On- and off-responses in the photopic electroretinogram in complete-type congenital stationary night blindness

Association of Missense Variants in VSX2 With a Peculiar Form of Congenital Stationary Night Blindness Affecting All Bipolar Cells

Importance

Congenital stationary night blindness (CSNB) is an inherited stationary retinal disorder that is clinically and genetically heterogeneous. To date, the genetic association between some cases with CSNB and an unusual complex clinical picture is unclear.

Objective

To describe an unreported CSNB phenotype and the associated gene defect in 3 patients from 2 unrelated families.

Design, Setting, and Participants

This retrospective case series was conducted in 2021 and 2022 at a national referral center for rare ocular diseases. Data for 3 patients from a cohort of 140 genetically unsolved CSNB cases were analyzed clinically and genetically.

Exposures

Complete ocular examination including full-field electroretinography and multimodal fundus imaging (spectral-domain optical coherence tomography, color, infrared reflectance, and short-wavelength autofluorescence photographs) were performed. The gene defect was identified by exome sequencing and confirmed by Sanger sequencing and co-segregation analysis in 1 family. Screening was performed for genetically unsolved CSNB cases for VSX2 variants by direct Sanger sequencing.

Main Outcomes and Measures

Ocular and molecular biology findings.

Results

The series included 3 patients whose clinical investigations occurred at ages in the early 30s, younger than 12 years, and in the mid 40s. They had nystagmus, low stable visual acuity, and myopia from birth and experienced night blindness. Two older patients had bilateral lens luxation and underwent lens extraction. Full-field electroretinography revealed an electronegative Schubert-Bornschein appearance, combining characteristics of incomplete and complete CSNB, affecting the function of rod and cone ON- and OFF-bipolar cells. Exome sequencing and co-segregation analysis in a consanguineous family with 2 affected members identified a homozygous variant in VSX2. Subsequently, screening of the CSNB cohort identified another unrelated patient harboring a distinct VSX2 variant.

Conclusions and Relevance

This case series revealed a peculiar pan-bipolar cell retinopathy with lens luxation associated with variants in VSX2. Clinicians should be aware of this association and VSX2 added to CSNB diagnostic gene panels.

Comparing the RETeval® portable ERG device with more traditional tabletop ERG systems in normal subjects and selected retinopathies

Purpose

Our study aimed to determine if ISCEV standard-like ERGs recorded with the LKC RETeval® portable ERG unit compared to those obtained using the more traditional tabletop unit.

Methods

ERGs recorded from normal subjects and patients affected with retinal ON and OFF pathway anomalies were compared. Analysis included peak time and amplitude measurements as well as time–frequency domain analysis with the discrete wavelet transform of waveforms obtained with the two systems.

Results

Although both systems were similarly able to record reliable and highly reproducible ERG responses, there were major discrepancies in ERG responses between the portable and tabletop units, pointing toward a weaker stimulation of the retinal OFF pathway with the portable RETeval® unit.

Conclusion

The portable RETeval® unit appears to be able to record highly reproducible and diagnostically useful clinical ERGs, albeit with some significant differences in waveform composition compared to those obtained with more standard tabletop systems. Given the unknown origin of these waveform discrepancies, if left uncorrected, these differences could potentially lead to erroneous interpretation when used in the clinical context and/or compared to ERGs recorded using more traditional table top units. Clearly, more research is warranted before handheld devices, such as the RETeval®, can be homologated as a diagnostically sound ERG devices.

ON than OFF pathway disruption leads to greater deficits in visual function and retinal dopamine signaling

The visual system uses ON and OFF pathways to signal luminance increments and decrements. Increasing evidence suggests that ON and OFF pathways have different signaling properties and serve specialized visual functions. However, it is still unclear the contribution of ON and OFF pathways to visual behavior. Therefore, we examined the effects on optomotor response and the retinal dopamine system in nob mice with ON pathway dysfunction and Vsx1^-/- mice with partial OFF pathway dysfunction. Spatial frequency and contrast sensitivity thresholds were determined, and values were compared to age-matched wild-type controls. Retinas were collected immediately after visual testing to measure levels of dopamine and its metabolite, DOPAC. At 4 weeks of age, we found that nob mice had significantly reduced spatial frequency (19%) and contrast sensitivity (60%) thresholds compared to wild-type mice. Vsx1^-/- mice also exhibited reductions in optomotor responses (3% in spatial frequency; 18% in contrast sensitivity) at 4 weeks, although these changes were significantly smaller than those found in nob mice. Furthermore, nob mice had significantly lower DOPAC levels (53%) and dopamine turnover (41%) compared to controls while Vsx1^-/- mice displayed a transient increase in DOPAC levels at 4 weeks of age (55%). Our results show that dysfunction of ON pathways leads to reductions in contrast sensitivity, spatial frequency threshold, and retinal dopamine and DOPAC levels whereas partial loss of the OFF pathway has minimal effect. We conclude that ON pathways play a critical role in visual reflexes and retinal dopamine signaling, highlighting a potential association for future investigations.

