visual evoked potential

The visual evoked potential is an electrical response to the visual stimuli in the occipital region of the cerebral cortex. It is a potential change caused by the retina receiving stimulation and conduction to the occipital cortex through the visual pathway. The specific reaction potential that is strictly related to the stimulation signal is usually called a specific evoked potential. This specific evoked potential is a series of neuroelectric activities in which the induced information is continuously combined at different levels of the neural pathway. Since there is a constant temporal relationship between the evoked response and the induced stimulus, the level of the neural pathway represented by the different responses in the evoked potential can be determined based on the nerve impulse conduction time. If a lesion or dysfunction occurs at a certain level, the corresponding part of the evoked potential will have a change in latency, amplitude, and waveform. Basic Information Specialist Category: Ophthalmic Examination Category: Applicable gender: whether men and women apply fasting: not fasting Tips: Maintain a normal diet and schedule. Normal value In addition to the quantitative assessment of visual dysfunction, VEP has certain diagnostic and differential diagnostic value for various visual dysfunction lesions. Although VEP is an objective method for assessing visual function, it also pays attention to the following problems in forensic identification: (1) VEP belongs to cortical potential, and mental state has a certain influence on the outcome of VEP, so subjects should be kept in the test. In a state of waking and quiet. (2) For the test results of P-VEP, it is necessary to pay special attention to the degree of gaze of the subject. Poor gaze can cause the latency of P-VEP to prolong, and the amplitude is reduced or even disappeared. Do not mistake the visual function for this; 3) Patients with severely damaged individual visual field, although sometimes have good visual acuity (0.1 to 0.3), can also cause no wave of VEP. Therefore, attention should be paid to the central visual function and peripheral visual function while analyzing the VEP results. (4) Patients with low vision may not have abnormalities in VEP and ERG. This can be used as a means of identification for pseudo-blindness. The pseudo-blind VEP and ERG are normal. Clinical significance Visual evoked potentials (VEP) are known to detect functional integrity from the retina to the visual cortex, ie the entire visual pathway. The evoked potentials (P100) were recorded in the visual cortex by the left and right eyes, respectively, by a specific checkerboard flip mode. According to the P100 latency and amplitude analysis, the level of lesion damage in the retina, pre-optical or post-optical crossover, objective assessment of the degree of damage, treatment effect and prognosis. Because VEP is a sensitive means to detect subclinical damage of the optic nerve, the diagnosis and identification of some clinical diseases in neurology and ophthalmology, visual evoked potential (VEP) has unique advantages. 1, optic neuritis In optic neuritis, the VEP latency is prolonged and the amplitude is reduced. Usually, the amplitude variability is large, the latency variability is small, the P100 of the optic nerve fiber is delayed, the average peak latency is almost extended by 30%, and the amplitude is reduced by 50%. The VEP of the affected side eyes was normal. 2, other abnormal optic nerve diseases The pathological involvement of optic nerve caused by multiple causes can affect VEP. In Leber's hereditary optic neuropathy, there may be VEP abnormalities. Many patients with severe visual impairment do not record VEP, or show small response, waveform dispersion and delay. In patients with ischemic optic neuropathy, VEP may occur. Delay, but the reduction of amplitude is usually more characteristic; VEP amplitude is significantly reduced in toxic amblyopia, but the incubation period is usually normal; VEP in glaucoma patients is often abnormal in latency. 3, multiple sclerosis VEP has a high diagnostic value for the diagnosis of multiple sclerosis, which has been repeatedly confirmed by a large number of studies, the positive rate is usually 70% -97%. The diagnosis of multiple sclerosis depends on clinical and laboratory evidence that multiple lesions are present in the central nervous system. In this disease, the optic nerve is one of the most frequently affected sites. VEP latency is an objective means of detecting visual pathway damage. Even when these lesions are in a subclinical state, VEP techniques may suggest a subclinical basis for visual pathway involvement in patients with multiple sclerosis. Therefore, when a lesion has been clinically confirmed, especially below the level of the foramen magnum, the detection of visual system lesions by VEP is very valuable for the diagnosis of multiple sclerosis. The pathophysiological basis of VEP changes remains unclear. Direct experiments have shown that complete conduction block may be the result of extensive demyelination of central nerve fibers, and the amplitude changes may largely reflect the complete conduction resistance of damaged fibers. In stagnation, in the less severe and less extensive demyelinating lesions, the VEP latency is often delayed, reflecting the slowing of the conduction velocity of the damaged optic fibers. 