Cone dystrophy and supernormal dark-adapted b-waves in the electroretinogram

Cone dystrophy with supernormal rod responses: A rare KCNV2 gene variant

PURPOSE

To describe the clinical, electrophysiological, and genetic findings of three Portuguese families with a rare variant in the KCNV2 gene resulting in "cone dystrophy with supernormal rod responses" (CDSRR).

METHODS

Retrospective clinical revision of five individuals from three unrelated families with CDSRR. Ophthalmological examination was described in all patients and included color vision testing, fundus photography, fundus autofluorescence (FAF) imaging, spectral domain-optical coherence tomography (SD-OCT), pattern electroretinogram (ERG), and full-field ERG. The mutational screening of the KCNV2 gene was performed with Sanger and Next Generation Sequencing.

RESULTS

All patients showed childhood-onset photophobia and progressive visual acuity loss with varying degrees of severity. In multimodal imaging, various degrees of retinal pigment epithelium disturbances and outer retinal atrophy, which tend to be worst with advancing age, were observed. Molecular screening identified a rare presumed truncating variant (p.Glu209Ter) in homozygosity in two families and in compound heterozygosity in a third family. Three patients showed ERG changes characteristic of CDSRR, however, two patients presented with incomplete electrophysiological features of the disease.

CONCLUSION

A rare variant in the KCNV2 gene was identified in five patients from three Portuguese families. This variant often leads to a severe and progressive form of retinopathy. Considerable variability in the ERG responses among patients with this KCNV2 variant was observed.

Rod and cone function in patients with KCNV2 retinopathy

BACKGROUND

To investigate rod and cone function and disease mechanisms in patients with KCNV2 retinopathy.

METHODOLOGY/PRINCIPAL FINDINGS

Psychophysical examinations as well as detailed electrophysiological examinations with Ganzfeld and multifocal electroretinogram (ERG) were performed to study response dynamics. Additionally, fundus photography, autofluorescence imaging and spectral domain OCTs were carried out for morphological characterization. Molecular genetic analysis revealed compound heterozygosity in five patients and homozygosity for the KCNV2 gene in one patient. The mutations resulted in complete absence of Kv8.2 subunits in three patients (no protein group, NOP), while the other three patients expressed mutant Kv8.2 subunits resulting in altered Kv2.1/Kv8.2 heteromeric or residual Kv2.1 homomeric potassium channel function (altered protein group, ALP). Although more advanced morphological changes were visible in the NOP group, a clear functional difference between the two groups could not be observed. All patients showed characteristic dynamics of the b-wave intensity-response function, however, scotopic b-wave response amplitudes were within normal limits. We also observed severely reduced oscillatory potentials.

CONCLUSIONS/SIGNIFICANCE

A specific genotype-phenotype correlation in retinal function could not be demonstrated. KCNV2 mutations cause a unique form of retinal disorder illustrating the importance of K(+)-channels for the resting potential, activation and deactivation of photoreceptors, while phototransduction remains unchanged. The reduced oscillatory potentials further suggest an altered function of the inner retina. Besides the characteristically steep amplitude-versus-intensity relationship, flicker responses at intermediate frequencies (5-15 Hz) are significantly reduced and shifted in phase.

PIII and Derived PII Analysis in a Patient with Retinal Dysfunction with Supernormal Scotopic ERG

PURPOSE

To present electroretinographic (ERG) findings in a patient with retinal dysfunction with supernormal scotopic ERG, and to analyze rod and cone PIII components and rod inner nuclear layer (derived PII) responses.

PATIENT

A Japanese 11-year-old girl complained of poor visual acuity. There was no parental consanguinity in her family. The corrected visual acuity was 0.7 in both eyes. No abnormal finding was observed in both fundi.

METHODS

The patient underwent full-field ERGs. Rod and cone a-waves were analyzed using photoreceptor models. The derived PII responses were analyzed using a technique described by Hood and Birch.

RESULTS

In the photopic ERG, responses to single flash and 30-Hz flicker were attenuated. In the scotopic ERG, b-wave was supernormal in amplitude in response to intense flashes, but smaller than normal and markedly delayed over a lower range of flash intensities. By the PIII analysis, phototransductions (values of S) of both rod and cone were remarkably decreased. The derived PII responses for this patient were larger than the responses for normal subjects, and the onset of the PII responses in this patient are significantly delayed compared to those in normal subjects.

