Visual Evoked Potential/ Response (VEP/VER)
Visual Evoked Potential/ Response (VEP/VER) measures the electrical signal generated at visual cortex in response to visual stimulation. The visual cortex is primarily activated by the central visual field and there is a large presentation of the macula at occipital cortex. VEP depends on integrity of visual pathway including eye, optic nerve, chiasma, optic tract, optic radiation and cerebral cortex. Standard International Society for Clinical Electrophysiology of Vision (ISCEV) protocols assess the anterior visual pathway (eye, optic nerve anterior to the optic chiasma). For dysfunctions of posterior visual pathway extended multi-channel protocols are needed.
Preparation of the patient
- Undilated pupil
- Refractive error corrected
- Uniocular recording Patch other eye
As per international 10/20 system of recording electroencephalogram. z denotes midline structures along sagittal plane.
- Occipital lobe(Oz, active/positive)
- Forehead(Fz, reference/negative)
- Earlobe/vertex/mastoid (ground/neutral)
ISCEV Standard protocol
- Pattern reversal VEPs - The blocks of checkerboard (white and black) reverse keeping the luminance same. Standard reversal rate is 2 reversals per second (rps). Both large (1 degree) and small (0.25 degree) should be checked.
- Pattern onset/offset VEPs- The blocks of checkerboard are on and off with time. During off time there is a diffuse gray background. However, there should be no change in luminance. This protocol should also be checked using both stimuli with large, 1 degree, and small, 0.25 degree checks. This protocol is useful for malingering and patients with nystagmus, The checkerboard is on for 200 ms and off (diffuse gray background) for 400ms.
- Flash VEPs use a flash which should subtends visual field of at least 20 degree. The flash rate should be 1 per second. The luminance of the stimulus is 3 photopic candela.second/meter squared (cd.s.m-2).
- Pattern reversal VEPs - 2 negative and one positive wave (N75, P100 and N135 peaks). These waves appear at 75ms, 100ms and 135ms after the stimulus respectively. Amplitude of P100 = peak of N75 to peak of P100.
- Pattern onset/offset VEPs- Positive wave c1 at 75ms, Negative c2 at 125ms, Positive c3 at 150 ms.
- Flash VEPs- Multiple positive and negative waves- most consistent N2 and P2. Amplitude of P2 (120 ms)= peak of N2 (90ms) to peak of P2
ISCEV reporting guidelines
The report should include
- Minimum 2 recordings of each protocol
- Stimulus- field size, strength of flash or mean luminance of pattern, size of pattern elements, contrast of pattern, frequency of reversal,
- Eye tested
- Filter setting
- Location of positive and negative electrodes
- Peak time and amplitude measurements along with reference values
- Any deviation from ISCEV standard
The peak time and amplitude of the waveforms denote the function of optic nerve anterior to chiasma and eye. Main clinical uses are
- Prognosticate eyes with poor vision before planning surgery
- Monitoring visual function of babies
- Investigation of optic neuropathy
- Rule out malingering
Evaluation of chiasma and retrochiasmal visual pathway
Multichannel VEP recording is needed. Active/positive elestrodes are placed
- One at midline over the occiput Oz
- 2 lateral electrodes O1 and O2
Negative/Reference electrode on forehead (Fz).
Additional active elctrodes at PO7 and P08 referred to Fz may increase sensitivity to lateral asymmetries.
Chiasmal misrouting as in albinism causes an asymmetric distribution of VEP over the posterior scalp. Chismal disorder causes a crossed asymmetry and retrochiasmal dysfunction causes uncrossed asymmetry.
- Odom JV, Bach M, Brigell M, Holder GE, McCulloch DL, Mizota A, Tormene AP; International Society for Clinical Electrophysiology of Vision. ISCEV standard for clinical visual evoked potentials: (2016 update). Doc Ophthalmol. 2016 Aug;133(1):1-9. doi: 10.1007/s10633-016-9553-y. Epub 2016 Jul 21. PubMed PMID: 27443562.
- American Clinical Neurophysiology Society. Guideline 5: Guidelines for standard electrode position nomenclature. J Clin Neurophysiol. 2006 Apr;23(2):107-10. PubMed PMID: 16612226.