4. Compressive lesions of the anterior visual pathway The prolongation of VEP latency is not unique to multiple sclerosis and optic neuritis, and compression disorders in the pre-visual pathway can produce similar abnormalities. In the case of oppressive lesions, VEP may have an extended latency, and most of the early stages, even if the incubation period increases, is much less than demyelinating disease. Latency delays generally do not exceed the normal upper limit of 20ms, while optic neuritis and multiple sclerosis often have an average delay of 34-45ms, and individual cases have a delay of up to 100ms. In addition, VEP showed a much higher incidence of waveform abnormalities than demyelinating diseases, especially in the sellar region, which is characterized by the asymmetry of VEP. 5, fraud or rickets If VEP can reflect the ability to "see", then those who claim "invisible" but no pathological changes can be tested by VFP. If the VEP is normal, the pathway from the retina to the visual cortex is intact. If the patient complains that the single eye is blind, and indeed VFP can not be recorded, there are two possibilities to consider: either the lesion or the random inhibition of VEP. VEP can be arbitrarily suppressed, and VEP can be eliminated by taking random activities such as excessive meditation, inattention to the checkerboard plane, and eyeball convergence. Therefore, when suspicion of fraud, VEP should be used with caution. By using large field of view, large checkerboard and binocular stimulation, VEP can be minimized due to random effects. When it is suspected that VEP is caused by random inhibition, the stimulation can also be randomly applied by the "start-stop" stimulation method, so that the subject does not know when the stimulus will appear, and the deception means is difficult to perform. Precautions 1. Standardization of instrument testing conditions, patient cooperation directly affects the results of VEP. 2. Sometimes it is necessary to use a half-stimulation field to achieve the purpose of diagnosis. 3. When performing single-eye VEP, the shading of the eye that is not subject to the eye must be strict, otherwise it is easy to draw a wrong conclusion. 4. The patient is required to look at the "+" mark in the center of the light ball or in the center of the screen during the examination to avoid eye movement and blinking. Inspection process stimulate (1) The subject takes a sitting position or a reclining position and performs in a shielded room. Non-inspected eyes should be shaded with a black eye mask during monocular stimulation. (2) Electrode placement: The working electrode is placed at 2-3 cm (Oz) on the occipital trochanter. When performing binocular half-field stimulation, it needs to be around Oz (ie, O1 and O2 positions of EEG 10-20 system). Place the electrode, the reference electrode is placed on the forehead, and the ground wire is attached to the earlobe. The lead connection causes the working electrode to have a positive waveform upward. (3) Stimulus mode: divided into two types: flash stimuli and graphic stimuli. The flash stimuli are the same as the ERG test, and the graphic stimuli are square or strip-shaped bright and dark patterns with variable size. (4) The main parameters affecting the size of the reaction and the spatial frequency of the stimulus (the number of square or strip gratings per unit, unit: week/degree), the repetition rate of the stimulus (the number of times the pattern changes per unit time) and the contrast of the graphic . (5) The amplifier frequency can be selected from 0.1-200Hz, sensitivity is 2-5μV, recording time is 200ms, and the number of superposition is 64-128 times. When superimposing, it must work with automatic rejection artifact. recording The recording electrode is placed on the midline Oz 5 cm above the occipital trochanter and 5 cm from the left and right sides of the occipital bone are respectively O1 and O2. The reference electrode is placed on the forehead Fz, the filter band is 1 to 100 Hz, the analysis time is 400 ms, and the average is superimposed 200 times, repeating two rounds. Main waveform and naming The main waveform components of the full-field mode flipping visual evoked potential are N75, P100, and N145, which are determined by the average peak latency and polarity of the respective waves, and are called NPN complex waves. The above three waves are from different parts of the cortex, and P100 is considered to be the action potential from the first viewing zone (17 zone) or the central zone. Because N75 is difficult to identify, N145 latency and amplitude variation are large, while P100 negative peaks are most obvious and stable. Therefore, P100 is the most important analytical tooth volume. The normal P100 latency (PL) is 102.3±5ms, the difference between the two eyes is 1.3±2.0ms, the amplitude is 10.1±4.2mv, and the time course is 63±8.7. The waveforms recorded by O1 and O2 are similar to Oz and are basically symmetrical, but the amplitude is low. The half-field stimuli recorded from the midline and the stimulating side of the occipital plexus were the NPN complex and the representative component P100, and the amplitudes recorded from the contralateral side were low, variability or PNP patterns with opposite polarities. Semi-field stimuli are useful for evaluating optometry and optic chiasm and can be used to identify blunt-blind lesions. Each laboratory should collect the data of each age group according to the type and method of using the evoked potential machine, and statistically process the data of the normal range of brain age-evoked potentials of the laboratory. VEP abnormal standard VEP usually has 3-5 waves in the process of 200ms, and there is a positive wave of about 100ms in clinical diagnosis. The latency and volatility of the left and right eyes are generally reported and compared with each other. The latency and amplitude of flash VEP (flashVEP, FVEP) vary greatly in different individuals, and the repeatability is poor. The waveform and latency of the pattern VEP (pattern VER, PVEP) with relatively constant parameters are relatively stable. The difference in amplitude of PVEP can also reflect the difference in binocular sharpness. There may be small changes in the incubation period of PVEP in normal eyes, but this change in latency is particularly sensitive in optic neuritis and demyelinating diseases. In patients with optic neuritis, the eyes can be significantly prolonged or within the normal range. The absolute value of the normal person and its scope must be measured according to the respective instrument and selection parameters. The main analysis range of visual evoked potentials includes P100 latency, amplitude, P100 latency difference between the two eyes, and P100 head distribution. The criteria are as follows: (1) The P100 latency period is longer than the normal ±2.5SD or the difference between the two eyes is a reliable and sensitive indicator of VEP abnormality. Prompt visual pathway conduction disorder, common demyelinating diseases. (2) The P100 wave completely disappears. (3) The amplitude is abnormally reduced. (4) The head distribution of P100 is abnormal, that is, the records of the left and right occiputs are obviously asymmetrical or non-crossing asymmetry.

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