A review of electroretinography waveforms and models and their application in the dog

Electroretinography (ERG) is a commonly used technique to study retinal function in both clinical and research ophthalmology. ERG responses can be divided into component waveforms, analysis of which can provide insight into the health and function of different types and populations of retinal cells. In dogs, ERG has been used in the characterization of normal retinal function, as well as the diagnosis of retinal diseases and measuring effects of treatment. While many components of the recorded waveform are similar across species, dogs have several notable features that should be differentiated from the responses in humans and other animals. Additionally, modifications of standard protocols, such as changing flash frequency and stimulus color, and mathematical models of ERG waveforms have been used in studies of human retinal function but have been infrequently applied to visual electrophysiology in dogs. This review provides an overview of the origins and applications of ERG in addition to potential avenues for further characterization of responses in the dog.

Ring analysis of multifocal oscillatory potentials (mfOPs) in cCSNB suggests near-normal ON-OFF pathways at the fovea only

PURPOSE

To investigate the center-periphery distribution of ON and OFF retinal responses in complete congenital stationary night blindness (cCSNB).

METHODS

Photopic full-field flash ERGs (photopic ffERGs) and OPs (photopic ffOPs) and slow m-sequence (to enhance OP prominence) mfERGs (and filtered mfOPs) evoked by a 37 hexagon stimulus array were recorded from normal subjects and cCSNB patients. Discrete wavelet transform (DWT) analysis of photopic ffERGs and mfERGs was also performed in order to assess the contribution of the ON and OFF retinal pathways (i.e., OFF-to-ON ratio) in both cohorts.

RESULTS

As expected, the photopic ffERG (and ffOPs) responses in cCSNB were devoid of the first two of the three OPs (i.e., OP2 and OP3 and OP4) normally seen on the ascending limb of the b-wave. A similar finding was also noted in the mfERGs (and mfOPs) of ring 4. In contrast, the mfERGs (and mfOPs) of ring 1 included all three OPs. DWT analysis revealed that while in normal subjects, the OFF-to-ON ratio of mfERGs slightly increased from rings 1 to 4 (from 0.61 ± 0.03 to 0.78 ± 0.04; p < 0.05; median: from 0.62 to 0.79; p < 0.05), in cCSNB this ratio increased significantly more [from 0.73 ± 0.13 (ring 1) to 1.18 ± 0.17 (ring 4); p < 0.05; median: 0.78 to 1.22; p < 0.05], hence from a normal ON-dominated ratio (central ring) to an OFF-dominated ratio (peripheral ring).

CONCLUSIONS

Our results show a clear discrepancy of ON and OFF mfOP components in cCSNB. Responses originating from the most central ring (i.e., ring 1) disclosed a near-normal electrophysiological contribution (as revealed with the presence of OP2, OP3 and OP4 as well as with the DWT OFF-to-ON ratio) of the retinal ON and OFF pathways in mfERG (and mfOPs) responses compared to responses from the more peripheral ring (and ffOP) which are devoid of the ON OPs (i.e., OP2 and OP3).

