The human electroretinogram: DC recordings at low and conventional stimulus intensities. Description of a new method for clinical use

From basic to clinical research: a journey with the retina, the retinal pigment epithelium, the cornea, age-related macular degeneration and hereditary degenerations, as seen in the rear view mirror

PURPOSE

This Acta Ophthalmologica Award and Gold Medal Honorary Lecture (the Lundsgaard Gold Medal Honorary Lecture) reviews some of the work I have carried out with my mentors and many of my wonderful collaborators and research students over more than 40 years, also including related work by other groups. It concentrates on the basic electrophysiology and ultrastructure of the retina and the retinal pigment epithelium (RPE), as well as covering basic and clinical aspects of the cornea, contact lenses, age-related macular degeneration (AMD) and hereditary diseases.

METHODS

The review describes research performed using light and electron microscopy, basic and clinical electrophysiology, genetics and biochemistry in animal experiments and in research on patients. It also outlines clinically used techniques, such as laser and photodynamic treatment and scanning laser ophthalmoscopy.

RESULTS

The paper reports on the following subjects: the mechanisms behind some of the electrical potentials originating in the retina and the RPE and the use of these potentials in hereditary diseases; corneal receptors for lectins and presumably for bacteria; the turnover of the photoreceptor outer segment and the formation of lipofuscin, including the relation of these processes to AMD; certain treatments for AMD, and hereditary degenerations in animal models, such as the RPE65 gene mutation in Briard dogs, which makes them a model of Leber's congenital amaurosis. The dogs are now treated successfully with gene therapy in the USA, and a clinical trial is in preparation.

CONCLUSIONS

During the last 40 years we have had the good fortune to experience a dramatic growth in knowledge and understanding within ophthalmic science of basic mechanisms. Huge progress has been made in diagnostics and clinical ophthalmological treatments, much to the benefit of our patients. Even a small contribution made by my group to these developments has been well worth the effort, particularly as scientific work is not just deeply satisfying: it is also fun!

A simple and stable d.c.electrode for ocular electrophysiology

We describe the fabrication of a simple silver-silk electrode which permits remarkably stable d.c.recording of the electroretinogram (ERG) and the optic nerve response (ONR). A saline soaked wick of surgical silk, guided into a polyethylene tube connects the tissue to a coil of Ag/AgCl wire placed in a small glass vial, which is filled with 0.9% NaCl. The vial that holds the tube and the wire is closed with a rubber cap allowing easy refilling with NaCl. Examples of the usefulness of the new silver-silk electrode are shown. We applied it in experimental work in the isolated arterially perfused cat eye for d.c.recordings of the ERG and the optic nerve response (ONR), and also in vivo, in anesthetized mice to record c-waves.

The late positive retinal potential in dogs

The late positive potential of the mammalian electroretinogram has been called the 'PI' or the 'c-wave' potential. It is unusual among retinal potentials because its peak implicit time increases in response to increasing stimulus intensity and because it cannot be demonstrated consistently in small samples of normal humans or normal dogs. We recorded wideband (DC-1 kHz) responses from 34 normal Beagles or dogs of similar size. Of the 34, 11 produced a late positive potential set that satisfied the criteria for c-waves. Multiple aspartate injections always increased c-wave amplitude and stimulus-response linearity in all 'producers'. Non-producers were never converted to producer status by aspartate blocking of the inner retina. Interaction of late positive and negative potentials and the possible influence of normal individual variations in the trans-epithelial potential are discussed. Individual mammal c-wave production is controlled by outer retinal phenomena which vary between individuals.

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

A male patient suffering from cone dystrophy was followed over 9 years. In addition to the typical clinical and electrophysiologic signs, supernormal b-waves were found in the dark-adapted electroretinogram. Our case is compared with 12 similar patients described in the literature. Our patient differed from the other patients in the following aspects: he was male and had a congenital stationary disease with a small pigment epithelial scar in the left eye only and no other fundus changes up to the age of 22 years. He did not complain of night blindness. The dark-adapted electroretinogram of our patient showed a normal b-wave threshold with increased b-wave amplitudes and markedly prolonged b-wave latencies and implicit times. This combination of signs has not been reported to date in any other patient and points towards a postreceptoral defect of the interneuronal connection.

