Dysfunction of transmission in the inner retina: incidence and clinical causes of negative electroretinogram

A novel mutation in RS1 and clinical manifestations in a Chinese twin family with congenital retinoschisis

Purpose: We aim to analyze the clinical and genetic features in a Chinese family with congenital retinoschisis by whole-exome sequencing and comprehensive clinical examination.

Methods: Six members were recruited from a Chinese family. Three of them were diagnosed as congenital retinoschisis, including two twin siblings. All subjects received a full eye examination. Whole-exome sequencing (WES) and Sanger sequencing were performed on two twin probands and all participants, respectively.

Results: A novel splice site mutation RS1.c.53-1G>A was identified in a Chinese congenital retinoschisis family. The mean onset age was 16.7 ± 2.4 years old. The average BCVA in patients was 0.37 ± 0.05. A typical spoke-wheel pattern was observed in all affected eyes. OCT examination results showed fovea schisis and schisis cavities were located in the inner nuclear layer in 100% eyes (6/6). ERG b/a ratio was decreased markedly, but was still more than 1 in the four eyes that were available.

Conclusion: The present study discovered a new pathogenic splice cite variant of RS1 in congenital retinoschisis, which expands the mutational spectrum. In contrast to previous research, the phenotype of patients with the same mutation within one family was highly similar. Early molecular testing is crucial for early diagnosis, clinical management, and genetic counseling of patients with congenital retinoschisis.

Electronegative electroretinogram in the modern multimodal imaging era

Background

The electronegative electroretinogram (ERG) reflecting inner retinal dysfunction can assist as a diagnostic tool to determine the anatomical location in eye disease. The aim of this study is to determine the frequency and aetiology of electronegative ERG in a tertiary ophthalmology centre and to develop a clinical algorithm to assist patient management.

Methods

Retrospective review of ERGs performed at the Save Sight Institute from January 2011 to December 2020. ERGs were performed according to ISCEV standard. The b:a ratio was analysed in dark adapted (DA) 3.0 or 12.0 recordings. Patients with ratio of ≤1.0 were included.

Results

A total of 4421 patients had ERGs performed during study period, of which 139 patients (3.1%) had electronegative ERG. The electronegative ERG patients' median age at referral time was 37 (0.7–90.6) years. The causative aetiologies were photoreceptor dystrophy (48, 34.5%), Congenital Stationary Night Blindness (CSNB) (33, 23.7%), retinal ischemia (18, 12.9%), retinoschisis (15, 10.8%), paraneoplastic autoimmune retinopathy (PAIR) and nonPAIR (14, 10.1%), batten disease (4, 2.9%), and inflammatory retinopathy (4, 2.9%). There were three patients with an unclassified diagnosis. Thirty‐two patients (23%) had good vision and a normal fundus appearance. Eleven patients (7.9%) had good vision and normal results in all multimodal imaging.

Conclusions

The frequency of electronegative ERG in our referral centre was 3.1% with photoreceptor dystrophy as the main aetiology. A significant number of the cases had good vision with normal fundus or normal multimodal imaging. This further highlights the value of an ERG in this modern multimodal imaging era.

The electrophysiological features of X-linked juvenile retinoschisis in a young male: a case report

This case report describes the detailed electrophysiological features and the corresponding relationship with the structural changes in a case of X-linked juvenile retinoschisis (XLRS). A 25-year-old male presented with a history of several years of decreased visual acuity in both eyes. The best corrected visual acuity was 20/200 in oculus dexter (OD) and 20/80 in oculus sinister. Retinoschisis was found in the macula by optical coherence tomography, which was more severe in OD. Electroretinogram revealed a similar electronegative waveform in both eyes. Visual evoked potential detected a reduced amplitude and delayed phase in P100-wave, which was worse in OD. The patient was diagnosed as XLRS and advised to undergo continuous medical observation. He was followed up for the next year, with no significant change in retinal function and structure being observed. These current findings suggest that electrophysiology permits the detailed analysis of the clinical picture of XLRS and helps to gain a deeper understanding of the pathogenesis.