Extracting the ON and OFF contributions to the full-field photopic flash electroretinogram using summed growth curves

Under cone-mediated (photopic) conditions, an "instantaneous" flash of light, including both stimulus onset and offset, will simultaneously activate both "ON" and "OFF" bipolar cells, which either depolarize (ON) or hyperpolarize (OFF) in response and, respectively, produce positive-going and negative-going deflections in the electroretinogram (ERG). The stimulus-response (SR) relationship of the photopic ON response demonstrates logistic growth, like that manifested in the rod-mediated (scotopic) b-wave, which is driven by a single class of depolarizing bipolar cell. However, the photopic b-wave SR function is importantly shaped by OFF responses, leading to a "photopic hill." Furthermore, both on and off stimuli elicit activity in both ON and OFF bipolar cells. This has made it difficult to produce meaningful parameters for ready interpretation of the photopic b-wave SR relationship. Therefore, we evaluated whether the sum of sigmoidal SR functions, as descriptors of the depolarizing and hyperpolarizing components of the photopic flash ERG, could be used to elucidate and quantitate the mechanisms that produce the photopic hill. We used a novel fitting routine to optimize a sum of simple sigmoidal curves to SR data in five groups of subjects: Healthy adult, 10-week-old infant, congenital stationary night blindness (CSNB), X-linked juvenile retinoschisis (XJR), and preterm-born, both without and with a history of retinopathy of prematurity (ROP). Differences in ON and OFF amplitude, sensitivity, and implicit time among the groups were then compared using parameters extracted from these fits. We found that our modeling procedure enabled plausible derivations of ON and OFF pathway contributions to the ERG, and that the parameters produced appeared to have physiological relevance. In adult subjects, the ON and OFF amplitudes were similar in magnitude with respectively longer and shorter implicit times. Infant, CSNB, and XJR subjects showed significant ON pathway deficits. History of preterm-birth, without or with a diagnosis of ROP, did not much affect cone responses.

ISCEV extended protocol for the photopic On-Off ERG

The International Society for Clinical Electrophysiology of Vision (ISCEV) standard for full-field electroretinography (ERG) describes a minimum procedure, but encourages more extensive testing. This ISCEV extended protocol describes an extension to the ERG standard, namely the photopic On-Off ERG, and outlines common clinical applications. A light stimulus duration of 150-200 ms is used in the presence of a rod-suppressing background to elicit cone-driven On- and Off-system ERG components. The On-response occurs after the stimulus onset and has a negative a-wave and positive b-wave. The Off d-wave is a positive component evoked by stimulus offset. Common diagnoses that may benefit from additional photopic On-Off ERG testing include retinal dystrophies and retinal disorders that cause dysfunction at a level that is post-phototransduction or post-receptoral. On-Off ERGs assess the relative involvement of On- and Off-systems and may be of use in the diagnosis of disorders such as complete and incomplete congenital stationary night blindness (complete and incomplete CSNB), melanoma-associated retinopathy, and some forms of autoimmune retinopathy. The photopic On-Off ERGs may also be useful in X-linked retinoschisis, Batten disease, Duchenne muscular dystrophy, spinocerebellar degeneration, quinine toxicity, and other retinal disorders.

Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics

Background

Amyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).

Results

Using histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.

Conclusions

These findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0245-z) contains supplementary material, which is available to authorized users.

Full-field electroretinogram response to increment and decrement stimuli

PURPOSE

The d-wave is typically elicited after the termination of an increment flash, but a decrement flash provides an alternative, and perhaps more appropriate, stimulus to elicit the d-wave. Here, we investigated the affects of stimulus polarity on the electroretinogram (ERG) response.

METHODS

ERG responses elicited to increment and decrement flashes of varying intensity and duration from different background levels were measured from human participants to assess the b-wave and d-wave responses as a function of adaptation level and flash polarity. Response amplitudes were measured using standard metrics for waveform analysis.

RESULTS

The amplitude of the b-wave is larger than the d-wave regardless of flash polarity when using different background levels which maximized the dynamic range of the two waveforms. However, when response amplitudes are measured from a common background, the d-wave elicited with decrement flash was larger than the b-wave elicited by an increment flash. This trend was evident across a range of background levels. The b-wave and d-wave become separate entities when flash duration reaches approximately 50 ms. Rapid-on and rapid-off sawtooth stimuli were also tested against increment and decrement step stimuli that were matched in mean luminance. These two stimulus types produced different amplitude b-wave and d-wave responses, suggesting asymmetric effects of the two stimulus types on the retinal response.

CONCLUSIONS

We conclude that the response properties of the b-wave and d-wave are influenced by the duration, polarity and waveform of the stimulus, as well as the background from which the stimuli arise.

