Higher Visual Pathways by Visually Evoked Potential
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Single Flash VEP – Basic
OVERVIEW: This test is designed to determine the cortical response to simple flashes of light to the eye. It is typically done only after retinal evaluation show that the retina is fully responsive. This VEP is the simplest determination of whether and how much light stimulus is reaching the visual cortex from each eye.
HOW DONE: Electrodes are placed on the scalp over the occipital cortex with easily removed water-soluble gel and are connected to a recording device. For the flash VEP the patient is positioned in front of a flash device. A bright light is repeatedly presented and the electrical responses are summated and recorded. The test takes about 15 minutes.
- Evaluating basic retino-cortical integrity in an infant suspected of having poor vision or with roving eye movements that preclude fixation.
- Patients with visual loss, but normal retinal anatomy and function tests, such as persons suspected of optic nerve dysfunction, including optic neuritis, multiple sclerosis, papilledema, optic nerve gliomas.
- Evaluating visual acuity potential in patients with ocular media opacities.
- Evaluating the possible misrouting of optic nerve fibers in patients suspected of having albinism.
HOW REPORTED: The flash VEP test typically produces two negative (N1, N2) and two positive peaks (P1, P2). The amplitude and time to onset (latency) are reported with a comparison to normal values.
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Pattern VEP – Macular Component of Cortical Response
OVERVIEW: This test is designed to evaluate the electrical response in the brain to a stimulus that elicits macular function. For the pattern-reversal VEP the patient fixates a checkerboard pattern, the white and black checks reverse their positions every ½ sec. Multiple check sizes may be presented.
HOW DONE: The testing situation, other than the character of the stimulus, is the same as for a flash VEP.
HOW REPORTED: The latency and amplitude of the prominent positive voltage that typically peaks at 100 microseconds (P100) after a pattern change.
USEFUL FOR: This test allows assessment of the speed of transmission from ganglion cells along the optic nerve to the visual cortex. The P100 is delayed in optic neuritis, papilledema, tumors effecting the optic nerve, and as a result of cortical insult e.g. anoxia.
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Multifocal VEP – Cortical aspect of visual field abnormality
OVERVIEW: This test records the cortical response to retinal stimulation by similar type of temporal stimulus that is used in the multifocal ERG. The difference is that a 4×4 checkerboard pattern that reverses according to the ‘m-sequence’ replaces the hexagonal patch that flashes on and off by the same sequence. The mfVEP provides an objective measurement of a subjectively measured visual field defect involving the central 20 to 30 degrees of the field
HOW DONE: The patient is positioned in front of the computer screen that has the capability of presenting an array of hexagonal patches that are flashed on and off according to a semi-random sequence and electrodes are placed over the occipital cortex. The electrical responses of the visual cortex are recorded. The variable intensities of stimulation of different areas of the retina are reflected in the intensity of response of different areas of the cortex thus validating the presence of a visual field defect.
HOW REPORTED: The result is reported as the symmetry of response recorded over horizontally and vertically positioned electrodes.
USEFUL FOR: This test provides objective validation for the subjective report of a defect in the central region on a visual field test.
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Sweep VEP for Objective Visual Acuity Estimation
OVERVIEW: This VEP test is designed to provide an objective approximation of visual acuity in pre-verbal infants and in adults with unexplained subjective visual acuity loss. In this test, cortical responses are generated by a flickering ‘grating’ or stripe pattern then reducing the width of the stripes to determine the minimum stripe width that is ‘seen’ by the visual system.
HOW DONE: The testing arrangement is the same as the single flash VEP described above with the exception that the light stimulus is presented in a series of light and dark stripes that vary in width. The estimation of acuity based on the cortical responses to these stimuli has been shown to correlate well with visual acuity. It is most commonly done on and well tolerated by infants and children sitting in the mother’s or care-giver’s laps.
HOW REPORTED: The test results are reported as the visual acuity in cycles per degree, which can be converted to Snellen scale for qualified eye-care professionals, but should not be used by social service agencies for determining benefits.
USEFUL FOR: The test is useful in any situation where an objective approximation of visual acuity is desired, especially in cases of infantile amblyopia before reliable acuities can be measured. The test may also be useful for determining vision in children with significant neurological impairment that may limit their behavioral response, e.g. cerebral palsy, global developmental delay, ocular motor apraxia. It is also useful in any situation where an objective approximation of visual acuity is desired, such as in adults whose subjective measurements of acuity are in doubt.
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Albinism VEP – Assesses Retino-Geniculo-Cortical Misrouting
OVERVIEW: This test is based on the unique anatomical fact that in albinism a larger-than-normal number of the axons from the temporal retina cross over at the optic chiasm to join those from the nasal retina of the opposite eye in projecting to the contralateral cerebral hemisphere. Normally, approximately one-half of the retino-geniculo-cortical neurons cross, resulting in VEPs from the left and right cerebral hemispheres that are identical. However, in albinism due to the asymmetric projections, stimulation of one eye results in a larger amplitude VEP from the contralateral visual cortex compared with the VEP from the ipsilateral cerebral hemisphere. This cross-over asymmetry is a feature of all types of albinism.
HOW DONE: This test uses the standard set-up for a VEP. Each eye is stimulated separately and the relative intensity of the response is measured over each cortex. Additional occipital electrodes may be added to confirm asymmetry. For infants, the stimulus is a strobe flash. For older children and adults, a checkerboard pattern presented in ‘on-off’ mode is used. Recordings are made from each eye to verify that the VEP asymmetry reverses cerebral hemispheres with eye stimulated.
HOW REPORTED: This test is reported as the amplitude of response of each side of the visual cortex to stimulus delivered to one eye at a time. Normally the responses are symmetric but in albinism the amplitude of the responses is higher from the contralateral visual cortex.
USEFUL FOR: This test supports a diagnosis of albinism.