outcome was not reliably associated with the fundus appearance in the acute phase. All subjects with a negative ERG waveform had severe vision loss on follow-up.

CONCLUSIONS

Retinal dysfunction was common during the acute phase of NAT. A near normal appearing fundus did not imply normal retinal function, and ERG abnormality did not always predict a poor visual acuity outcome. However, a negative ERG waveform was associated severe visual acuity loss. Potential artifacts of retinal hemorrhages and anesthesia could not fully account for multifocal ERG abnormalities. Retinal function can be preserved in areas adjacent to traumatic retinoschisis.

Cognitive Effects of Transcranial Direct Current Stimulation in Healthy and Clinical Populations: An Overview

Transcranial direct current stimulation (tDCS) is a neuromodulatory approach that is affordable, safe, and well tolerated. This review article summarizes the research and clinically relevant findings from meta-analyses and studies investigating the cognitive effects of tDCS in healthy and clinical populations. We recapitulate findings from recent studies where cognitive performance paired with tDCS was compared with performance under placebo (sham stimulation) in single sessions and longitudinal designs where cognitive effects were evaluated following repeated sessions. In summary, the tDCS literature currently indicates that the effects of tDCS on cognitive measures are less robust and less predictable compared with the more consistent effects on motor outcomes. There is also a notable difference in the consistency of single-session and longitudinal designs. In single-session tDCS designs, there are small effects amid high variability confounded by individual differences and potential sham stimulation effects. In contrast, longitudinal studies provide more consistent benefits in healthy and clinical populations, particularly when tDCS is paired with a concurrent task. Yet, these studies are few in number, thereby impeding design optimization. While there is good evidence that tDCS can modulate cognitive functioning and potentially produce longer-term benefits, a major challenge to widespread translation of tDCS is the absence of a complete mechanistic account for observed effects. Significant future work is needed to identify a priori responders from nonresponders for every cognitive task and tDCS protocol.

VEP analysis methods in children with optic nerve hypoplasia: relationship to visual acuity and optic disc diameter

PURPOSE

Assessing vision in young children with optic nerve hypoplasia (ONH) is challenging due to multi-directional infantile nystagmus, the range of optic nerve loss, and cognitive delay. This study examined visual evoked potential (VEP) responses and averaging techniques in children with ONH. The assumption is that EEG epochs with inconsistent temporal phase would be associated with nystagmus, signal reduction due to axon loss, and visual inattention.

METHODS

A retrospective chart review was performed on 44 children (average age 2.2 years; SD 1.9). Optic disc diameter was estimated by ophthalmoscopy. Visual function was measured under binocular viewing and then compared to the eye with the larger optic disc to exclude secondary amblyopia. Visual acuity was measured by Teller cards or by recognition optotypes, and both measures were converted into log minimum angle of resolution (logMAR). VEPs were recorded to onset/offset of horizontal gratings and to reversing checkerboards. Signal-to-noise ratios (SNRs) were estimated from phase consistency across epochs in the Fourier domain. VEPs were also averaged after (1) correction of epochs for phase shifts across a limited bandwidth, or (2) selection of only epochs showing phase consistency.

RESULTS

Optic disc diameter, logMAR, VEP amplitudes, and VEP SNR were all significantly inter-correlated. Optic disc diameter correlated best with VEP SNR (Spearman rho = 0.82; p < 0.001). Age-corrected logMAR correlated with optic disc diameter and VEP SNR (Spearman rho = -0.695 and 0.70, respectively; p < 0.001). VEP latency poorly correlated with optic disc diameter or logMAR. Correction of phase shifts or selection of epochs based on phase consistency significantly increased VEP amplitude and SNR for children with optic disc diameters <1000 microns. Correction of phase inconsistency did not improve the correlation of VEP parameters with optic disc diameter or with logMAR.

CONCLUSIONS

In ONH, the size of the optic nerve is correlated with VEP SNR and logMAR. The results imply a direct relationship between the reduction in optic nerve axons and generalized reduction in visual function. Our calculation of VEP SNR provides objective assessment of optic nerve function that is independent of subjective scoring of VEP peaks.

