Electroretinographic findings in a patient with congenital stationary night blindness due to a novel NYX mutation

Luminance white noise electroretinograms (wnERGs) in mice

Purpose

To record and analyse electroretinograms (ERGs) to luminance stimuli with white noise temporal profiles in mice. White noise stimuli are expected to keep the retina in a physiologically more natural state than, e.g., flashes. The influence of mean luminance (ML) was studied.

Methods

Electroretinograms to luminance temporal white noise (TWN) modulation (wnERGs) were measured. The white noise stimuli contained all frequencies up to 20 Hz with equal amplitudes and random phases. Responses were recorded at 7 MLs between -0.7 and 1.2 log cd/m². Impulse response functions (IRFs) were calculated by cross correlating the averaged white noise electroretinogram (wnERG) responses with the stimulus. Amplitudes and latencies of the initial trough and subsequent peak in the IRFs were measured at each ML. Fourier transforms of the IRFs resulted in modulation transfer functions (MTFs). wnERGs were averaged across different animals. They were measured twice and the responses at identical instances in the 1st and 2nd recordings were plotted against each other. The correlation coefficient (r ² _repr) of the linear regression quantified the reproducibility. The results of the first and second measurement were further averaged. To study the underlying ERG mechanisms, the ERG potentials at the different MLs were plotted against those at the lowest and highest ML. The correlation coefficients (r ² _ML) were used to quantify their similarities.

Results

The amplitudes of the initial (a-wave-like) trough of the IRFs increased with increasing ML. The following positive (b-wave-like) peak showed a minimum at -0.4 log cd/m² above which there was a positive correlation between amplitude and ML. Their latencies decreased monotonously with increasing ML. In none of the IRFs, oscillatory potential (OP)-like components were observed. r ² _repr values were minimal at a ML of -0.1 log cd/m², where the MTFs changed from low-pass to band-pass. r ² _ML values increased and decreased with increasing ML when correlated with responses obtained at the highest or the lowest ML, respectively.

Conclusion

White noise electroretinograms can be reliably recorded in mice with luminance stimuli. IRFs resemble flash ERGs superficially, but they offer a novel procedure to study retinal physiology. New components can be described in the IRFs. The wnERGs are either rod- or cone-driven with little overlap.

Complete congenital stationary night blindness associated with a novel NYX variant (p.Asn216Lys) in middle-aged and older adult patients

BACKGROUND

Complete congenital stationary night blindness (CSNB) is a retinal disorder thought to be non-progressive. The purpose of this study was to characterize the clinical and genetic findings of middle-aged and older adult patients with X-linked complete CSNB.

METHODS

Three male CSNB patients (aged 62, 72, and 51 years) and one unaffected female carrier in a Japanese family were included in this study. Whole-exome sequencing (WES) was performed to determine the disease-causing variants. Co-segregation was confirmed in the family members. We performed a comprehensive ophthalmic examination on each patient.

RESULTS

In the 62-year-old patient, a novel hemizygous variant (c.648 C > A; p.Asn216Lys) of the NYX gene was identified by WES analysis. The other two patients carried the variant hemizygously, and the unaffected carrier harbored the variant heterozygously. The clinical and electroretinography (ERG) findings were very similar among all three patients. Fundus images exhibited high myopic chorioretinal atrophy with long axial length. Ultra-wide field fundus autofluorescence images showed no retinal degenerative changes except for changes resulting from high myopia and previous retinal diseases. The ERG findings showed no response in rod ERG, electronegative configuration with preserved a-waves in standard/bright-flash ERG, and preserved responses in cone and 30-Hz flicker ERG, which were compared with age-matched controls with high myopia.

CONCLUSIONS

We identified a novel missense NYX variant in a Japanese family with complete CSNB. Our clinical findings indicated that photoreceptor mediated ERG responses are well preserved even in middle-aged and older adult patients.

Macular sensitivity in patients with congenital stationary night-blindness

AIM

To evaluate and correlate mean light sensitivity thresholds (MLST) in patients with congenital stationary night-blindness (CSNB) in comparison with healthy subjects using microperimetry (MP1).

METHODS

Eleven patients with CSNB and 13 healthy subjects were compared. In all subjects, static threshold perimetry was performed using MP1 evaluating the central 6 mm of the retina. This central retinal area was divided into three rings through using the ETDRS grid algorithm with an innermost (1 mm), inner (3 mm) and outer ring (6 mm). The MLSTs were acquired in nine sectors of the ETDRS grid. A comparison of MLST was performed between both groups using a t-test (significance level p<0.005).

RESULTS

A significant reduction of MLST in the fovea (innermost ring, 1 mm) was observed for patients with CSNB (7.2±3.90 dB) in comparison to healthy subjects (19.7±0.75; p<0.0001). Similarly, comparison of MLST in all other sectors (superior/inferior/temporal and nasal) within the inner and outer ring revealed a statistically significant reduction in patients with CSNB compared with healthy subjects (p<0.001).

CONCLUSIONS

Examination of macular retinal sensitivity intensity using MP1 revealed for the first time a significant reduction of MLST within the central 6 mm of the retina in patients with CSNB compared with healthy subjects. This finding supports MP1 as an additional diagnostic tool when examining patients with retinal dysfunctions such as CSNB.

Poststimulus response characteristics of the human cone flicker electroretinogram

At certain temporal frequencies, the human cone flicker electroretinogram (ERG) contains multiple additional responses following the termination of a flicker train. The purpose of this study was to determine whether these poststimulus responses are a continuing response to the terminated flicker train or represent the oscillation of a resonant system. ERGs were recorded from 10 visually normal adults in response to full-field sinusoidally modulated flicker trains presented against a short-wavelength rod-saturating adapting field. The amplitude and timing properties of the poststimulus responses were evaluated within the context of a model of a second-order resonant system. At stimulus frequencies between 41.7 and 71.4 Hz, the majority of subjects showed at least three additional ERG responses following the termination of the flicker train. The interval between the poststimulus responses was approximately constant across stimulus frequency, with a mean of 14.4 ms, corresponding to a frequency of 69.4 Hz. The amplitude and timing characteristics of the poststimulus ERG responses were well described by an underdamped second-order system with a resonance frequency of 70.3 Hz. The observed poststimulus ERG responses may represent resonant oscillations of retinal ON bipolar cells, as has been proposed for electrophysiological recordings of poststimulus responses from retinal ganglion cells. However, further investigation is required to determine the types of retinal neurons involved in the generation of the poststimulus responses of the human flicker ERG.