Visual Evoked Potential

Visual Evoked Potential by Dr Vikas Khatri

1. VISUAL EVOKED POTENTIAL Dr. Vikas Khatri

2. Measurement of the electrical signal recorded at the scalp over the occipital cortex in response to light stimulus Measure the functional integrity of the visual pathways from retina via the optic nerves to the visual cortex of the brain whereas imaging techniques such as MRI evaluate mostly their anatomical and structural basis Any abnormality that affects the visual pathways or visual cortex in the brain can affect the VEP

3. Since Macular projection is magnified at cortex, it is an objective indication of macular function. VEP recorded from this location exaggerates the contribution of the macular portion of the retina due to the fact that only the macula is projected onto the occipital pole

4. Sources of Visual Evoked Potentials Electrical Potential – Peristriate and Striate Occipital Cortex Central 10° - Visual field - Surface of the Occipital pole Rest   Primary Visual Cortex - Calcarine fissure M aximum electroencephalographic (EEG) activity of occipital cortex during foveal pattern reversal stimulation.

5. Types of Stimuli Flash Stimuli Occipital cortex is relatively insensitive to flash Pattern Stimuli Cortex is sensitive to edges of contrast Source : Strobe Flash Flash LED Flash LED Goggle Source : LED Monitor Projector Oscilloscope

6. Recommended Recording Parameters (IFCN and ISCEV) Sensitivity - 2𝜇V Amplification - 20,000–50,000 times Background luminance - 20–40 cd/m2 Analysis time (Sweep duration) - 250–300ms . Averaging – number of sweeps (Epochs) per average - 200 Contrast - 50 - 80 % Rate of presentation/sec – <3Hz (transient VEP), 4–8Hz (steady state ), 5-15Hz (Sweep) IFCN : International Federation of Clinical Neurophysiology ISCEV : International Society for Clinical Electrophysiology of Vision

7. Steady State VEP . Stimulation rate is more than 3/sec produce a waveform which is sinusoidal Assessment of visual field and Acuity of infants Transient State VEP . Stimulation rate is less than 3/sec, Most commonly Employed

8. Response to diffusely flashing light stimulus that subtends a visual field of 20 degrees Best for patient’s whose acuity 6/120 or worse Cruder response than pattern VEP Indicates that light has been perceived by cortex Indications – Media haze, Infants ( not maintain fixation on a pattern), Uncooperative patient, S edated or Anesthetized patients. Flash VEP Types of VEP

9. Checkerboard pattern Stimulus ( m.c. ), which reverses every half-second Preferred technique for most clinical purposes, give us the estimate of Visual Acuity in preverbal children. Checkerboard stimulus is preferable when the eye is optically correctable because the occipital cortex is very sensitive to sharp edges and contrast, whereas it is relatively insensitive to diffuse light. Pattern Reversal VEP Types of VEP

10. Best choice in cooperative patients with good visual acuity, particularly when testing for possible effects of : O ptic neuritis, Optic atrophy, N eurofibromatosis C ompression of the optic pathways.

11. Pattern Onset/Offset VEP Types of VEP A pattern is abruptly exchanged with an equilluminant diffuse background More intersubject variability than pattern reversal VEP Preferred in patients with Corrected Visual acuity 6/60 – 6/120 or the patient has nystagmus or both. Most productive stimulus for monitoring amblyopia and in detection of patients with malingering

12. COMPONENT PEAK TIME N1 (N70) Prominent negative component 70 msec P1 (P100) Larger amplitude positive component 100 msec N2 (N140) Variable negative component 140 msec P1 – Sensitive to defocusing can be used with different check sizes to estimate refractive error. Reliable between individuals and stable from about age 5 years to 60 years. Only slows about 1msec per decade from 5 years old until 60 years old. N1 & N2 – Highly variable and is not used for standard test interpretation

13. Latency Amplitude If acuity of the patient is in question , the amplitude is more important If detection of a lesion in visual pathway is in question , latency is more important

14. Latency Amplitude Amplitude reduction: Amplitude of P100 show wide individual variation, therefore interocular amplitude ratio is used to detect abnormalities. Reduced amplitude indicates axonal lesions( ex: AION).

