L- and M-cone driven large-field and multifocal electroretinograms in sector retinitis pigmentosa

Selective Stimulation of the Different Photoreceptor Classes by Silent Substitution in Psychophysical and Electroretinographic Measurements

The silent substitution technique allows creating photoreceptor-selective stimuli for psychophysical and electrophysiological tests. In contrast to other techniques, the purpose of silent substitution is not to make the targeted photoreceptor type more sensitive in comparison to the other types, but to make the stimulus invisible ("silent") to the other photoreceptor types. This allows selectivity independent of the retinal state of adaptation and enables comparing photoreceptor types under identical conditions. The foundations of these techniques will be explained in this paper. Furthermore, the importance of postreceptoral processing for the perception of photoreceptor-selective stimuli is discussed here. Although this technique is currently only available in specialized vision science labs, there is an enormous potential for clinical application.

Responses of Postreceptoral Pathways Elicited by L- and M-Cone Isolating ON- and OFF-Electroretinograms in Glaucoma Patients

Purpose

To compare the electroretinographical (ERG) responses elicited by L- and M-cone isolating ON- and OFF-sawtooth stimuli in normal subjects and glaucoma patients.

Methods

Twenty-one normal subjects and 44 primary open-angle glaucoma patients participated in the study. L- and M-cone isolating (18% cone contrast; 284 cd/m²) rapid ON- and rapid OFF-sawtooth (4 Hz) stimuli with two stimulus sizes (full-field (FF) and central 70° diameter) were generated using the triple silent substitution technique. ON- and OFF-response asymmetries were studied by adding the two (to obtain L-add and M-add responses). The initial positive (P) and subsequent late negative (LN) components of the L-add and M-add ERGs were compared between the subject groups and correlated with retinal nerve fiber layer thickness (RNFLT) and pattern ERG responses.

Results

The responses to L-ON and to M-OFF stimuli and vice versa resembled each other particularly with 70° stimuli. The P_L-add amplitudes were not significantly different between the normal subjects and glaucoma patients, whereas the LN_L-add amplitude was significantly (P < 0.01) smaller in the glaucoma patients. Both P_M-add and LN_M-add were not significantly different between the subject groups. The PERG amplitude with 0.8° check sizes and the 0.8°/16° amplitude ratio (PERG ratio) were significantly (P < 0.05) different between the subject groups. The 70° LN_L-add amplitude and the 0.8° PERG amplitude were significantly correlated with RNFLT.

Conclusions

The ERGs to 70° cone isolating sawtooth stimuli reflect cone opponency. The cone opponent ERG responses were not significantly different between glaucoma patients and normal subjects. Luminance driven L-add responses were significantly different, indicating that central luminance signals are mainly affected in glaucoma.

Multifocal electroretinographic responses in sector retinitis pigmentosa

HEADINGS

Multifocal electroretinography (mfERG) may be useful in the management of sector retinitis pigmentosa (SRP).

AIM

To compare multifocal electroretinographic responses in SRP, generalised retinitis pigmentosa (GRP), and healthy controls.

METHODS

Eighteen patients with SRP, twelve with GRP, and fifteen controls were included in the study. All participants underwent: complete ophthalmological examination, Humphrey visual field testing, full-field ERG, and mfERG. The mean P1 amplitude, the implicit time (IT), and the mapping of the local responses were evaluated.

RESULTS

The mean P1 amplitude was higher in the SRPs than in GRPs (p < 0.001), while it did not differ between SRPs and controls (p = 0.913). In the SRPs, the P1 amplitude in pathologic areas was higher than in the GRPs (p < 0.001). In normal areas, this parameter did not differ from the controls (p = 0.499). Moreover, in the SRPs, no differences in the P1 amplitude and the IT between pigmented and non-pigmented areas were found.

CONCLUSION

In the present study, the mfERG examination displayed significant differences between sector and generalised RP, showing normal values in sector RP even in pigmented areas. Considering the patients included in this study, SRP seems to represent a favourable variant of the disease, characterised by a limited retinal involvement and apparently mild functional damage. It is still unclear how these results can be extended to other forms of SRP.

Early diagnosis of Stargardt disease with multifocal electroretinogram in children

To present two pediatric cases where multifocal electroretinogram (mfERG) was able to establish an earlier diagnosis compared to full field electroretinogram (ERG) Case 1: an 11-year-old boy with reduced visual acuity, pale discs, macular pigmentation with white dots bilaterally. Case 2: a 12-year-old girl with reduced vision in her right eye, slight pallor of the right optic disc, intense pigmentation at both maculae and scattered punctate lesions throughout the peripheral fundi. Both had been investigated with electrodiagnostic tests according to the International Society of Clinical Electrophysiology for Vision protocol. Full-field ERGs for both children showed normal responses. Case 1: mfERG revealed a severe reduction in function in the inner 20°. Case 2: mfERG showed attenuated responses in each eye. Clinical examination and mfERG were consistent with Stargardt disease. mfERG is applicable to children and is a sensitive tool for early diagnosis of retinal dystrophies.