X-linked congenital retinoschisis

The natural history and electrophysiological findings of 52 patients with X-linked congenital retinoschisis with a follow-up of up to 26 years are described. The mean visual acuity was reduced to 0.24 +/- 0.2 and remained unchanged in most patients during this time. If visual loss occurred, it usually happened in the first decennium. The complications were retinal detachments in 11% and vitreous hemorrhages in 4% of the eyes. In general, the vitreous hemorrhages resolved spontaneously. Retinal detachments were treated successfully with conventional buckling procedures. Redetachments occurred in about 40%. Prophylactic laser coagulation was of no use because it was complicated by detachment in 43% of our series. The electro-oculogram was usually normal. In addition to the known electrorentinographic findings of normal a-wave and reduced b-wave amplitudes, we found prolonged b-wave latencies and implicit times, as well as a reduced 30 Hz flicker response.

The C-wave in hereditary degenerations of the ocular fundus

The c-wave of the human direct current ERG may give new insights about the involvement of the retinal pigment epithelium in hereditary degenerations. In our single-sweep recordings from 236 alert patients we saw reduced c-waves not only in vitelliforme macular degenerations but also in dominantly inherited drusen, Stargardt's disease, cone dystrophies, and x-linked retinoschisis. In achromatopsia the c-wave was close to normal. The relationship of the b- and c-wave, however, was altered only in Best's disease, cone dystrophy, and x-linked retinoschisis. We postulate that the c-wave when more severely reduced than the b-wave reflects not merely the dysfunction of the pigment epithelium but more precisely whether this retinal layer is involved earlier than the photoreceptors.

Changes in corneal DC-potentials associated with changes in pupillary diameter

Using light-emitting diodes we stimulated monocularly with light intensities of between 7.5 and 1,800 cd/m2, and recorded simultaneously the Dc-electroretinogram and pupillary movements of the stimulated and the contralateral eye. In some investigations, the visual evoked potential and the activity from the periorbicular muscles were also recorded. Various drugs acting on the autonomous nervous system were topically applied and their efforts studied. In the eye with an untreated pupil, stimulated or contralateral, a corneo positive potential coincident with pupillary constriction was seen (cu-wave), provided the pupil was large beforehand, a corneonegative deflection dominated, which was also coincident with pupillary constriction (mu-wave). Parasympathicolytics or -mimetics abolished both the cu and mu-wave. We conclude that the cu-wave is related to depolarization of the sphincter pupillae during constriction, whereas the mu-wave might be related to a modification of the potential distribution between the pigment epithelium and the tissue surrounding the eye in the case where the pupil is constricted beyond a critical point.

[Information conveyed by electroretinography (author's transl)]

Clinical evaluation of the function of the human retina by subjective, psychophysical tests can be complemented by objective, electrophysiological methods such as electroretinography. The electroretinogram (ERG) is a transient action potential generated by the vertebrate retina in response to light. Using standardized stimulation, after assessing the normal range of the ERG the investigator can establish early diagnosis, improve prognostic statements and document the course of retinal diseases. A method to selectively stimulate the rod- and cone-system, developed by Gouras and co-workers, has proved to allow further differentiation of heredodegenerative diseases of the retina. The ERG monitors objectively the function of the retina in infants uncapable of cooperating in psychophysical tests and in patients suffering from opacities of the optic media. ERG results should be evaluated synoptically with psychophysical data, ophthalmoscopy, fluoresceinangiography and possibly with EOG and VER recordings. Under experimental conditions the ERG serves to monitor the sensitivity of the retina in vivo as well as in vitro in physiological, pharmacological and toxicological studies.