Negative electroretinograms: genetic and acquired causes, diagnostic approaches and physiological insights

The dark-adapted human electroretinogram (ERG) response to a standard bright flash includes a negative-going a-wave followed by a positive-going b-wave that crosses the baseline. An electronegative waveform (or negative ERG) results when the b-wave is selectively reduced such that the ERG fails to cross the baseline following the a-wave. In the context of a normally sized a-wave, it indicates a site of retinal dysfunction occurring after phototransduction (commonly at the photoreceptor to bipolar cell synapse). This is an important finding. In genetic disease, the pattern of ERG abnormality can point to variants in a small group of genes (frequently those associated with congenital stationary night blindness and X-linked retinoschisis, but negative ERGs can also be seen in other conditions including syndromic disease). In acquired disease, there are numerous causes, but specific features may point to melanoma-associated retinopathy (MAR). In some cases, the visual symptoms precede the diagnosis of the melanoma and so the ERG findings can initiate investigations facilitating early detection and treatment. Negative ERGs can occur in other paraneoplastic conditions, and in a range of other diseases. This review will outline the physiological basis for the negative ERG, report prevalences in the literature from different cohorts, discuss the range of causes, displaying examples of a number of ERG phenotypes, highlight features of a clinical approach to patients, and briefly discuss further insights relating to current flows shaping the a-wave trough and from single-cell transcriptome analysis.

Prevalence of electronegative electroretinograms in a healthy adult cohort

Objective

An electronegative electroretinogram (ERG) can indicate important ocular or systemic disease. This study explored the prevalence of electronegative responses to dark-adapted stimuli in a largely healthy cohort.

Methods and Analysis

211 participants recruited from the TwinsUK cohort underwent ERG testing incorporating international standard (International Society for Clinical Electrophysiology of Vision (ISCEV)) protocols and additional stimuli. Responses were recorded using conductive fibre electrodes, following pupil dilation and 20 min dark adaptation. Responses analysed were to the ISCEV standard and strong flashes (3.0 and 10 cd/m² s), and to additional white flashes (0.67–67 cd/m² s). A-wave and b-wave amplitudes were extracted; b:a ratios were calculated and proportions of eyes with ratios<1 were noted.

Results

Mean (SD) age was 62.4 (11.4) years (median, 64.3; range 23–86 years). 93% were female. Mean (SD) b:a ratios for right and left eyes, respectively, were 1.86 (0.33) and 1.81 (0.29) for the standard flash, and 1.62 (0.25) and 1.58 (0.23) for the stronger flash; average b:a ratio was lower for the stronger flash (p<0.0001). No waveforms were electronegative. For additional flashes, b:a ratio decreased with increasing flash strength. No electronegative waveforms were seen except in three eyes (0.7%) for the strongest flash; in some cases, drift in the waveform may have artefactually reduced the b:a ratio.

Conclusion

For standard dark-adapted stimuli, no participants had electronegative waveforms. The findings support the notion that electronegative waveforms (in response to standard flash strengths) are unusual, and should prompt further investigation.

Impaired Ca2+ Sensitivity of a Novel GCAP1 Variant Causes Cone Dystrophy and Leads to Abnormal Synaptic Transmission Between Photoreceptors and Bipolar Cells

Guanylate cyclase-activating protein 1 (GCAP1) is involved in the shutdown of the phototransduction cascade by regulating the enzymatic activity of retinal guanylate cyclase via a Ca²⁺/cGMP negative feedback. While the phototransduction-associated role of GCAP1 in the photoreceptor outer segment is widely established, its implication in synaptic transmission to downstream neurons remains to be clarified. Here, we present clinical and biochemical data on a novel isolate GCAP1 variant leading to a double amino acid substitution (p.N104K and p.G105R) and associated with cone dystrophy (COD) with an unusual phenotype. Severe alterations of the electroretinogram were observed under both scotopic and photopic conditions, with a negative pattern and abnormally attenuated b-wave component. The biochemical and biophysical analysis of the heterologously expressed N104K-G105R variant corroborated by molecular dynamics simulations highlighted a severely compromised Ca²⁺-sensitivity, accompanied by minor structural and stability alterations. Such differences reflected on the dysregulation of both guanylate cyclase isoforms (RetGC1 and RetGC2), resulting in the constitutive activation of both enzymes at physiological levels of Ca²⁺. As observed with other GCAP1-associated COD, perturbation of the homeostasis of Ca²⁺ and cGMP may lead to the toxic accumulation of second messengers, ultimately triggering cell death. However, the abnormal electroretinogram recorded in this patient also suggested that the dysregulation of the GCAP1–cyclase complex further propagates to the synaptic terminal, thereby altering the ON-pathway related to the b-wave generation. In conclusion, the pathological phenotype may rise from a combination of second messengers’ accumulation and dysfunctional synaptic communication with bipolar cells, whose molecular mechanisms remain to be clarified.

