Diabetic retinopathy . Features of the application of electrophysiological methods.
Standards ISCEV, why it is important to follow ?Which errors can result if these standards ?

Frisky SV MRI GB them . Helmholtz

According to the World Health Organization in 2011, the number of diabetics has exceeded 220 million people in Russia today there are more than 8 million people with diabetes. Prevalence and social significance of diabetes ranks third after cancer and cardiovascular diseases.

Diabetes causes damage to the cardiovascular and nervous systems of the body , and leads to partial or complete loss of vision . If the duration of diabetes more than 15 years , approximately 10% of patients are visually impaired and 2% - blind.The most common cause of visual loss in diabetes is a disease of the retina - diabetic retinopathy . Currently, it is the main cause of irreversible blindness among working-age population of developed countries.

Effective treatment for proliferative diabetic retinopathy is panretinal laser photocoagulation . However, its main side effect is a reduction of a few weeks after the completion of laser photocoagulation . This decrease is due to the appearance of or worsening of macular edema . In some instances, this edema is reversible in some - not. Therefore, in modern ophthalmology particularly acute problem of early diagnosis of diabetic retinopathy , its prevention and treatment. Diagnosis in the early stages will increase the effectiveness of treatment and reduce the likelihood of loss of vision .

It is now established that, even before the onset of clinical signs of diabetic retinopathy is a change of bioelectric potentials of neurons and glial cells of the retina Muller , which can be registered by using electrophysiological techniques, in particular electroretinography . These methods have a number of characteristics that make them indispensable in the evaluation of functional abnormalities in the retina.

Electroretinography based on the total registration of bioelectric potentials of the retina in response to light stimulation .Light stimuli with different parameters duration , brightness and wavelength form bioelectric potentials from the various layers of the retina and cellular structures . These potentials - elektroretinograficheskie ( ERG ) signals - recorded using three electrodes : corneal (corneal electrode), indifferent (reference electrode) and common (ground electrode). To interpret the results of research and forming an opinion should be analyzed for the ERG signals , which limit automated thanks to the software that came with the recording equipment .

Terms of stimulation parameters amplifiers and methods of the study are standardized by the International Society of Clinical Electrophysiology of (International Society for Clinical Electrophysiology of Vision (ISCEV)), which ensures the representativeness of the results. However, there are many factors affecting the quality of the recording ERG signals, and thus , the reliability of the study results of patients with diabetic retinopathy that physician electrophysiologist should consider .

Photic stimulation . In accordance with standard ISCEV light stimulant should form the luminous flux in all directions to ensure uniform illumination of the retina. For these purposes the integrating sphere ( Ganz - feld sphere) - hollow ball -lined matte white paint with a high coefficient of diffuse reflection . If you do not take into account the instability of the parameters of a light stimulant , then the end result will affect primarily the size of the pupil of the patient , the level of dark ( or light ) adaptation, and too often follow the light flashes when dark adaptation , which can lead to maladjustment of the visual system and misleading results.

Parameters of light stimuli strictly regulated standard ISCEV, in the last edition ( from 2008 ), the light stimulus and background lights set a particular value (rather than the range of acceptable values ​​, as it was in earlier versions of the standard ) . Compliance with the standard conditions of light stimulation to avoid errors when comparing and interpreting the results obtained in different clinics .

Electrodes . Unfortunately, the electrodes , which are the primary converters of bioelectric potentials of the retina, neglected during electrophysiological studies . Despite the fact that the market offers a large number of possible electrodes , their selection may not be evident . It should immediately be noted that it is mostly about corneal electrodes.

In accordance with selection ISCEV corneal electrode lies on the investigator , however, their further studies , it should use only the selected type of electrode , because they differ in structure , material and shape of the current collector surface, which leads to different results ( relative change in amplitude of the signal can be up to 50 % when applied at the different electrodes of the same patients ) . However, not only reduced the amplitude can skew the results. Having a contact between the electrode and the corneal surface of the eye leads to complicated electrochemical processes between the lacrimal fluid and the surface of the electrode current collector . The effect of these processes on the ERG signals expressed as uncontrolled shifting contours , signal drift and high-amplitude noise.

Based on these studies , it was found that all of what is shown most adequate corneal electrodes for recording bioelectric potentials of the retina are the ERG-jet, DTL and HK loop. It should be borne in mind that the use of very thin and " invisible " to the eye of the electrodes may lead to difficult to control the displacements of the signal curve , which is fraught with incorrect interpretation of the results .

Registration method . On the one hand , the methodology of the study is extremely automated and requires a doctor - only electrophysiologist strict implementation of all its stages. On the other hand , each of the stages may have some difficulties to be solved quickly and efficiently . Consider some of the possible problems with registration and methods of solving them .

Lack of ERG signal. One of the main reasons - contact failure of one of the three electrodes . Monitor their position on the patient should be using the built - in Ganz feld field infrared camera , as well as by measuring the electrode impedance. It is likely that one of the electrodes simply come unstuck . In any case, the impedance value should achieve no more than 5 ohms . Another possible reason for the lack of signal - movement of the eyeballs or eyelids that cause significant interference of high amplitude . These false signals are not perceived by the system as ERG signals and blocked. Eye movement or blinking should be monitored by a video camera inside the Ganz - feld sphere.

The high level of interference in the signal. The reasons can be many , but the main are: the absence of a ground fault or recording equipment ; availability of technical origin interference ( electronic equipment, located in the area where the research conducted ERG ); supply noise 50/100 Hz ; micromotion eyes or offset electrodes during registration.Unfortunately , no universal solution to this problem does not exist. If the ERG signal contains non-harmonic noise ( not 50 Hz), the increase in the number of synchronous averaging can reduce noise and highlight the desired signal. If long-term re- recording is not possible , it is permissible to use the smoothing filters . For example, in the software system Roland Consult you can configure any of bandpass filters , which allows to select individual settings for each type of ERG signals.

Harmonic interference ( 50/100 Hz) can not be eliminated by using synchronous averaging , as power supply noise at each averaging will develop herself. To resolve this issue , a special amplifier with high common mode suppression ratio , which allows almost completely get rid of network interference. However, you should remember that the most common mode is effectively suppressed only if equality (or similar terms) the impedance of the electrodes. Therefore, when measuring the impedance is important not only to monitor its value , but also to control the difference between the values ​​of the impedance of the stratum and the indifferent electrode .

Assess the quality of registration ERG signal should be made as follows. Before the start of the study parameters in the software necessary to set the time interval to the light stimulus ( not less than 20 ms ), which will be preceded by the ERG signal. In this time interval will be no useful signal as an action potential is caused by the potential of the retina and is generated after a flash of light . If after registration ERG signal in the time domain to the level of the light stimulus contour line behaves "calmly " and much less noise ERG signal, the recording quality can be considered high and the results of the study are reliable.

Diabetic retinopathy . For a sample of 72 patients with diabetes at different stages of diabetic retinopathy ( non-proliferative stage ( NPDR ) preproliferative step ( PPDR ) , proliferative stage (PDR ) ) have shown that the use of different corneal electrodes within a single laboratory , and lack of quality control registration ERG signals leads to significantly depressed values ​​of sensitivity and specificity determining stages of diabetic retinopathy. Verification stages of diabetic retinopathy in diabetic patients was performed using ophthalmoscopy and fluorescein angiography .