Estimating ON and OFF contributions to the photopic hill: normative data and clinical applications

BACKGROUND

With progressively brighter stimuli, the amplitude of the b-wave of the human photopic electroretinogram (ERG) first increases to a maximal value (Vmax) and then decreases to finally reach a plateau, a phenomenon known as the photopic hill (PH). A mathematical model combining a Gaussian (G) and a logistic (L) growth function was previously proposed to fit this unusual luminance-response curve, where the G and L functions were suggested to represent, respectively, the OFF and ON retinal pathway contributions to the building of the PH.

METHOD

The PHs of patients presenting stationary diseases affecting specifically the ON (3 CSNB-1) or OFF (4 CPCPA) retinal pathways as well as patients affected with retinitis pigmentosa (14 RP) of different stages or etiology were analyzed using this mathematical model and compared to the PHs of a group of 28 normal subjects.

RESULTS

The PH of the CSNB-1 patients had a much larger contribution from the G function compared to normal subjects, whereas the opposite was observed for the CPCPA patients. On the other hand, analysis of data from RP patients revealed variable G-L contributions to the building of their PH.

CONCLUSION

In this study, we confirm the previous claim that the luminance-response function of the photopic ERG b-wave can be decomposed into a Gaussian function and a logistic growth function representing, respectively, the OFF and ON retinal pathways. Furthermore, our findings suggest that this mathematical decomposition could be useful to further segregate and potentially follow the progression of retinopathies such as RP.

Asymmetrical growth of the photopic hill during the light adaptation effect

In response to progressively stronger flashes delivered against a rod saturating background light, the amplitude of the photopic ERG b-wave first increases, reaches a maximal value (V (max)) and then decreases gradually to a plateau where the amplitude of the b-wave equals that of the a-wave, a phenomenon known as the photopic hill (PH). The purpose of this study was to investigate how the PH grew during the course of the light adaptation (LA) process that follows a period of dark adaptation (DA): the so-called light adaptation effect (LAE). Photopic ERG (time-integrated) luminance-response (LR) functions were obtained prior to (control-fully light adapted) and at 0, 5 and 10 min of LA following a 30-min period of DA. A mathematical model combining a Gaussian and a logistic growth function, suggested to reflect the OFF and ON retinal contribution to the PH respectively, was fitted to the LR functions thus obtained. Our results indicate that the magnitude of the cone ERG LAE is modulated by the stimulus luminance, with b-wave enhancements being maximal for luminance levels that result in the descent of the PH. The Gaussian function grew significantly with LA while the logistic growth function remained basically unchanged. Our findings would therefore suggest that the LAE reflects primarily an increase in the retinal OFF response during LA.

Modulation of ERG retinal sensitivity parameters with light environment and photoperiod

It has been reported that the sensitivity to light of the circadian system of animals and human subjects can be modulated following long-term exposure to a given light environment. Animal studies have also shown that long-term exposure to a light regimen, or light history, will have a significant impact on the retinal structure and function, the objective of which being to regulate the number of photons processed daily by the retina, a phenomenon referred to as photostasis. The existence of such a mechanism has never been explored in humans. In the present study, daily light exposure was continuously recorded in two populations of full-time workers: one group working indoors, in a relatively dim environment without access to natural light, and one group working mainly outdoors in natural bright light. The effect of seasonal changes in the length of the photoperiod was also examined. Retinal sensitivity, as determined with scotopic and photopic electroretinograms (ERG), was compared between these two groups. Indoor workers received less light than outdoor workers, the difference being significant only during work hours. A dim work environment was associated with greater retinal sensitivity in scotopic conditions and lower retinal sensitivity in photopic conditions when compared to a bright work environment. The above differences in retinal sensitivity were evidenced only in workers studied during the months with the shortest photoperiod (Fall-Winter). These results support the hypothesis that, similar to what was previously demonstrated with animal models, the human retina adapts its sensitivity to light according to previous chronic light history, suggesting the existence of a photostasis phenomenon in the human retina as well.

The luminance-response function of the human photopic electroretinogram: a mathematical model

The luminance-response function of the brief flash full-field photopic electroretinogram (ERG) rises to a peak before falling to a sub-maximal plateau -- the 'photopic hill'. The combination of on- and off-responses inherent in the brief flash photopic ERG suggests that this luminance-response function could be modelled by the sum of a Gaussian function and a logistic growth function. Photopic ERGs to a luminance series of brief flashes against three different background luminances recorded from seven healthy adults showed the characteristic 'photopic hill' function for b-wave amplitudes which were satisfactorily fitted with the sum of a Gaussian curve and a logistic growth curve. As background luminance increased, both components shifted to the right on the luminance axis. The Gaussian component increased in amplitude while the logistic growth function component decreased in amplitude. The luminance-response function of a complete congenital stationary night blindness patient had almost no logistic growth component.