15. Latency Amplitude Latency prolongation : P100 Latency prolongation > 3 SD or interocular latency difference > 10msec is significant. Latency depends on fast conducting fibers . Prolonged P100 latency seen in demyelinating lesions, retinopathies & glaucoma

16. Reference electrode ( Fz ) – forehead Ground electrode ( Cz ) - vertex Active electrode (Oz) - 3-4cm above the inion Electrode locations on the Scalp Fz Cz O z NASION VERTEX INION

17. Electrode locations on the Scalp Occipital scalp overlying the Calcarine fissure ( closest location to Primary Visual Cortex / Brodmann’s area 17 ). Inion Electrode 3-4 cm 3-4 cm 3-4 cm 10-20 International System Distance between Electrode and Inion - 10% of the distance between the Inion and Nasion

18. Electrode locations on the Scalp Occipital scalp overlying the Calcarine fissure ( closest location to Primary Visual Cortex / Brodmann’s area 17 ). Inion Electrode 5 cm 5 cm 5 cm Queen Square system Further off the midline - better able to lateralize anomalies

19. There should be no distracting sound or light waves Pattern and flash must both be done in all patients as pattern cannot be detected in patients with media opacities Pattern VEP followed by flash VEP Significantly affected by eccentric fixation, excessive blinking of eyes and partial closure of eyes Prerequisites

20. The room should be dark Undilated pupil Refractive correction 1m distance from monitor Monocular recording (Test One eye at a time) Skin preparation by abrading and degreasing Apply electrodes (silver coated silver chloride disc electrodes) using conducting jelly Stimulus: Checkerboard pattern , Strobe Flash Procedure

21. Factors Affecting VEP Age Children (<3 years) have poor fixation – Delayed and Low amplitude wave. Children (>5 years) – Adult VEP waveforms Adults (>55 years) – Attenuation in amplitude and Slowing of the P1 component Scattergram of pattern reversal VEP “P100” component peak times recorded from normal individuals ages 5 to 90 years.

22. Gender P100 latency longer in Adult Males > Adult Females larger head size and lower body temperature in males compared to adult females Eye movements Eye movement reduces the amplitude of P100 but its latency is not affected. Patients with nystagmus having normal visual pathway have normal P100 latency. Size of Pupil Pupillary constriction increase p100 latency which is attributed to decreased area of retinal illumination . Refractive error Refractive error of patient increases – Amplitude of VEP decreases

23. Person with 6/6 or better- largest amplitude, fastest VEP components using a small check size (15-20′ of arc or 5-6mm viewed at 1m). Person with 6/60 or Worse- largest amplitude, fastest components with a larger check size (>50′ or >20mm viewed at 1m distance)

24. VEP recordings in different Retina-Brain Pathologies

25. Multiple Sclerosis I nitially Right Nerve yields normal range evoked potentials. The left nerve, affected with retrobulbar optic neuritis shows a delayed P100 component. Asymmetric Optic Neuritis

26. Patients with MS usually develop optic neuritis later in the other nerve O ptic Neuritis

27. Later Stage Optic Neuritis

28. Trauma Flash VEPs recorded soon after occipital trauma in 2 year old showing small amplitude VEP in the right eye and no recordable VEP in the left eye.

29. Tumors Children with neurofibromatosis type 1 are vulnerable to development of optic nerve gliomas. VEP can be a more sensitive and cost effective test to follow the progress of nerve pathology than MRI tests alone

30. Optic Nerve Toxicity Optic nerve Ethambutol toxicity showing slow P100 peak times.

31. Infections Visual Cortices effected by Meningeal tuberculosis Any pathology affecting visual cortices can be quantified using VEPs

32. Malingering and Hysteria Patients with Hysterical Blindness. VEP remains normal with vision as low as 1/60. VEP can be enhanced by using Large fields, Large checks and Binocular vision. Continuous record of optic nerve function in form of VEP to prevent inadvertent damage to the nerve during surgical manipulation During Orbital or Neurosurgical Procedures

33. Multifocal Visual Evoked Potential To detect small or localized optic pathway dysfunction. A Computer program (Binary m-sequencing) that can extract hundreds of VEPs from occipital scalp Multifocal VEPs best evaluate asymmetry of visual function caused by optic nerve dysfunction mfVEP Stimuli - dartboard pattern with each sector a contrast reversing check pattern Dartboard Pattern stimulus

34. Four occipital scalp electrodes Just below the inion 3-4cm above the inion 2 Laterally place electrodes 4cm off the midline several centimetres above inion. The Ground Electrode can be anywhere on the body 4 cm 4 cm 3-4 cm Electrode locations on the Scalp Inion Electrode

35. Multifocal VEPs recorded stimulating each eye in a normal adult. Right eye ( RED ) traces ---- Left eye ( BLUE ) traces ---- mfVEP Normal Adult

36. Multifocal VEPs showing Right optic nerve (RED) versus L eft optic nerve (BLUE) during acute phase of optic neuritis(Right Eye). Optic Neuritis

37. Multifocal VEPs showing significant recovery in amplitudes of mfVEPs , 2 months after acute phase of optic neuritis, Persisting delays in peak times in the Right optic nerve (RED). Optic Neuritis

38. Multifocal VEPs recorded from a patient Unilateral Right ( RED ) Optic N erve G laucoma.

39. Thank You