Investigation of P300 response characteristics through human color vision-based visual stimulation

In this study, we propose visual stimulation based on the primary colors (red, green, and blue) in order to investigate the characteristics of the P300 response. Eleven healthy volunteers participated in our experiment, and their brain signals were recorded by electroencephalography (EEG). Using two basic measures referred to as `on-peak' and `off-peak' for comparison of the P300 response among the participants, we found that the P300 response varies depending on the color of the stimulus. The results of this investigation are expected to contribute to various existing and future EEG-based applications.

Visual sensory and ocular motor function in children with polymicrogyria: relationship to magnetic resonance imaging

PURPOSE

To assess visual and ocular motor function in children with polymicrogyria (PMG).

METHODS

The medical records of 15 children (0.4-4 years of age) with PMG documented by magnetic resonance imaging (MRI) and with age-corrected visual acuity measured by Teller acuity cards were reviewed retrospectively. Cortical function was assessed by pattern visually evoked potentials (VEP). Ocular motor function was assessed by video-oculography or clinical assessment. Results were compared to age-matched controls.

RESULTS

Extent of PMG involvement varied from bilateral fronto-parietal to bilateral-diffuse. Nine children had involvement of the occipital lobe. Visual acuity at presentation was normal in 5 children (≥20/40 Snellen equivalent for age) and subnormal in 10 (average 20/200 equivalent). Visual acuity was similar in children with or without involvement of the occipital lobe (P = 0.4). Follow-up visual acuity was available for 9 children; 3 improved and 6 failed to improve (5 of whom had seizures). PMG involving the occipital lobe significantly reduced VEP amplitude and signal-to-noise ratios. Three infants without visually-guided behaviors had VEP responses. All 3 children with cytomegalovirus-related PMG without retinal disease had preserved visual function despite generalized MRI abnormalities.

CONCLUSIONS

All children with PMG had recordable visual function either by visual acuity or VEP testing, however the majority did not show longitudinal improvement in acuity. Seizures may impose limits on visual acuity development. Children with cytomegalovirus-related PMG, microcephaly, and developmental delay can have normal visual acuity. Children with a recordable VEP but without visually guided behaviors may have a defect in sensorimotor transformation.

The cone dysfunction syndromes

The cone dysfunction syndromes are a heterogeneous group of inherited, predominantly stationary retinal disorders characterised by reduced central vision and varying degrees of colour vision abnormalities, nystagmus and photophobia. This review details the following conditions: complete and incomplete achromatopsia, blue-cone monochromatism, oligocone trichromacy, bradyopsia and Bornholm eye disease. We describe the clinical, psychophysical, electrophysiological and imaging findings that are characteristic to each condition in order to aid their accurate diagnosis, as well as highlight some classically held notions about these diseases that have come to be challenged over the recent years. The latest data regarding the genetic aetiology and pathological changes observed in the cone dysfunction syndromes are discussed, and, where relevant, translational avenues of research, including completed and anticipated interventional clinical trials, for some of the diseases described herein will be presented. Finally, we briefly review the current management of these disorders.

Dark-adaptation functions in molecularly confirmed achromatopsia and the implications for assessment in retinal therapy trials

PURPOSE

To describe the dark-adaptation (DA) functions in subjects with molecularly proven achromatopsia (ACHM) using refined testing conditions with a view to guiding assessment in forthcoming gene therapy trials.

METHODS

The DA functions of nine subjects with ACHM were measured and compared with those of normal observers. The size and retinal location of the stimuli used to measure DA sensitivities were varied in four distinct testing condition sets, and the effect of altering these parameters assessed.

RESULTS

In three of the four testing condition sets, achromats had significantly higher mean final thresholds than normal observers, whereas in the fourth condition set they did not. A larger, more central stimulus revealed the greatest difference between the final DA thresholds of achromat and normal subjects, and also demonstrated the slowest rate of recovery among the achromat group.