CONCLUSIONS

The ophthalmological findings in this patient are consistent with previous publications of this disease. Although it has been reported that the sites of disease action were beyond the outer segment (values of S were within the normal range), our results suggest that photoreceptors could be involved in sites of disease action in at least some patients with this disease.

Evidence supportive of a functional discrimination between photopic oscillatory potentials as revealed with cone and rod mediated retinopathies

We report on a family where four of the eleven children presented with reduced visual acuities, a red-green deficit at the Farnsworth-Munsel FM 100-hue test, normal appearing fundi and unexpected electroretinographic findings. Light- (photopic) and dark- (scotopic) adapted electroretinograms (ERG) and oscillatory potentials (OPs) were obtained following an accepted standard protocol. The b-wave of their photopic ERG was significantly more attenuated than the a-wave due to the specific abolition of OP4, while the amplitudes of OP2 and OP3 were within the normal range, giving to the b-wave a truncated appearance reminiscent of that seen in congenital stationary night blindness (CSNB) with myopia. Interestingly in the latter condition, which is believed to result from an ON-retinal pathway anomaly, it is OP2 and OP3 which are specifically abolished while OP4 is of normal amplitude thus resulting in an OP response pattern which complements that seen with our patients. Also of interest is the fact that, in our patients, the amplitude of the dark-adapted OP2 was, on average, 240% larger than that measured in light-adaptation while, in normal, a non-significant 14% increase is noted; a finding which is in keeping with other studies reporting supernormal scotopic ERGs in some forms of cone dystrophies. Based on the photopic OP response pattern, our patients represent the electrophysiological complement of patients affected with CSNB. Interestingly their symptoms are also complementary, a finding which could support a functional discrimination between the photopic OPs.

Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: applications and methods

The impact of a disease on phototransduction can be assessed by fitting the leading edge of the rod a-wave to high-energy flashes with a quantitative expression. Two parameters of rod receptor activity are obtained, S (sensitivity) and Rm (maximum response). In this study, the meaning of these parameters and examples of conditions that change them were examined. In addition, a new protocol was developed for obtaining these parameters. A set of three to five white flashes were first presented in the dark and then on an adapting field (30 cd/m2). Subtracting the light-adapted responses from the dark-adapted responses yielded isolated rod a-wave responses. A clinical protocol was developed based on a single white flash energy. It is possible to determine whether a disease is producing a change in S and/or Rm with this single flash energy without the use of any equations.

Cone dystrophies with negative photopic electroretinogram

A scotopic electroretinogram with an a-wave amplitude larger than the b-wave amplitude traditionally is termed 'negative'. Six male patients with negative photopic electroretinograms were examined; three of them suffered from progressive cone dystrophy, in which negative electroretinograms are unusual. Another patient without symptoms was the brother of a patient with cone dystrophy. These patients are compared with others who characteristically have negative electroretinograms-one patient with incomplete congenital stationary night blindness and another with X linked congenital retinoschisis. Differential diagnosis between these unusual cases of cone dystrophies and X linked retinoschisis or congenital stationary night blindness was possible with funduscopy, adaptometry, and evaluation of progression, but not with the electroretinogram. Inner retinal defects may occur in cone dystrophies as indicated by the negative electroretinogram. The waveform variations between our patients may be due to different inner retinal defects. The findings in two brothers indicate that cone dystrophy and inner retinal defects may be inherited separately.

Cone dysfunction and supernormal scotopic electroretinogram with a high-intensity stimulus. A report of three cases

An unusual form of scotopic electroretinogram with a bright white stimulus, which consisted of a rectangular a-wave of normal amplitude and a b-wave of supernormal amplitude, was recorded in three patients with cone dysfunction. In addition to poor visual acuity, abnormal color vision and reduced amplitude of the photopic electroretinogram, these patients showed a 2-log unit elevation of the dark-adaptation threshold. Funduscopic examination and fluorescein angiography revealed fine granular pigment disturbances at the macula. The relationship between the response of the dark-adapted electroretinogram versus stimulus intensity was unique to these patients. The b-wave thresholds were elevated by 1 log unit. The b-waves were reduced in amplitude and markedly delayed in implicit time to dim stimuli, but supernormal in amplitude and normal in implicit time to bright stimuli.