Neuroscience
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Assessing Early Stage Open-Angle Glaucoma in Patients by Isolated-Check Visual Evoked Potential
Chapters
Summary May 25th, 2020
The isolated-check visual evoked potential (icVEP) method is implemented here to assess the magnocellular ON pathway that is initially damaged in glaucoma. The study shows standard operative procedures using icVEP to obtain reliable results. It is proved to serve as a useful objective diagnosing technology for the early detection of glaucoma.
Transcript
This method can help answer how this unique objective electrophysiologic radio field functional measure work in the diagnosis of early stage open-angle glaucoma. This technique can selectively stimulate aim on pathway, which initially is damaged in glaucoma, especially for early stage. To set up the icVEP system, click the setting button, and confirm that the frame rate is set to 6 Hertz.
The luminance of the static background of the display is set to 51 candela's per square meter. And the total cycles are set to 20. To differentiate the OAG patients and control subjects, confirm that the sign and total temperature signals are set to 10 Hertz at six frames per cycle with a 15%positive contrast.
To ensure the acquisition of reliable results, confirm that the refractive error is corrected to adapt for a distance of 114 centimeters, that the intraocular pressure is no more than 30 millimeters of mercury on the day of examination, that the pupil diameters are at least two millimeters and without mydriasis, and that each subject is at rest and is quiet at least 30 minutes before the examination. To avoid the influence of a study curve, check the right and the left eyes. Then, check the right and left eyes again and record the second result.
Initiate a retest after at least a 30 minute rest when the R value between both eyes shows a difference of greater 0.2, indicating that the result is unreliable. To accrue facilitated and careful fixation on the center of the screen, click the IC button to confirm that the spacial pattern is a 24 by 24 array of isolated checks to sub 10 and 11 degree visual field. With a two by two array fixation cross, without sinusoidal temporal signals, place the gold cup electrodes, filled with electrolytic water-soluble paste, at the midline sites on the scalp, based on the international 10/20 system.
And note that the grounding electrode can be the parietal midline electrode or the frontal parietal midline electrode. Take care that the testing distance is 114 centimeters. When all of the electrodes have been placed, click the start button.
One run will last for two seconds. With the first second presenting half of the test contrast level as an adaptation condition. And the second second presenting the full test contrast.
Note the prompt from the program and repeat the run when noise is detected and the EEG epoch is rejected. The EEG data will be displayed with the monitor when the run is determined to be valid after prompting to either accept or reject the data based on the reliability. Once the data has been accepted, the program will indicate what the next run should be until a set of 8 valid runs are accumulated.
Each run will produce a fundamental frequency component. When all eight runs have been completed, the program will calculate the mean fundamental frequency component and the radius of a 95%confidence circle using the T squared circular statistic that is automatically produced from the eight fundamental frequency components within less than one minute after the end of the test. The changing of retinal nerve fiber layer thickness in the temporal superior and inferior quadrants were calculated by subtracting the retinal nerve fiber layer thickness value from the standard value obtained from a database of healthy individuals.
In this representative analysis, a total of 44 OAG patients and 39 control subjects were included. No statistical differences in age, sex, right, or left eye, best corrected visual acuity, spherical equivalent, or pupil diameter were measured between patients and controls. But the signal to noise ratio was significantly lower in patients.
Using an A priori signal to noise ratio criteria on 0.93, the specificity of the receiver operating characteristic analysis reached 100%with a sensitivity of 65.9%For the patients, abnormalities in the central 11 degree visual field test were calculated by the numbers of abnormal points with different possibility criteria. With a criteria level of p 0.5, the amount of abnormal test points in the central 11 degree visual field was significantly negatively correlated with the signal to noise ratio. Thickness changing at the retinal nerve fiber layer in the temporal superior quadrant was significantly positively correlated with the signal to noise ratio, while the standard automated perimetry mean deviation of the other eye, thickness changing of the retinal nerve fiber layer in the temporal inferior quadrant, and the baseline interocular pressure, and central corneal thickness, were not correlated.
If you find a person who may be diagnosed as glaucoma suspect or open-angle glaucoma, you can use this procedure and make sure that step 1.2 and 1.3, sitting for isolated check, Ocular hypertension, or subjects with extension of a cap to distal ratio can also be performed with this procedure to differentiate open-angle glaucoma. Fortunately, the second generation of icVEP has been designed. It was developed for smaller screen and HD display.
Also, shorter exam distance. The machine is much smaller than first generation, which is much more convenient.
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