The c-wave of the human electroretinogram in central retinal artery occlusion

The pigment epithelium-receptor complex, which is supplied from the choroidal circulation, is the main source of the c-wave of the ERG. Occlusion of the central retinal artery (OCRA), which causes serious damage to the inner retinal layers, should therefore not primarily be expected to affect the c-wave amplitude. Nevertheless, conflicting reports of increased, decreased and unchanged c-waves have appeared in the literature. In the present study on four patients with OCRA the affected eyes showed diminished b- and c-waves. On the other hand, both the a-wave and the trough between the b- and c-waves were accentuated in the diseased eyes. Experiments in progress on Cynomolgus monkeys with experimentally induced unilateral OCRA show similar results.

The influence of stimulus duration on the human d.c. registered c-wave. A quantitative study

The c-wave of the human ERG was studied at different stimulus durations with a d.c. technique which permitted stable and reproducible recordings. With increasing stimulus lengths the implicit time increased up to a maximum of about 5.5 s. Also the amplitude of the c-wave rose. However, it was influenced by positive and negative off-effects, seen in most volunteers, and at several stimulus lengths superimposed upon the peak of the c-wave. This fact must be considered when developing a standardized method for measuring the c-wave amplitude proper.

Experiments concerning the human C-wave

The dependence of the human c-wave from the step amplitude was studied qualitatively. The maximum of the c-wave is achieved with light stimuli longer than 10 sec. After a dark period of 10 sec a fully developed c-wave appears. Also, in response to a brilliant photoflash a c-wave could be recorded. The influence of a mobile pupil on the response in the DC-ERG is demonstrated. Responses of the retinal potential to square wave and sinusoidal stimuli of different period times have been registered.

A method for DC-ERG recording of alert humans

We developed a technique for DC-coupled ERG recording of the alert human. The equipment consists of three units: a new cornea suction glass, a separate electrode, and a vacuum control device. We get stable recordings of more than one-half hour. By comparison with the EOG the identity of the EOG "on"-peak and ERG c-wave is assured. A fine structure of DC-ERG recording of 100 muV amplitude is closely correlated to the variation of the blood pressure in the ophthalmic artery. It may either be caused by the variable electric conductivity of the eyeball due to the blood pressure or it may reflect a variation of the retinal potential itself.

Effects of ethyl alcohol on the directly recorded standing potential of the human eye

The effects of ethanol on the human standing potential (SP) were studied with a recently developed method, which allows direct SP recordings by means of a suction contact lens, temperature stabilized calomel electrodes and d.c. amplification. It is well known that the human SP oscillates with a frequency of about 2/hour in response to a sudden change in illumination. In the present paper marked cyclic variations of the SP, resembling damped oscillations, were provoked by a small oral dose of ethyl alcohol. A first maximum was reached after about 10 min. The difference in amplitude between the peak and the trough of the first oscillation was of the order of 4 mV. The oscillatory frequency was about 2/hour. The length of a cycle varied between 25 and 34 min in different volunteers, being fairly constant in the same subject on different occasions. The SP response to ethanol was similar both under scotopic and photopic conditions. The results correlate well with earlier findings of 2/hour oscillations in c-wave amplitude in response to ethanol, as may be expected considering the partly common origin of the c-wave and the SP.

Covariation of the simultaneously recorded c-wave and standing potential of the human eye

The c-wave and the directly recorded standing potential (SP) of the human eye were studied with the aid of a recently developed method including matched temperature stabilized calomel electrodes, d.c. amplifiers and a suction contact lens. This technique, which does not require general anaesthesia, permits simultaneous direct d.c. recordings of the SP and the c-wave in human volunteers during long-term experiments. Upon repetitive light flashes (stimulus duration 1 sec, interval 20 sec and flash intensity 4.5 rel. log units above b-wave threshold) both variables responded with slow amplitude oscillations with a frequency of about 2/hour. The oscillations were similar as to phases and frequencies. Both the potentials are held to be generated mainly in the pigment epithelium. Considering this partly common origin the observed covariation was an interesting finding.