visual evoked potentials

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Information about visual evoked potentials

Published on May 21, 2010

Author: ranjithpolusani


Visual evoked potentials : Visual evoked potentials P. Ranjith kumar Resident in neurology Gandhi medical College. Slide 2: VEPs (visual evoked potentials) are visually evoked electrophysiological signals extracted from the electroencephalographic activity in the visual cortex recorded from the overlying scalp. As visual cortex is activated primarily by the central visual field, VEPs depend on functional integrity of central vision at any level of the visual pathway including the eye, retina, the optic nerve, optic radiations and occipital cortex Slide 3: International Federation of Clinical Neurophysiology (IFCN) International Society for Clinical Electrophysiology of Vision (ISCEV A visual stimulus is presented to the subject for a selected number of times, and the cerebral responses are amplified, averaged by a computer, and displayed on an oscilloscope screen or printed out on paper Averaging : Averaging “Averaging” : “Averaging” EPs - repetitively stimulated -time locked - same configuration Background noise - Random time - Changing morphology Visual system : Visual system VISUAL SYSTEM : VISUAL SYSTEM Visual pathway : Visual pathway The visual system processes information along multiple parallel channels The magno cellular system is involved primarily with motion analysis. The parvo cellular system is associated with color selectivity and shows a preference for high spatial-frequency stimuli. The two major processing systems, directed toward posterior parietal and inferior temporal cortex, exist in all primates Slide 9: Steady-state VEP: waveform of a VEP depends upon the temporal frequency of the stimulus. At rapid rates of stimulation, the waveform becomes approximately sinusoidal . Transient VEP: At low temporal frequencies, the waveform consists of a number of discrete deflections and . All ISCEV standard VEPs are transient electrodes : electrodes Sintered silver-silver chloride, standard silver-silver chloride, or gold disc electrodes skin preparation to ensure good, stable electrical connection. The electrode impedances should be below 5 k measured between 10 and 100 Hz and, to reduce electrical interference, they should not differ by more than 20% between electrode sites. Electrode placement- International 10/20 system The active electrode at Oz with the reference electrode at Fz Ground at the forehead, vertex (Cz), mastoid, earlobe (A1 or A2) or linked earlobes. Slide 11: The ISCEV standard VEP protocols are defined for a single recording channel with a midline occipital active electrode. These protocols are intended for assessment of prechiasmal function; If chiasmal or retrochiasmal disease is suspected, a three channel montage, using the midline and two lateral active electrodes, is recommended in addition to the basic standard tests Stimulus parameters : Stimulus parameters Patterned -Pattern-reversal VEPs Pattern Onset/Offset VEPs 2. Un patterned-Flash VEP Pattern stimuli : Pattern stimuli The two most commonly used patterns are checks and gratings. The pattern should be achromatic (black and white). The size of the individual checks should be expressed in terms of visual angle. where B is the visual angle in minutes of are, W is the width of the checks in millimeters, and D is the distance of the pattern from the corneal surface in millimeters Pattern stimuli : Pattern stimuli Although checks are customarily reported in minutes of arc ('), gratings are usually reported in cycles per degree. Measurements in cycles per degree (abbreviated as c/deg) define the spatial frequency of the stimulus. Measurements of the visual angle in minutes of arc can be converted to cycles per degree by the formula where W is the width of the grating in minutes. In the case of checks, however, W represents the diagonal measure of the checks in minutes of arc. Pattern stimuli : Pattern stimuli a high contrast black and white checkerboard two check element sizes should be used: 1° ± 20% and 0.25° ± 20% of arc per side. All checks should be square and equal number of light and dark checks. Field size- ratio between width and height should not exceed 4:3 and at least 15 deg in its narrowest dimension. Rectangular or circular with fixation point in center. luminescence : luminescence subdued room lighting with no bright sources visible to the subject. The luminance of the white checks should be 100 ± 20 candelas per meter squared (cd·m-2). The luminance of the black checks should be low enough to achieve a Michelson contrast of greater than 80%. mean luminance of the checkerboard will be between 40 and 67 cd·m-2. The luminance and contrast of the stimulus should be uniform