Electronegative Electroretinograms in the United Arab Emirates

PURPOSE

An electronegative electroretinogram (ERG), defined as having a b:a wave ratio ≤1 in the scotopic flash ERG response, indicates relative inner retinal dysfunction. Causes vary depending upon the study population. In the Arabian Gulf, where inherited retinal disease is relatively prevalent, common diagnoses associated with electronegative ERGs have not been described. In this study, we report the frequency and causes of electronegative ERGs in a cohort of Emirati patients with inherited retinal disease.

METHODS

A retrospective review was performed of all full-field ERGs done for Emirati patients in the Ocular Genetics Service of Cleveland Clinic Abu Dhabi from January 2017 to December 2019. Those who had an electronegative ERG in at least one eye were included in the study.

RESULTS

Out of 137 patients, 9 probands (6.6%) had an electronegative ERG. The mean age at presentation was 24 years (range 5-48 years), and five patients (55.6%) were male. The final clinical diagnoses were congenital stationary night blindness (CSNB) (two TRPM1-related and one Oguchi disease), X-linked retinoschisis (XLRS) (one genetically confirmed and two not genetically tested), cone-rod dystrophy (one CRX-related and one not genetically tested), and enhanced S-cone syndrome (ESCS) (one NRL-related). The one patient who did not have bilateral electronegative ERGs was a male with XLRS whose fellow eye had an unrecordable ERG.

CONCLUSIONS

In this series of Emirati patients, an electronegative ERG was most commonly associated with the inherited retinal diseases recessive CSNB and XLRS. An electronegative ERG was noted in a case of NRL-related ESCS.

Retinal dystrophies with bull's-eye maculopathy along with negative ERGs

PURPOSE

The aim of this study was to examine the ophthalmological characteristics and genotypes of patients with congenital retinal pathologies, who display a bull's-eye maculopathy in the fundus, along with a negative scotopic electroretinogram.

METHODS

We analysed the results of five patients showing both a bull's-eye maculopathy, as well as a negative scotopic ERG evoked by a bright flash. Their median age was 39 years (range 11-63 years): three males and two females. All underwent a comprehensive examination with determination of distant visual acuity (ETDRS) and recording of the full-field ERG (scotopic and photopic). Fundus, OCT, and FAF images were obtained, the kinetic visual field was determined, and colour vision (D-15) was tested in most patients. Targeted gene panel sequencing was performed on peripheral blood.

RESULTS

One patient carried a homozygous ABCA4 mutation and an additional heterozygous variant in CRX. Two of the five patients were shown to have a heterozygous mutation in the CRX gene, one of whom had an additional heterozygous ABCA4 mutation. Two patients had the common heterozygous mutation c.2413G>A;p.Arg838His in GUCY2D. In all of the patients, there was a reduction in the amplitude of the b-wave with a regular a-wave amplitude in the scotopic bright-flash ERG.

CONCLUSIONS

The five patients with bull's-eye maculopathy along with a negative ERG had differing genotypes. Mutations were found in the CRX gene (2 patients), the ABCA4 gene (1 patient), and the GUCY2D gene (2 patients).

Phenotypic characterization of complete CSNB in the inbred research beagle: how common is CSNB in research and companion dogs?

PURPOSE

Although congenital stationary night blindness (CSNB) has been described in a Japanese beagle dog research colony, certain clinical correlates with human CSNB have not yet been described, nor has an estimate of frequency of the condition been made in inbred and outbred beagle populations.

METHODS

A beagle with CSNB obtained from a commercial research dog supplier in the USA and matched control dogs (n = 3) underwent examination, refraction, ocular imaging, assessment of visual navigation ability and detailed electroretinography (ERG). Retrospective review of ERGs in two independent groups of inbred (n = 15 and 537, respectively) and one group of outbred dogs (n = 36) was used to estimate CSNB frequency in these populations.

