Electroretinography

Electroretinography is a method for recording the total bioelectrical activity of all retinal neurons: negative a-wave - photoreceptors and positive b-wave - hyper- and depolarizing bipolar and Mueller cells. Electroretinogram (ERG) occurs when light stimuli of different sizes, shapes, wavelengths, intensities, durations, frequencies of light and dark adaptation are applied to the retina.

The electroretinogram records the action potential of the retina in response to light stimulation of a corresponding intensity, i.e. The potential between the active corneal electrode mounted in the contact lens (or a gold-plated gold electrode fixed on the lower eyelid) and the reference electrode on the forehead of the patient. Electroretinogram is recorded under conditions of light adaptation (photopic electro-retinogram) and tempo adaptation (scotopic electro-retinogram). Normally, the electroretinogram is two-phase.

• a-wave is the first negative deviation from the isoline, the source of which is the photoreceptors.
• b-wave - positive deviation, which is generated by Muller cells and reflects the bioelectrical activity of bipolar cells. The amplitude of the b-wave is measured from the negative peak of the a-wave to the positive peak of the b-wave, increases with the dark adaptation and with an increase in the brightness of the light stimulus; The b-wave consists of subcomponents: b1 (reflects the activity of rods and cones) and b2 (cone activity). A special registration technique makes it possible to distinguish between rod and cone responses.

The practical value of electroretinography is determined by the fact that it is a very sensitive method of assessing the functional state of the retina, which allows to determine both the most insignificant biochemical disorders and gross dystrophic and atrophic processes. Electroretinography helps to study the mechanisms of the development of pathological processes in the retina, facilitates early differential and topical diagnostics of retinal diseases, it is used to monitor the dynamics of the pathological process and the effectiveness of treatment.

The electroretinogram can be recorded from the entire area of the retina and from the local area of various sizes. The local electroretinogram, recorded from the macular area, allows to evaluate the functions of the conical system of the macular area. An electroretinogram caused by a reversible chess stimulus is used to characterize a neuron of the second order.

The isolation of the functions of the photopic (scobular) and scotopic (rod) systems is based on the difference in the physiological properties of the cones and rods of the retina, and therefore use the appropriate conditions in which each of these systems dominates. The cones are more sensitive to bright red stimuli presented in photographic conditions of illumination after preliminary light adaptation, suppressing rod activity, to a frequency of flickering above 20 Hz, rods to weak achromatic or blue stimuli under dark adaptation conditions, to a flicker frequency of up to 20 Hz.

The different degree of involvement in the pathological process of rod and / or cone systems of the retina is one of the characteristic features of any retinal disease, vascular, inflammatory, toxic, traumatic and other genesis, which determines the nature of electrophysiological symptoms.

The electroretinogram classification of electroretinograms is based on the amplitude characteristics of the main a and b-waves of the electroretinogram, as well as their temporal parameters. There are the following types of electroretinograms: normal, supernormal, subnormal (plus and minus negative), extinct, or unregistered (absent). Each type of electroretinogram reflects the localization of the process, the stage of its development and pathogenesis.

Normal electroretinogram

Includes 5 types of response. The first 3 species are recorded after 30 mimes of dark adaptation (scotopic), and 2 species - after 10 minutes of adaptation to diffuse illumination of medium brightness (photopic).

Scotopic electroretinogram

• rod-like response to a small, low-intensity white flash or to a blue stimulus: a high-amplitude b-wave and a low-amplitude or non-registered a-wave;
• mixed rod and cone response to a white flash of high brightness: pronounced a- and b-waves;
• Oscillatory potentials for a bright flash and with special registration parameters. Oscillations are recorded on the ascending "bend" of the b-wave and are generated by cells of the inner layers of the retina.

Photopic Electroretinogram

• The cone response to a single bright flash consists of an a-wave and a b-wave with small oscillations;
• the cone response is used to record an isolated cone response when stimulated with a flickering stimulus at a frequency of 30 Hz, to which the rods are insensitive. The cone response is recorded normally at a flash up to 50 Hz, above which individual responses are not detectable (critical flicker frequency).

The supernormal electroretinogram is characterized by an increase in a- and b-waves, which is noted at the first signs of hypoxia, drug intoxications, sympathetic ophthalmia, etc. The supernormal bioelectrical response in traumatic optic nerve puncture and its atrophy is caused by a violation of the excitation of retino-talamic centrifugal inhibitory fibers. In some cases, it is difficult to explain the nature of the supernormal electroretinogram.

Subnormal electroretinogram is the most frequently detected type of pathological electroretinogram, which is characterized by a decrease in a- and b-waves. It is recorded in dystrophic diseases of the retina and choroid, retinal detachment, uveitis with the involvement of the retina in the 1st and 2nd retinal neurons, chronic vascular insufficiency with microcirculation disorder, some forms of retinosis (X chromosome, sex-linked, Wagner syndrome) and and so forth.

Negative electroretinogram is characterized by an increase or preservation of a-wave and a small or significant decrease in the b-wave. Negative electroretinogram can be observed in pathological processes, in which changes are localized in the distal parts of the retina. Negative electroretinogram occurs with ischemic thrombosis of the central vein of the retina, drug intoxication, progressive myopia and congenital stationary night blindness, Ogushi's disease, X-chromosomal juvenile retinoschisis, retinal metallosis and other pathologies.

The extinct, or unrecorded (absent) electroretinogram is an electrophysiological symptom of severe irreversible changes in the retina with its total detachment, developed metallosis, inflammatory processes in the eye membranes, occlusion of the central artery of the retina, and pathognomonic signs of retinitis pigmentosa and amberusa Lebera. The absence of an electroretinogram was noted in the case of gross irreversible changes in neurons, which can be observed in dystrophic, vascular and traumatic lesions of the retina. The electroretinogram of this type is recorded in the terminal stage of diabetic retinopathy, when a coarse proliferative process extends to the distal parts of the retina, and with vitreoretinal dystrophy of Favre-Goldman and Wagner.