between the center and the periphery of the field. Pattern-reversal stimuli : Pattern-reversal stimuli black and white checks change phase abruptly (i.e., black to white and white to black) and repeatedly at a specified number of reversals per second. No overall change in the luminance of the screen, The large check (1°) and small check (0.25°) stimuli are specified by the check width (visual angle), the stimulus rate (in reversals per second) number of reversals, the mean luminance, the pattern contrast and the field size. A reversal rate of 2 reversals per second (+/- 10%) should be used to elicit the standard pattern reversal VEP. Pattern onset/offset stimuli : Pattern onset/offset stimuli the checkerboard pattern is abruptly exchanged with a diffuse gray background. The mean luminance of the diffuse background and the checkerboard must be identical. Pattern onset duration should be 200 ms separated by 400 ms of diffuse background. The ISCEV standard onset/offset response is the onset response. At least two pattern element sizes should be used: checks of 60 min and 15 min per side Flash stimulus : Flash stimulus The flash VEP should be elicited by a brief flash that subtends a visual field of at least 20 deg, presented in a dimly illuminated room. The strength (time-integrated luminance) of the flash stimulus should be 3 (2.7-3.3) photopic candelas seconds per meter squared (cd·s·m-2). A hand held stroboscopic light or by positioning an integrating bowl (ganzfeld) . The flash rate should be 1 per second (1.0 Hz ±20%) stimuli : stimuli Pattern-reversal is the preferred stimulus for most clinical purposes. Pattern-reversal VEPs are less variable in waveform and timing than the VEPs elicited by other stimuli. The pattern onset/offset stimulus is best suited for the detection of malingering and for use in patients with nystagmus. Flash VEPs are useful when poor optics, poor cooperation or poor vision makes the use of pattern stimulation inappropriate Recording parameters : Recording parameters Amplification- 20,000-50,000 times, input impedance of 100 Mohm and the common mode rejection ratio should exceed 120dB. Filters-Analogue high pass and low pass filters should be set at <1Hz ( time constant 0.16 s or more) and at >100 Hz . Averaging-minimum number of sweeps per average should be 64. At least two averages should be performed to verify the reproducibility of each VEP Analysis Time: The minimum analysis time (sweep duration) for all adult transient flash and pattern reversal VEPs is 250 ms and 500 ms for pattern onset/offset VEP -IFCN : VEP -IFCN Preparation of the patient : Preparation of the patient Cleaning Seating Refraction correction Pupil size Electrodes Viewing distance monocular VEP nomenclature : VEP nomenclature VEPs to a pattern-reversing checkerboard (the most commonly used stimulus in clinical laboratories) consist of a set of sequential waveforms . The waveforms are alternately positive and negative and are designated in accordance with their polarity and latency. Positive waves are designated P, followed by a number indicating the peak latency in milliseconds (e.g., P60 and PlOO) negative waves are designated N, followed by a number indicating the peak latency N70 and NI45 VEP : VEP Slide 31: ISCEV-recommend that VEP traces be presented as positive upwards. P-100:pattern size, pattern contrast, mean luminance, signal filtering, patient age, refractive error, poor fixation and miosis. Pattern onset: typically consists of three main peaks in adults; C1 (positive approximately 75 ms), C2 (negative approximately 125 ms) and C3 (positive, approximately 150ms) Flash:N2 and P2 peaks,90 ms and120 ms. Pattern reversal VEP : Pattern reversal VEP Pattern onset/offset VEP : Pattern onset/offset VEP Flash VEP : Flash VEP P-100 : P-100 considerable variation in the morphology of normal VEPs, but the dominant wave is the PIOO component. some subjects the initial negativity (N70) is absent, whereas in other subjects N70 is as large as PIOO. 0.5 percent of normal cases, PlOO has a W-shaped configuration (i.e., PIOO is subdivided into two peaks). In these normal subjects, both peaks have latencies within the boundaries of normality. The best way to determine which peak corresponds to PIOO is to obtain VEPs to patterns of three different sizes. Usually the larger checks will yield only one PIOO peak P100 morphology : P100 morphology Slide 37: Pupil constriction affects the amplitude and the latency of N70 and PIOO in the same manner as decreased luminance of the stimulus does. Check size also influences the latency and amplitude of the responses. A decrease in check size is usually associated with a prolongation of N70 and PIOO latency. However, the relationship between pattern size and PIOO latency is not linear certain studies suggest