RESULTS

In the affected dog, there were absent dark-adapted b-waves in response to dim-light flashes, severely reduced dark-adapted b-waves in response to bright-light flashes, and normal light-adapted b-waves with a-waves that had broadened troughs. Long-flash ERGs confirmed a markedly reduced b-wave with a preserved d-wave, consistent with cone ON-bipolar cell dysfunction. There was evidence of normal rod photoreceptor a-wave dark adaptation, and rapid light adaptation. In the wider beagle populations, five inbred beagles had a b/a wave ratio of < 1 in dark-adapted bright-flash ERG, whereas no outbred beagles had ERGs consistent with CSNB.

CONCLUSIONS

The identified dog had clinical findings consistent with complete type CSNB, similar to that described in the Japanese colony. CSNB appears to be a rare disorder in the wider beagle population, although its detection could confound studies that use retinal function as an outcome measure in research dogs, necessitating careful baseline studies to be performed prior to experimentation.

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.

The diagnostic usefulness of the negative electroretinogram

The definition of the negative response of the full field electroretinogram is the presence of a b-wave with less amplitude than the a-wave (b/a ratio<1) in the combined response of cones and rods. The presence of this pattern reflects an alteration in the bipolar cells, the Müller cells, or in the transmission of the stimulus from the photoreceptors to the bipolar cells, with preserved photoreceptor function. This finding can be seen bilaterally and symmetrically in different hereditary conditions, such as congenital stationary night blindness, juvenile X-linked retinoschisis, and Duchenne and Becker muscular dystrophies. On the other hand, it can also be found unilaterally (or asymmetrically) in acquired pathologies, such as some types of immuno-mediated retinitis (Birdshot retinochoroiditis), autoimmune retinopathies, cancer/melanoma associated retinopathy, or retinal toxicity. The objective of this review is to summarise the characteristics of the pathologies in which this finding can be observed, in order to highlight its usefulness in the differential diagnosis of retinal conditions.

[Electrophysiology in ophthalmology]

Electrophysiology is an objective functional test of the visual pathway and allows the location of visual dysfunctions to be detected. The flash electroretinogram (ERG) allows recognition of large area damage to the retina and can distinguish between rod and cone diseases by recording under both dark and light-adapted conditions. Specific stimulation techniques are used for the multifocal ERG (mfERG) which reveals localized retinal dysfunction, e. g. in maculopathies. The pattern ERG (PERG) is an indicator of ganglion cell function and can be used for early detection of glaucoma. The visual evoked potential (VEP) is a cortical response and serves as a functional test of the entire visual pathway from the eye to the visual system of the brain. After presenting each of these methods individually, the article gives assistance in situations where the appropriate electrophysiological method for a given clinical hypothesis is to be selected and explains how the methods can be combined in a reasonable way.

Characterization of a Case of Pigmentary Retinopathy in Sanfilippo Syndrome Type IIIA Associated with Compound Heterozygous Mutations in the SGSH Gene

PURPOSE

To report longitudinal phenotypic findings in a patient with Sanfilippo syndrome type IIIA, harboring SGSH mutations, one of which is novel.

METHODS

Heparan-N-sulfatidase enzyme function testing in skin fibroblasts and white blood cells and SGSH gene sequencing were obtained. Clinical office examinations, examinations under anesthesia, electroretinogram, spectral domain optical coherence tomography (SD-OCT), and fundus photography were performed over a 5-year period.

RESULTS

Fundus examination revealed a progressive breadcrumb-like pigmentary retinopathy with perifoveal pigmentary involvement. SD-OCT showed loss of normal neuroretinal lamination and cystic macular changes responsive to treatment with carbonic anhydrase inhibitors. Electroretinography exhibited complex characteristics indicative of a generalized retinal rod > cone dysfunction with significant ON > OFF postreceptoral response compromise. Sequencing revealed compound heterozygous mutations in the SGSH gene, the novel c.88G > C (p.A30P) change and a second, previously reported one (c.734G > A, p.R245H).

CONCLUSIONS

We have identified ocular features of a patient with Sanfilippo syndrome type IIIA harboring a novel SGHS mutation that were not previously known to occur in this disease - namely, a progressive retinopathy with distinctive features, cystic macular changes responsive to carbonic anhydrase inhibitors, and complex electroretinographic abnormalities consistent with postreceptoral dysfunction. SD-OCT imaging revealed retinal lamination changes consistent with previously reported histologic studies. Both the SD-OCT and the electroretinogram changes appear attributable to intraretinal deposition of heparan sulfate.