CONCLUSIONS

In this, the largest study of DA functions in molecularly proven ACHM to date, we have identified optimal testing conditions that accentuate the relative difference between achromats and normal observers. These findings can help optimize DA testing in future trials, as well as help resolve the dichotomy in the literature regarding the normality or otherwise of DA functions in ACHM. Furthermore, the shorter testing time and less intense adaptation light used in these experiments may prove advantageous for more readily and reliably probing scotopic function in retinal disease, and be particularly valuable in the frequent post therapeutic assessments required in the context of the marked photophobia in ACHM.

The value of clinical electrophysiology in the assessment of the eye and visual system in the era of advanced imaging

Electrophysiological techniques allow clinical investigations to include a 'dissection' of the visual system. Using suitable electrophysiological techniques, the 'dissection' allows function to be ascribed to the different photoreceptors (rod and cone photoreceptors), retinal layers, retinal location or the visual pathway up to the visual cortex. Combined with advances in genetics, retinal biochemistry, visual fields and ocular imaging, it is now possible to obtain a better understanding of diseases affecting the retina and visual pathways. This paper reviews core electrophysiological principles that can complement other examination techniques, including advanced ocular imaging, and help the interpretation of other clinical data and thus, refine and guide clinical diagnosis.

Waveform variance and latency jitter of the visual evoked potential in childhood

PURPOSE

Recording the visual evoked potential (VEP) in young children is challenging due to movement artifacts with variable fixation or attention. This study examined the effects of latency jitter, noise, and waveform consistency on the averaging of the VEP across childhood age.

METHODS

Stimuli were contrast-reversing (1.4 Hz) checkerboards of 163 arc minutes and pattern-onset-offset of 0.5 cycle/degree horizontal sine-wave gratings. Subjects were 79 normal children (0.3-16 years age; mean 6.9). Results were compared to recordings of EEG noise only (noise controls). Epochs underwent four averaging methods: (1) latency jitter correction using cross-correlation, (2) correction of phase shifts across a limited bandwidth in the Fourier domain, (3) selection of epochs based on consistency in the time domain, and (4) selection of epochs based on phase consistency in the Fourier domain. Signal-to-noise ratios (SNR) were estimated in both the time and Fourier domains.

RESULTS

Compared to standard averaging, all methods improved the amplitude of the primary peak (P100) while generating mild changes in latency. All methods also increased amplitudes of residual peaks in noise controls. In VEPs with an adequate SNR, selective averaging in the Fourier domain provided the greatest improvement in amplitude (61 % increase; p < 0.0001) without prolongation in latency. Correction of latency jitter did not consistently improve amplitude but caused latency prolongation in 24 % of subjects. There was no age-related effect of any averaging method for either stimulus.

CONCLUSIONS

Since latency jitter correction does not improve VEP amplitude more than selective averaging, recording artifacts in children are dominated by random phase components rather than inducing latency jitter.

Reduced L- and M- and increased S-cone functions in an infant with thyroid hormone resistance due to mutations in the THRβ2 gene

BACKGROUND

To document an infant with a cone photoreceptor disorder associated with severe thyroid hormone resistance due to compound heterozygosity in the thyroid hormone receptor beta 2 (TRβ2) encoding gene THRβ2.

MATERIALS AND METHODS

Cone photoreceptor function was assessed using standard electroretinography (ERG) including colored flashes. We PCR-amplified and sequenced exons 8-10 of the THRβ2 gene. We cloned and sequenced a genomic segment (exons 9-10) of the THRβ2 gene to confirm a compound heterozygote mutation. We investigated whether mutations in the (OPN1LW-OPN1MW) gene array were responsible for the phenotype.

RESULTS

ERG testing showed a normal scotopic response and severely reduced photopic response. Spectral testing showed a small amplitude b-wave to a red flash and a larger amplitude b-wave to the blue flash. Molecular analysis revealed this child was a compound heterozygote for p.R338W and p.R429W mutations in exons 9 and 10 of the THRβ2 gene. These two mutations lie within the ligand-binding domain that is known to selectively inhibit Trβ2 binding as homodimers to the thyroid hormone receptor response elements (TREs). No mutations were found within the OPN1LW and OPN1MW genes or the locus control region that regulates expression of these opsin genes.

