An extended 15 Hz ERG protocol (2): data of normal subjects and patients with achromatopsia, CSNB1, and CSNB2

Caring for Hereditary Childhood Retinal Blindness

Inherited retinal diseases (IRDs) are a major cause of incurable familial blindness in the Western world. In the pediatric population, IRDs are a major contributor to the 19 million children worldwide with visual impairment. Unfortunately, the road to the correct diagnosis is often complicated in the pediatric population, as typical diagnostic tools such as fundus examination, electrodiagnostic studies, and other imaging modalities may be difficult to perform in the pediatric patient. In this review, we describe the most significant IRDs with onset during the pediatric years (ie, before the age of 18). We describe the pathogenesis, clinical presentation, and potential treatment of these diseases. In addition, we advocate the use of a pedigree (family medical history), electroretinography, and genetic testing as the 3 most crucial tools for the correct diagnosis of IRDs in the pediatric population.

The morphology of human rod ERGs obtained by silent substitution stimulation

Purpose

To record transient ERGs from the light-adapted human retina using silent substitution stimuli which selectively reflect the activity of rod photoreceptors. We aim to describe the morphology of these waveforms and examine how they are affected by the use of less selective stimuli and by retinal pathology.

Methods

Rod-isolating stimuli with square-wave temporal profiles (250/250 ms onset/offset) were presented using a 4 primary LED ganzfeld stimulator. Experiment 1: ERGs were recorded using a rod-isolating stimulus (63 ph Td, rod contrast, C_rod = 0.25) from a group (n = 20) of normal trichromatic observers. Experiment 2: Rod ERGs were recorded from a group (n = 5) using a rod-isolating stimulus (C_rod = 0.25) which varied in retinal illuminance from 40 to 10,000 ph Td. Experiment 3: ERGs were elicited using 2 kinds of non-isolating stimuli; (1) broadband and (2) rod-isolating stimuli which contained varying degrees of L- and M-cone excitation. Experiment 4: Rod ERGs were recorded from two patient groups with rod monochromacy (n = 3) and CSNB (type 1; n = 2).

Results

The rod-isolated ERGs elicited from normal subjects had a waveform with a positive onset component followed by a negative offset. Response amplitude was maximal at retinal illuminances <100 ph Td and was virtually abolished at 400 ph Td. The use of non-selective stimuli altered the ERG waveform eliciting more photopic-like ERG responses. Rod ERGs recorded from rod monochromats had similar features to those recorded from normal trichromats, in contrast to those recorded from participants with CSNB which had an electronegative appearance.

Conclusions

Our results demonstrate that ERGs elicited by silent substitution stimuli can selectively reflect the operation of rod photoreceptors in the normal, light-adapted human retina.

Rod Electroretinograms Elicited by Silent Substitution Stimuli from the Light-Adapted Human Eye

PURPOSE

To demonstrate that silent substitution stimuli can be used to generate electroretinograms (ERGs) that effectively isolate rod photoreceptor function in humans without the need for dark adaptation, and that this approach constitutes a viable alternative to current clinical standard testing protocols.

METHODS

Rod-isolating and non-isolating sinusoidal flicker stimuli were generated on a 4 primary light-emitting diode (LED) Ganzfeld stimulator to elicit ERGs from participants with normal and compromised rod function who had not undergone dark-adaptation. Responses were subjected to Fourier analysis, and the amplitude and phase of the fundamental were used to examine temporal frequency and retinal illuminance response characteristics.

RESULTS

Electroretinograms elicited by rod-isolating silent substitution stimuli exhibit low-pass temporal frequency response characteristics with an upper response limit of 30 Hz. Responses are optimal between 5 and 8 Hz and between 10 and 100 photopic trolands (Td). There is a significant correlation between the response amplitudes obtained with the silent substitution method and current standard clinical protocols. Analysis of signal-to-noise ratios reveals significant differences between subjects with normal and compromised rod function.

CONCLUSIONS

Silent substitution provides an effective method for the isolation of human rod photoreceptor function in subjects with normal as well as compromised rod function when stimuli are used within appropriate parameter ranges.

TRANSLATIONAL RELEVANCE

This method of generating rod-mediated ERGs can be achieved without time-consuming periods of dark adaptation, provides improved isolation of rod- from cone-based activity, and will lead to the development of faster clinical electrophysiologic testing protocols with improved selectivity.

Achromatopsia: a review

PURPOSE OF REVIEW

The purposes of this article are to examine the literature published on achromatopsia and provide a comprehensive review of the clinical disease, genetic characteristics, and potential for therapy. Specifically, this article will describe recent advances in gene therapy in animal models, clinical features in human, and barriers to human translation.

RECENT FINDINGS

Building on prior success with adeno-associated virus (AAV) therapy in mice models for achromatopsia with mutations in the CNGB3, CNGA3, or GNAT2 genes, multiple cone-specific promoters have recently been developed and shown success in mice and nonhuman primates. A sheep CNGA3 model has also been characterized. Two clinical trials are under way: one to better characterize humans with achromatopsia and another to study a ciliary neurotrophic factor (CNTF) implant as a treatment for patients with the CNGB3 mutation.

SUMMARY

Genetic understanding and disease characterization of achromatopsia continues to evolve, as do gene therapy tools and animal models. The potential for the treatment of achromatopsia in humans with gene therapy shows great promise.