Prph2 mutations as a cause of electronegative ERG

PURPOSE

To describe the phenotypic and genotypic features in patients with PRPH2 mutations and negative electroretinograms.

METHODS

Retrospective observational case series. Records of patients with a confirmed molecular diagnosis of PRPH2 mutation, and an electronegative electroretinogram (reduced b-wave to a-wave amplitude ratio) under either photopic or scotopic conditions, were identified. Data examined included clinical history and retinal images, electrophysiology, and mutational analysis.

RESULTS

Six patients were ascertained. All had presented with clinically evident maculopathy and Snellen visual acuities in the range of 6/6 to 1/60. All had negative electroretinograms in scotopic or photopic electroretinograms or both. Four patients were heterozygous for a previously reported missense mutation c.514C>T, p.R172W; 2 were heterozygous for the frame-shifting mutations c.259_266del8, p.D87fs and c.394delC, p.Q132fs. No other cause of electronegative electroretinogram was identified in any patient. Photopic On- and Off-response recording was useful in identifying On-pathway dysfunction.

CONCLUSION

PRPH2 mutation can be associated with negative electroretinograms. This novel finding is not mutation specific and does not relate to the severity of the disease. The data add to the documented phenotypical variability of PRPH2 mutations and represent a further cause of negative electroretinogram.

Sorting out co-occurrence of rare monogenic retinopathies: Stargardt disease co-existing with congenital stationary night blindness

BACKGROUND

Inherited retinal diseases are uncommon, and the likelihood of having more than one hereditary disorder is rare. Here, we report a case of Stargardt disease and congenital stationary night blindness (CSNB) in the same patient, and the identification of two novel in-frame deletions in the GRM6 gene.

MATERIALS AND METHODS

The patient underwent an ophthalmic exam and visual function testing including: visual acuity, color vision, Goldmann visual field, and electroretinography (ERG). Imaging of the retina included fundus photography, spectral-domain optical coherence tomography (OCT), and fundus autofluorescence. Genomic DNA was PCR-amplified for analysis of all coding exons and flanking splice sites of both the ABCA4 and GRM6 genes.

RESULTS

A 46-year-old woman presented with recently reduced central vision and clinical findings of characteristic yellow flecks consistent with Stargardt disease. However, ERG testing revealed an ERG phenotype unusual for Stargardt disease but consistent with CSNB1. Genetic testing revealed two previously reported mutations in the ABCA4 gene and two novel deletions in the GRM6 gene.

CONCLUSIONS

Diagnosis of concurrent Stargardt disease and CSNB was made on the ophthalmic history, clinical examination, ERG, and genetic testing. This case highlights that clinical tests need to be taken in context, and that co-existing retinal dystrophies and degenerations should be considered when clinical impressions and objective data do not correlate.

X-linked juvenile retinoschisis: clinical diagnosis, genetic analysis, and molecular mechanisms

X-linked juvenile retinoschisis (XLRS, MIM 312700) is a common early onset macular degeneration in males characterized by mild to severe loss in visual acuity, splitting of retinal layers, and a reduction in the b-wave of the electroretinogram (ERG). The RS1 gene (MIM 300839) associated with the disease encodes retinoschisin, a 224 amino acid protein containing a discoidin domain as the major structural unit, an N-terminal cleavable signal sequence, and regions responsible for subunit oligomerization. Retinoschisin is secreted from retinal cells as a disulphide-linked homo-octameric complex which binds to the surface of photoreceptors and bipolar cells to help maintain the integrity of the retina. Over 190 disease-causing mutations in the RS1 gene are known with most mutations occurring as non-synonymous changes in the discoidin domain. Cell expression studies have shown that disease-associated missense mutations in the discoidin domain cause severe protein misfolding and retention in the endoplasmic reticulum, mutations in the signal sequence result in aberrant protein synthesis, and mutations in regions flanking the discoidin domain cause defective disulphide-linked subunit assembly, all of which produce a non-functional protein. Knockout mice deficient in retinoschisin have been generated and shown to display most of the characteristic features found in XLRS patients. Recombinant adeno-associated virus (rAAV) mediated delivery of the normal RS1 gene to the retina of young knockout mice result in long-term retinoschisin expression and rescue of retinal structure and function providing a 'proof of concept' that gene therapy may be an effective treatment for XLRS.