CONCLUSION

We document a congenital disorder of cone function characterized by severely reduced L- and M-cone responses and increased S-cone responses caused by deleterious mutations in the THRβ2 gene in thyroid resistant patients. Thyroid hormone, via TRβ2, is critical for determining cone-type differentiation in humans.

Relationship of chromatic visual-evoked potentials and the changes of foveal photoreceptor layer in central serous chorioretinopathy patients

PURPOSE

To investigate the relative involvement of chromatic and achromatic visual subsystems in central serous chorioretinopathy, and to correlate the function changes with the changes of the foveal photoreceptor layer using Fourier domain optical coherence tomography (FD-OCT).

METHODS

Central serous chorioretinopathy patients and control subjects were tested and compared. Equiluminant Chromatic sinusoidal gratings were presented in a pattern onset-offset mode. We measured the distance between the internal limiting membrane (ILM) and the external limiting membrane (ELM) on FD-OCT images.

RESULTS

Visual evoked potential mean latency was delayed significantly in central serous chorioretinopathy patients compared to controls. The visual evoked potential delay was greater for chromatic than for achromatic stimuli. Retinal thickness from ILM to ELM was associated significantly and selectively with short-wavelength-sensitive (S) cone visual evoked potential latency (r = -0.40, p = 0.032).

CONCLUSIONS

These results are consistent with data that indicate the loss of photoreceptors in central serous chorioretinopathy resulting in a reduced thickness of the outer nuclear layer. Additionally, they are consistent with data which indicate that the photoreceptor loss is relatively greater in cones. The S-cone pathway was affected the most.

Decreased catalytic activity and altered activation properties of PDE6C mutants associated with autosomal recessive achromatopsia

Mutations in the gene encoding the catalytic subunit of the cone photoreceptor phosphodiesterase (PDE6C) have been recently reported in patients with autosomal recessive inherited achromatopsia (ACHM) and early-onset cone photoreceptor dysfunction. Here we present the results of a comprehensive study on PDE6C mutations including the mutation spectrum, its prevalence in a large cohort of ACHM/cone dysfunction patients, the clinical phenotype and the functional characterization of mutant PDE6C proteins. Twelve affected patients from seven independent families segregating PDE6C mutations were identified in our total patient cohort of 492 independent families. Eleven different PDE6C mutations were found including two nonsense mutations, three mutations affecting transcript splicing as shown by minigene assays, one 1 bp-insertion and five missense mutations. We also performed a detailed functional characterization of six missense mutations applying the baculovirus system to express recombinant mutant and wildtype chimeric PDE6C/PDE5 proteins in Sf9 insect cells. Purified proteins were analyzed using Western blotting, phosphodiesterase (PDE) activity measurements as well as inhibition assays by zaprinast and Pγ. Four of the six PDE6C missense mutations led to baseline PDE activities and most likely represent functional null alleles. For two mutations, p.E790K and p.Y323N, we observed reduction in PDE activity of approximately 60% and 80%, respectively. We also observed differences for Pγ inhibition. The p.E790K mutant, with an IC₅₀ value of 2.7 nm is 20.7-fold more sensitive for Pγ inhibition, whereas the p.Y323N mutant with an IC₅₀ of 158 nm is 3-fold less sensitive when compared with the wildtype control.

Rod and rod-driven function in achromatopsia and blue cone monochromatism

PURPOSE

To evaluate rod photoreceptor and postreceptor retinal function in pediatric patients with achromatopsia (ACHR) and blue cone monochromatism (BCM) using contemporary electroretinographic (ERG) procedures.

METHODS

Fifteen patients (age range, 1-20 years) with ACHR and six patients (age range, 4-22 years) with BCM were studied. ERG responses to full-field stimuli were obtained in scotopic and photopic conditions. Rod photoreceptor (S(rod), R(rod)) and rod-driven postreceptor (log sigma, V(max)) response parameters were calculated from the a-wave and b-wave. ERG records were digitally filtered to demonstrate the oscillatory potentials (OPs); a sensitivity parameter, log SOPA(1/2), and an amplitude parameter, SOPA(max), were used to characterize the OP response. Response parameters were compared with those of 12 healthy control subjects.