Rod- and cone-isolated flicker electroretinograms and their response summation characteristics

This study defined the amplitude and phase characteristics of rod- and cone-isolated flicker electroretinograms (ERGs) and determined how these responses summate to generate the nonreceptor-specific ERG. Full-field ERGs were obtained from six normally sighted subjects (age 26 to 44 years) using a four-primary LED-based photostimulator and standard recording techniques. The four primaries were either modulated sinusoidally in phase to achieve simultaneous rod and cone activation (ERGR+C; nonreceptor-specific) or in different phases to achieve rod-isolated (ERGR) and cone-isolated (ERGC) responses by means of triple silent substitution. ERGs were measured at two mean luminance levels (2.4 and 24 cd/m2), two contrasts (20 and 40%), and four temporal frequencies (2-15 Hz). Fundamental amplitude and phase for each condition were derived by Fourier analysis. Response amplitude and phase depended on the stimulus conditions (frequency, mean luminance, and contrast), however, for all conditions: 1) response phase decreased monotonically as stimulus frequency increased; 2) response amplitude tended to decrease monotonically as stimulus frequency increased, with the exception of the 24 cd/m2, 40% contrast ERGR+C that was sharply V-shaped; 3) ERGR phase was delayed (32 to 210 deg) relative to the ERGC phase; 4) ERGR amplitude was typically equal to or lower than the ERGC amplitude, with the exception of the 2.4 cd/m2, 40% contrast condition; and 5) the pattern of ERGR+C responses could be accounted for by a vector summation model of the rod and cone pathway signals. The results show that the ERGR+C amplitude and phase can be predicted from ERGR and ERGC amplitude and phase. For conditions that elicit ERGR and ERGC responses that have approximately equal amplitude and opposite phase, there is strong destructive interference between the rod and cone responses that attenuates the ERGR+C. Conditions that elicit equal amplitude and opposite phase rod and cone responses may be particularly useful for evaluating rod-cone interactions.

Rod and cone contributions to the dark-adapted 15-Hz flicker electroretinogram

PURPOSE

To evaluate rod and cone contributions to the dark-adapted 15-Hz flicker electroretinogram (ERG) across a broad range of stimulus luminances by comparing rod-isolating (ERGR), cone-isolating (ERGC), and non-receptor-specific (ERGR+C) responses.

METHODS

Dark-adapted, full-field 15-Hz ERGs were obtained from four normally sighted subjects (ages 29-36 years) using a four-primary LED-based stimulating system. The primaries were either modulated sinusoidally in phase (ERGR+C) or were modulated in counter-phase to achieve rod isolation (ERGR) or cone isolation (ERGC) by means of triple silent substitution. Measurements were made for a broad range of luminances (-2.5 to 1.8 log scot. cd/m(2) in 0.2 log unit steps). Fourier analysis was used to obtain the amplitude and phase of the fundamental response component at each stimulus luminance.

RESULTS

Stimulus luminance had different effects on response amplitudes and phases under the three paradigms. Specifically, ERGC amplitude and phase increased monotonically as luminance increased. The effects on ERGR+C and ERGR were complex: ERGR+C and ERGR amplitude was small and the phase decreased for low luminances, whereas amplitude and phase increased sharply at moderate luminances. For high luminances, ERGR+C amplitude and phase increased, whereas ERGR amplitude decreased and phase was approximately constant.

CONCLUSIONS

At low luminances, the ERGR+C and ERGR functions can be attributed to interactions between two rod pathways. At high luminances, the functions can be accounted for by interactions between rod and cone pathways (ERGR+C) or rod insensitivity (ERGR). The ERGR paradigm minimizes cone intrusion, permitting assessment of rod function over a large range of luminance levels.

Ultrastructural localization of GPR179 and the impact of mutant forms on retinal function in CSNB1 patients and a mouse model

PURPOSE

Complete congenital stationary night blindness (CSNB1) is characterized by loss of night vision due to a defect in the retinal ON-bipolar cells (BCs). Mutations in GPR179, encoding the G-protein-coupled receptor 179, have been found in CSNB1 patients. In the mouse, GPR179 is localized to the tips of ON-BC dendrites. In this study we determined the ultrastructural localization of GPR179 in human retina and determined the functional consequences of mutations in GPR179 in patients and mice.

METHODS

The localization of GRP179 was analyzed in postmortem human retinas with immunohistochemistry. The functional consequences of the loss of GPR179 were analyzed with standard and 15-Hz flicker ERG protocols.

RESULTS

In the human retina, GPR179 is localized on the tips of ON-BC dendrites, which invaginate photoreceptors and terminate juxtaposed to the synaptic ribbon. The 15-Hz flicker ERG abnormalities found in patients with mutations in GPR179 more closely resemble those from patients with mutations in either TRPM1 or NYX than in GRM6. 15-Hz flicker ERG abnormalities of Gpr179(nob5) and Grm6(nob3) mice were comparable.

CONCLUSIONS

GRP179 is expressed on dendrites of ON-BCs, indicating that GRP179 is involved in the ON-BCs' signaling cascade. The similarities of 15-Hz flicker ERGs noted in GPR179 patients and NYX or TRPM1 patients suggest that the loss of GPR179 leads to the loss or closure of TRPM1 channels. The difference between the 15-Hz flicker ERGs of mice and humans indicates the presence of important species differences in the retinal activity that this signal represents.