Negative electroretinograms in the pediatric and adult population

Objective To assess the frequency of negative waveform electroretinograms (ERGs) in a tertiary referral center. Design Retrospective chart review. Participants All patients who had an ERG performed at the electrophysiology clinic at Emory University from January 1999 through March 2008 were included in the study. Methods Patients with b-wave amplitude ≤ a-wave amplitude during the dark-adapted bright flash recording, in at least one eye, were identified as having a "negative ERG". Clinical information, such as age, gender, symptoms, best corrected visual acuity, and diagnoses were recorded for these patients when available. Results A total of 1,837 patients underwent ERG testing during the study period. Of those, 73 patients had a negative ERG, for a frequency of 4.0%. Within the adult (≥ 18 years of age) and pediatric populations, the frequencies of a negative ERG were 2.5 and 7.2%, respectively. Among the 73 cases, negative ERGs were more common among male than female patients, 6.7% versus 1.8% (P < 0.0001). Negative ERGs were most common among male children and least common among female adults, 9.6% versus 1.1%, respectively, (P < 0.0001). Overall in this group of patients, the most common diagnoses associated with a negative ERG were congenital stationary night blindness (CSNB, n = 29) and X-linked retinoschisis (XLRS, n = 7). Conclusions The overall frequency of negative ERGs in this large retrospective review was 4.0%. Negative ERGs were most common among male children and least common among female adults. Despite the growing number of new diagnoses associated with negative ERGs, CSNB, and XLRS appear to be the most likely diagnoses for a pediatric patient who presents with a negative ERG.

Improvement in S-cone-mediated visual fields and rod function after correction of vitamin A deficiency

PURPOSE

To evaluate S-cone-mediated visual fields and full-field electroretinograms (ERGs) in a patient with vitamin A deficiency.

METHODS

A 65-year-old woman diagnosed with primary sclerosing cholangitis reported experiencing night blindness. The patient underwent comprehensive ophthalmic examination, including funduscopy, ERGs, Humphrey standard automated perimetry (SAP), and short-wavelength automated perimetry (SWAP). Serum vitamin A concentrations were measured.

RESULTS

The patient's best-corrected visual acuity was 1.2 in both eyes. The ERG results showed no rod b-waves, reduced combined rod-plus-cone responses (negative type), and normal cone and 30-Hz flicker responses. Serum vitamin A concentration was 18 IU/dL (normal range 97-316 IU/dL). The SAP mean deviation (MD) values were -1.09 dB (OD) and -0.97 dB (OS), whereas the SWAP MD values were -10.10 dB (OD) and -10.50 dB (OS). The rate of sensitivity decreased with increasing eccentricity in SWAP. Four months after starting oral administration of vitamin A, all ERG values were normalized, and SWAP MD values were greatly improved (OD -3.47 dB, OS -4.10 dB) compared with changes in SAP MD values (OD +0.67 dB, OS +0.41 dB). Rod dysfunction and impaired S-cone-mediated pathways were preferentially observed and found to be reversed after the treatment.

CONCLUSIONS

The findings in this patient suggest that rods and S cones are more susceptible to vitamin A deficiency than L and M cones. Vitamin A deficiency visual impairment may therefore be reversible with appropriate therapy.

Surgical treatment for complications of congenital retinoschisis

This study examined the clinical features of complications of congenital retinoschisis and the clinical efficacy of vitreoretinal surgery in the treatment of these complications. The clinical efficacy of surgical treatments was retrospectively analyzed in 10 patients with congenital retinoschisis (10 eyes) complicated with rhegmatogenous retinal detachment (n=5), vitreous hemorrhage (n=2) and macula- involving schisis (n=1). All the patients suffered foveal and peripheral schisis. They were treated with scleral buckling (n=1) or vitrectomy (n=9). After the surgical treatment, the retina was reattached in patients with rhegmatogenous retinal detachment; the refractive media became transparent in those with vitreous hemorrhage; the visual acuity in 80% of patients was improved; no remarkable progression of schisis was found; no severe operative complications occurred. It was concluded that vitreoretinal surgery in the treatment of complications of congenital retinoschisis is safe and effective, and helps improve and maintain the visual function.