ON- and OFF-response of the photopic electroretinogram in relation to stimulus characteristics

The aim of this study was to explore the effect of stimulus duration, stimulus intensity, stimulus wavelength and background luminance on the amplitude and waveform of the ON- (b-wave) and OFF- (d-wave) response of the photopic ERG. The following parameters were used in this study: stimulus duration from 5 to 200 ms, the flash intensities from 0.4 to 2.1 log cd s/m2, the background luminance from 20 to 50 cd/m2 and stimulus wavelengths 460, 508 and 667 nm. Prolonging the stimulus duration from 75 to 200 ms did not influence the amplitudes of a-, b- and d-waves significantly. Higher stimulus intensities (1.9 log cd s/m2) broadened the b-wave and increased variability of its implicit time. With a brighter background (50 cd/m2) the b-wave did not broaden and its optimal amplitude was preserved. Stimuli of longer wavelengths (667 nm) significantly reduced the d-wave amplitude. Our results suggest that high intensities and longer wavelengths (red) may not be suitable stimulus parameters in the routine clinical ON- and OFF-response recording.

Primate Retinal Signaling Pathways: Suppressing ON-Pathway Activity in Monkey With Glutamate Analogues Mimics Human CSNB1-NYX Genetic Night Blindness

Retinal on-pathway dysfunction is implicated in human complete-type congenital stationary night blindness (CSNB1), a Mendelian genetic condition that results from mutations in the NYX gene encoding the protein nyctalopin. We probed cone pathway dysfunction in four human genotyped CSNB1 affected males by electroretinogram (ERG) recordings elicited with photopic sinusoidal and rapid-on/off-ramp flicker stimuli that are reputed to elicit on/off-pathway activity selectively. Results were analyzed in relation to ERG abnormalities created in anesthetized non-human primates by intravitreal application of glutamate analogues that selectively suppress retinal on- or off-pathway bipolar cell activity. 2-amino-4-phosphonobutyric acid (APB), which selectively blocks light responses of on-pathway depolarizing bipolar cells, fully recreated the essential ERG abnormalities found for human CSNB1 under the condition that the off-pathway remained active. Both CSNB1-NYX humans and APB-treated monkey retina lacked the normal amplitude dip and the phase deflection that occurs in the fundamental component near 12 Hz for sinusoidal flicker stimuli. The off-pathway suppressing agent, cis-2,3-piperidine-dicarboxylic acid (PDA), gave results in monkey quite discordant to CSNB1 human for sinusoidal stimulation. The results implicated a specific on-pathway signaling deficiency in CSNB1-NYX males with no evidence of off-pathway involvement. Likewise, rapid-on/off ramping stimuli also indicated that the functional deficit was localized to the on pathway. Analysis of non-human primate retinal responses after drug application demonstrated a complexity to on/off-pathway contributions to ramping on/off ERG responses not previously anticipated. These results support the hypothesis that nyctalopin acts principally or exclusively within the on pathway at the level of depolarizing bipolar cells, and thus human CSNB1-NYX subjects provide an opportunity to probe the primate visual system for consequences of on-pathway deficits.

Control of late off-center cone bipolar cell differentiation and visual signaling by the homeobox gene Vsx1

Retinal bipolar cells are interneurons that transmit visual signals from photoreceptors to ganglion cells. Although the visual pathways mediated by bipolar cells have been well characterized, the genes that regulate their development and function are largely unknown. To determine the role in bipolar cell development of the homeobox gene Vsx1, whose retinal expression is restricted to a major subset of differentiating and mature cone bipolar (CB) cells, we targeted the gene in mice. Bipolar cell fate was not altered in the absence of Vsx1 function, because the pan-bipolar markers Chx10 and Ret-B1 continued to be expressed in inner nuclear layer neurons labeled by the Vsx1-targeting reporter gene, tauLacZ. The specification, number, and gross morphology of the subset of on-center and off-center (OFF)-CB cells defined by tauLacZ expression from the Vsx1 locus were also normal in Vsx1(tauLacZ)/Vsx1(tauLacZ) mice. However, the terminal differentiation of OFF-CB cells in the retina of Vsx1(tauLacZ)/Vsx1(tauLacZ) mice was incomplete, as demonstrated by a substantial reduction in the expression of at least four markers (recoverin, NK3R, Neto1, and CaB5) for these interneurons. These molecular abnormalities were associated with defects in retinal function and documented by electroretinography and in vitro ganglion cell recordings specific to cone visual signaling. In particular, there was a general reduction in the light-mediated activity of OFF, but not on-center, ganglion cells. Thus, Vsx1 is required for the late differentiation and function of OFF-CB cells and is associated with a heritable OFF visual pathway-specific retinal defect.