that as check size increases above 30', the latency of P lOO also increases. Slide 40: Age is another important variable that influences the VEP. PIOO latency increases with age. The effect of aging was more prominent when small checks were used to elicit the responses The age-related increase in latency of VEP was related to changes in the visual pathways or cortex. Ganglion cell loss, dysmyelination, axonal swelling, and nerve fiber loss have been described in the optic nerve Slide 41: Shorter-latency and larger-amplitude VEPs than in males have been described in females. Uncorrected refractory errors may affect the amplitude and latency of the VEP, especially for patterns of small size. Blurring of the pattern stimulus not only prolongs PlOO latency but often drastically changes VEP morphology, with elimination of N70 and broadening of the PlOO wave. Two diopters of blur reduces Snellen visual acuity from 20/20 to 20/120 Patients to be tested not infrequently have 1 or 2 diopters of uncorrected myopia PVEP – NORMAL DATA : PVEP – NORMAL DATA P 100 LATENCY ( m sec ) = 102  5 R-L difference ( msec) = 1.3  2.0 Amplitude (μV) =10  4.2 Duration = 63  8.7 FULL FIELD PVEP- CRITERIA FOR ABNORMALITY : FULL FIELD PVEP- CRITERIA FOR ABNORMALITY LATENCY CRITERIA PROLONGATION > 3 SD INTEROCULAR LATENCY OF P100>10 msec, LONGER LATENCY ABNORMAL AMPLITUDE CRITERIA INTEROCULAR AMPLITUDE RATIO>2 ABNOMALLY LOW OR HIGH AMPLITUDE ABSENCE OF IDENTIFIABLE VEP FROM MIDLINE AND LATERAL OCCIPITAL SITES. FULL FIELD PVEP- INTERPRETATION : FULL FIELD PVEP- INTERPRETATION Monocular abnormal full field stimulation suggest anterior visual pathway defect. Bilateral abnormal full field VEP is seen with bilateral optic nerve, tracts or chiasmal lesions Abnormal VEP : Abnormal VEP abnormal : abnormal Abnormal in 50% patients of glaucoma with field defects showing prolonged latency and decreased amplitude In hysteria, malingering- normal (can be normal in cortical blindness) glaucoma : glaucoma PVEP IN DEMYELINATION : PVEP IN DEMYELINATION Acute attack-decrease amplitude or absent VEP, prolonged latency P100 prolongation persists for many years even after clinical recovery Abnormal VEP is seen in 75-85% of multiple sclerosis VEP-clinical recovery : VEP-clinical recovery Non standard VEP : Non standard VEP Paediatric VEP : Paediatric VEP In infants the sweep duration should be at least 500 ms post-stimulus. By six months of age, the peak time of the main positive peak of the pattern reversal VEP for large checks (1°) is usually within 10% of adult values. improved if a recording trial can be paused or interrupted when fixation wanders Binocular pattern stimulation, flash VEP is easy Monocular testing to at least one stimulus is desirable to assess the function of each eye. the degree of cooperation and arousal of the child Multi-channel VEP recording : Multi-channel VEP recording Intracranial visual pathway dysfunction asymmetrical distribution of the VEP over the posterior scalp. Chiasmal dysfunction gives a “crossed” asymmetry whereby the lateral asymmetry obtained on stimulation of one eye is reversed when the other eye is stimulated. Retrochiasmal dysfunction gives an “uncrossed” asymmetry such that the VEPs obtained on stimulation of each eye show a similar asymmetrical distribution across the hemispheres field of 30 degrees ,minimum of three active electrodes, two lateral electrodes placed at O1 and O2, and a third midline active electrode at Oz. All three active electrodes should be referenced to Fz Crossed asymmetry : Crossed asymmetry Hemi field stimulation : Hemi field stimulation Chiasmatic or retro chiasmatic lesion Marked inter hemispheric asymmetry on monocular . Additional electrodes and channels are used -LT,LO,MO,RO,AND RT. Paradoxical localization. Paradoxical lateralisation : Paradoxical lateralisation Multimodal VEP : Multimodal VEP The visual system analyzes spatial, temporal, and chromatic aspects of objects via multiple, parallel channels. Isoluminant chromatic sinusoidal gratings (red green) at 2 cycled deg (cpd) appeared for 200 msec and were replaced by a yellowish background for 800 msec in recording chromatic pattern appearance VEPs. High contrast (90%) achromatic sinusoidal gratings (black white) at 5.3 cpd also appeared for 200 msec and were replaced by a grayish-white background for 800 msec for recording achromatic VEPs. . Motion VEPs. Two squares (60 minutes of arc) at opposite comers of a hypothetical square were presented together for a duration of 500 msec and then switched off, followed by two squares appearing simultaneously on the remaining two corners. Steady-state VEP- high-contrast (90%) achromatic gratings at 2 cpd were reversed at a rate of 4 Hz (8 reversal/sec) for recording

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