Sodium iodate selectively injuries the posterior pole of the retina in a dose-dependent manner: morphological and electrophysiological study

Sequential morphological and functional features of retinal damage in mice exposed to different doses (40 vs. 20 mg/kg) of sodium iodate (NaIO₃) were analyzed. Retinal morphology, apoptosis (TUNEL assay), and function (electroretinography; ERG) were examined at several time points after NaIO₃ administration. The higher dose of NaIO₃ caused progressive degeneration of the whole retinal area and total suppression of scotopic and photopic ERG. In contrast, the lower dose induced much less severe degeneration in peripheral part of retina along with a moderate decline of b- and a-wave amplitudes in ERG, corroborating the presence of regions within retina that retain their function. The peak of photoreceptor apoptosis was found on the 3rd day, but the lower dose induced more intense reaction within the central retina than in its peripheral region. In conclusion, these results indicate that peripheral area of the retina reveals better resistance to NaIO₃ injury than its central part.

A negative electroretinogram (ERG) in a case of probable multiple system atrophy (MSA)

Recent articles have described negative ERGs in a small number of patients with cerebellar degeneration. Five of the previously reported seven cases were hereditary (2/5 had spinocerebellar ataxia-1 (SCA-1) gene mutations) and the other two were sporadic. We report a negative ERG in a case of cerebellar degeneration that differs significantly from earlier cases. The 65-year-old man had a 5-year history of ataxia, unsteady gait, orthostatic hypotension, and bladder and erectile dysfunction, with no family history of neurological or retinal disease. Visual acuity was 20/30 OD, 20/40 OS, but reportedly was never 20/20. His fundus exam showed optic nerve pallor, but otherwise was normal. Visual fields had enlarged blind spots but no central scotomas. Autofluorescence was normal. Photopic flash and 30-Hz ERG responses were normal. Rod b-waves were reduced and delayed. Standard flash a-waves were normal, but the b-waves were smaller than the a-waves. Blood tests were negative for Leber's hereditary optic neuropathy, dominant optic atrophy, and for expansions in SCA genes including SCA-1. This is only the third reported case of sporadic ataxia with a negative ERG. The patient's prominent autonomic dysfunction differs from the previous cases, and meets the clinical criteria for probable multiple system atrophy (MSA). This introduces another possible diagnosis in cases of negative ERGs with ataxia, and suggests that the visual system may be affected in MSA.

The negative ERG: clinical phenotypes and disease mechanisms of inner retinal dysfunction

Inner retinal dysfunction is encountered in a number of retinal disorders, either inherited or acquired, as a primary or predominant defect. Fundus examination is rarely diagnostic in these disorders, although some show characteristic features, and careful electrophysiological assessment of retinal function is needed for accurate diagnosis. The ERG in inner retinal dysfunction typically shows a negative waveform with a preserved a-wave and a selectively reduced b-wave. Advances in retinal physiology and molecular genetics have led to a greater understanding of the pathogenesis of these disorders. This review summarizes current knowledge on normal retinal physiology, the investigative techniques used and the range of clinical disorders in which there is predominantly inner retinal dysfunction.

Clinical electrophysiology in quinine induced retinal toxicity

A 43-year-old woman was evaluated 9 months after a deliberate overdose of quinine. The patient's visual acuity was 20/20, but she had lost all peripheral vision beyond approximately 30 degrees eccentricity. The multifocal electroretinography (mfERG) and optical coherence tomography are described for the first time in this condition. The mfERG shows electronegative waveforms beyond 6 degrees . The mfERG response density is reduced at all retinal locations. Optical coherence tomography shows thinning of the middle and inner retina by 25 to 35%, with preservation of the photoreceptor layer. Recent regulatory restrictions in off-label uses of quinine products should help to reduce the incidence of adverse toxic reactions.

X-linked retinoschisis: an update

X-linked retinoschisis is the leading cause of macular degeneration in males and leads to splitting within the inner retinal layers leading to visual deterioration. Many missense and protein truncating mutations have now been identified in the causative retinoschisis gene (RS1) which encodes a 224 amino acid secretory retinal protein, retinoschisin. Retinoschisin octamerisation is implicated in cell-cell interactions and cell adhesion perhaps by interacting with beta2 laminin. Mutations cause loss of retinoschisin function by one of the three mechanisms: by interfering with protein secretion, by preventing its octamerisation or by reducing function in the secreted octamerised protein. The development of retinoschisis mouse models have provided a model system that closely resembles the human disease. Recent reports of RS1 gene transfer to these models and the sustained restoration of some retinal function and morphology suggest gene replacement may be a possible future therapy for patients.