Abnormalities of the scotopic threshold response correlated with gene mutation in X-linked retinoschisis and congenital stationary night blindness

STRs and dark-adapted ERGs were recorded in nine normal subjects, nine patients with XLRS, 11 patients with CSNB1 and one patient with CSNB2. In XLRS STR amplitude was significantly lower than normal at every intensity, but the response could be recorded in every patient and the maximum amplitude response was outside the 95% confidence limits in only four of the nine patients. STRs were significantly poorer in patients with CSNB and a responses was not measurable at any intensity in nine of the 11 patients with CSNB1. In both CSNB and XLRS the STR could only be recorded at higher stimulus intensities, suggesting reduced sensitivity of the STR. In XLRS onset and peak latencies were also significantly prolonged and the slope of the intensity-response functions for amplitude and onset latency differed significantly from normal. Maximum STR amplitude did not correlate with the maximum dark-adapted ERG response. The finding of abnormal STRs and dark adapted ERGs in all three dystrophies indicates that the different causative genes must have similar effects on the rod On-bipolar cell pathway. But there were also differences between the three clinical groups, particularly in the greater severity of the abnormality in CSNB1, which suggests that there may be multiple sites of abnormality.

Spontaneous occurrence of a potentially night blinding disorder in guinea pigs

Several hereditary retinal disorders such as retinitis pigmentosa and congenital stationary night blindness compromise, sometimes exclusively, the activity of the rod pathway. Unfortunately, there are few animal models of these disorders that could help us better understand the pathophysiological processes involved. The purpose of this report is to present a pedigree of guinea pigs where, as a result of a consanguineous mating and subsequent selective breeding, we developed a new and naturally occurring animal model of a rod disorder. Analysis of the retinal function with the electroretinogram reveals that the threshold for rod-mediated electroretinograms (ERGs) is significantly increased by more than 2 log-units compared to that of normal guinea pigs. Furthermore, in response to a suprathreshold stimulus, also delivered under scotopic condition, which yield a mixed cone-rod response in normal guinea pigs, the ERG waveform in our mutant guinea pigs is almost identical (amplitude and timing of a- and b-waves) to that evoked in photopic condition. The above would thus suggest either a structural (abnormal development or absence) or a functional deficiency of the rod photoreceptors. We believe that our pedigree possibly represents a new animal model of a night blinding disorder, and that this condition is inherited as anautosomal recessive trait in the guinea pig population.

The photopic ERG luminance-response function (photopic hill): method of analysis and clinical application

With progressively brighter stimuli, the amplitude of the photopic b-wave first increases, briefly saturates and then decreases gradually to reach a plateau, where the amplitude of the b-wave equals that of the a-wave; a phenomenon previously presented as the photopic hill. The unique presentation of this luminance-response function seriously complicates its analysis with curve fitting equations such as that of Naka-Rushton used for scotopic electroretinogram. We report a method of analysis of the photopic hill based on easily identifiable and reproducible features of the ascending and descending limbs of this function. The clinical usefulness of these parameters is illustrated with selected cases of retinal disorders.

Abnormalities of the long flash ERG in congenital stationary night blindness of the Schubert-Bornschein type

We investigated abnormalities of the retinal cone ON- and OFF-pathways in 24 males with Schubert-Bornschein congenital stationary night blindness (CSNB). Substantial differences were found between both CSNB types. In incomplete type, a-, b- and d-waves were reduced and delayed, whereas in complete type only the b-wave showed significant changes. Oscillatory potentials (OPs) were not discernible from noise in incomplete CSNB and showed significant peak alterations of the ON-OPs only in complete CSNB. In the complete CSNB type, the ON pathway appeared to be mainly affected. In the incomplete CSNB form marked involvement of both the ON and the OFF pathways was noted.