Optic disc pit with sectorial retinitis pigmentosa

Sectorial retinitis pigmentosa (RP) and optic disc pit (ODP) are rare clinical conditions. We present a 40-year-old woman with a history of mild night blindness and decreased vision in the right eye for about 5 years. Fundus examination revealed retinal pigmentary changes in the superior and inferotemporal sectors covering the macula and reduced arterial calibre and ODP at the temporal edge of the optic disc. In addition, fundus autofluorescence, spectral-domain optical coherence tomography, fluorescein angiography, and multifocal electroretinogram scans confirmed these clinical findings. Visual acuity was decreased due to an atrophic-appearing foveal lesion. No intervention was suggested because of the poor visual potential. To the best of our knowledge, the present study is the first to describe coexistent optic disc pit and sectorial RP in the superior and inferotemporal sectors covering the macula in the same eye with figures.

The clinical applications of multifocal electroretinography: a systematic review

Multifocal electroretinography (mfERG) is an investigation that can simultaneously measure multiple electroretinographic responses at different retinal locations by cross-correlation techniques. mfERG therefore allows topographic mapping of retinal function in the central 40-50 degrees of the retina. The strength of mfERG lies in its ability to provide objective assessment of the central retinal function at different retinal areas within a short duration of time. Since the introduction of mfERG in 1992, mfERG has been applied in a large variety of clinical settings. This article reviews the clinical applications of mfERG based on the currently available evidence. mfERG has been found to be useful in the assessment of localized retinal dysfunction caused by various acquired or hereditary retinal disorders. The use of mfERG also enabled clinicians to objectively monitor the treatment outcomes as the changes in visual functions might not be reflected by subjective methods of assessment. By changing the stimulus, recording, and analysis parameters, investigation of specific retinal electrophysiological components can be performed topographically. Further developments and consolidations of these parameters will likely broaden the use of mfERG in the clinical setting.

Progressive cone dystrophy with deutan genotype and phenotype

PURPOSE

To study the electroretinographic signals originating in the long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cone pathways by means of large-field and multifocal cone type-specific electroretinograms (ERGs) in a patient with progressive cone dystrophy.

METHODS

A 65-year-old male patient with colour vision disturbances (age at onset 10 years), loss of visual acuity (14 years), and central visual field defects (40 years) was investigated. Large-field flicker-ERG responses to stimuli that exclusively modulated the L-cones or the M-cones, or the two simultaneously (both in-phase and in counter-phase), were measured. Short-wavelength-sensitive (S) cones were not modulated. Multifocal ERGs (mfERGs) were also recorded, with a pattern-reversing display that modulated only the L- or the M-cones at equal cone contrasts and average quantal catches. Genetic analysis of L- and M-pigment genes was performed on genomic DNA isolated from peripheral venous blood.

RESULTS

The patient showed a normal rod-driven ERG but reduced cone-driven electroretinographic amplitudes with normal implicit times in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG. The large-field flicker-ERG responses to pure L-cone modulation were significantly above noise level but were substantially reduced in comparison with both normal trichromatic subjects and (otherwise normal) deuteranopes. The L-cone driven electroretinographic implicit times and phases were within normal limits. The M-cone driven electroretinographic responses were not detectable. A model fit of all the L- and M-cone driven flicker-ERG data revealed that the responses were exclusively driven by the L-cones. Consistently, the cone type-specific mfERGs showed severely reduced but detectable responses to L-cone-isolating stimuli. The M-cone driven multifocal-ERG responses were undistinguishable from noise. The L- and M-pigment gene array consisted of only a single L-pigment gene. The complete coding sequence of this gene was determined and showed no abnormality.

CONCLUSIONS

This patient exhibits a coincidence of progressive cone dystrophy and deuteranopia. The molecular genetic data of the L/M-pigment gene array is consistent with the deutan phenotype. It cannot be excluded that the rearrangement of the X-chromosome pigment gene array is responsible for the cone dystrophy in this patient. It is, however, suggested that the dichromacy and the cone dystrophy have different and independent genetic origins.

The assessment of L- and M-cone specific electroretinographical signals in the normal and abnormal human retina

Electroretinography (ERG) is a non-invasive method that can contribute to a description of the functional organization of the human retina under normal and pathological circumstances. The physiological and pathophysiological processes leading to an ERG signal can be better understood when the cellular origins of the ERG are identified. The ERG signal recorded at the cornea is initiated by light absorption in the photoreceptors which leads to activity in the photoreceptors and in their post-receptoral pathways. Light absorption in distinct photoreceptor types may lead to different ERG responses caused either by differences between the photoreceptors or between their post-receptoral pathways. The description of contributions of the different photoreceptor types to the ERG may therefore give more detailed insight in the origins of the ERG. Such a description can be obtained by isolating the responses of a single photoreceptor type. Nowadays, careful control of differently colored light sources together with the relatively well-known cone and rod fundamentals enables a precise description and control of photoreceptor excitation. Theoretically, any desired combination of photoreceptor excitation modulation can be achieved, including conditions in which the activity in only one photoreceptor type is modulated (silent substitution). In this manner the response of one photoreceptor type is isolated without changing the state of adaptation. This stimulus technique has been used to study the contribution of signals originating in the different photoreceptor types to the human ERG. Furthermore, by stimulating two or more photoreceptor types simultaneously, the interaction between the different signals can be studied. With these new techniques results of measurements in healthy subjects and patients with retinal diseases can be compared. This approach should ultimately help to develop better diagnostic tools and result in a fuller description of the changes and the pathophysiological mechanisms in retinal disorder. Finally, data obtained with cone and rod specific stimuli may lead to a reinterpretation of the standard ERG used in a clinical setting.