RESULTS

As expected, photopic responses were nondetectable in patients with ACHR and BCM. In addition, mean scotopic photoreceptor (R(rod)) and postreceptor (V(max) and SOPA(max)) amplitude parameters were significantly reduced compared with those in healthy controls. The flash intensity required to evoke a half-maximum b-wave amplitude (log sigma) was significantly increased.

CONCLUSIONS

Results of this study provide evidence that deficits in rod and rod-mediated function occur in the primary cone dysfunction syndromes ACHR and BCM.

The key role of electrophysiology in the diagnosis of visually impaired children

PURPOSE

To describe the outcome of specialized electrophysiology in visually impaired children.

METHODS

We carried out a retrospective evaluation of 340 electrophysiological examinations performed in 298 children over a 3-year period (2001-2003), with regard to demographic data, referral pattern, degree of compliance, and diagnostic results. Electrophysiology was performed without sedation or anaesthesia. In electroretinograms, DTL electrodes were used in combination with online selection of responses. Visual evoked potentials testing was performed with seven active occipital electrodes.

RESULTS

The mean age of the children was 7 +/- 5 years; 72 (24%) of the children were mentally as well as visually impaired. Main reasons for referral were suspected posterior segment disease, abnormal visual development, unexplained low vision, high myopia, and suspected albinism. Compliance was good in 302/340 (88%), partial in 24/340 (7%), and absent in 14/340 (4%) of the examinations. Of the 326 successful procedures, 215 (66%) showed abnormal results. Tapetoretinal dystrophy (22%), opticopathy (16%), congenital stationary night blindness (13%), and cone dystrophy (11%) were the most frequently established diagnoses. Albinism was confirmed in 14 of 24 suspected patients; additionally, unsuspected misrouting was found in six. In 26 (9%) of the patients, a previously established diagnosis was changed.

CONCLUSIONS

In a specialized setting, electrophysiological examinations can be performed successfully in visually impaired children. The results are essential for the final ophthalmological diagnosis and have important consequences for rehabilitation.

Comparison of pattern visual-evoked potentials to perimetry in the detection of visual loss in children with optic pathway gliomas

PURPOSE

We sought to compare visual evoked potentials (VEPs) with standard visual field testing in children with visual pathway gliomas.

METHODS

Fifteen of 40 children with visual pathway gliomas verified on magnetic resonance imaging scan who cooperated with Goldmann visual field (GVF) and 3-channel VEPs were studied. GVFs were obtained in 25 eyes with adequate vision. VEP amplitudes, latencies, and signal-to-noise ratios (SNRs) were compared with control subjects. Four of the patients (5 eyes) also had Humphrey visual field testing.

RESULTS

Twenty-two of 25 eyes had a field defect, 15 eyes showed a relative or absolute hemianopia, 7 eyes showed a central or generalized depression, and 3 eyes were normal. In hemianopic eyes, 87% showed a depression (GVF) or reduced sensitivity (Humphrey field) in the opposite hemifield. VEP amplitudes and SNRs, normally largest at the midline electrode, were significantly reduced in all eyes with visual field loss. By comparison, lateral electrodes showed significantly lower amplitudes and SNRs in patients and controls. Interhemispheric VEP asymmetry (>2:1 ratio) was seen in 67% of patients with hemianopia and 53% of controls.

CONCLUSIONS

Reduction of amplitude and SNR at the midline VEP electrode was a sensitive indicator of visual field loss. Interhemispheric VEP asymmetry was not reliable in detection of a hemianopic field defect. VEPs can be a reliable and objective alternative for the detection of visual loss due to optic pathway glioma in children who are intolerant to visual field testing. We recommend the test protocol include pattern-onset and check reversal stimuli of at least one high and one low spatial frequency.