Electrophysiological testing of VEP

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Electrophysiological testing of VEP – ERG and EOG

The electrophysiological tests consisting of VEP, ERG, and EOG provide objective evidence of function at different levels of the visual system. These tests enable us to diagnose the accurate localization of dysfunction in the vast majority of patients

The VEP can be elicited by various stimuli, usually pattern reversal, pattern onset/offset (where a contrast pattern appears from a uniform background of identical mean luminance, ). In clinical practice the reversing checkerboard is perhaps the most common and useful stimulus, but pattern appearance and diffuse flash both have their uses. The pattern reversal VEP was delayed in patients with demyelinating optic neuritis, and patients with multiple sclerosis could show delayed VEP from eyes with no signs or symptoms of optic nerve disease, i.e. the VEP was able to detect sub-clinical demyelination. It soon became apparent that the VEP was a sensitive indicator of optic nerve dysfunction, but that the delay found in association with optic nerve demyelination was not pathognomonic. The additional information provided by PERG may be crucial to the accurate interpretation of an abnormal pattern VEP. There is also an abnormal distribution of the pattern appearance VEP in association with the intra-cranial misrouting of ocular albinism where the majority of fibres from each eye decussate to the contralateral hemisphere. VEPs, together with the other electrophysiological tests, are of crucial importance in the diagnosis of non-organic or "functional" visual loss, which may reflect psychological disturbance or malingering. In such cases there are normal electrophysiological findings in association with symptoms which should suggest otherwise.

The ERG measures the mass response of the whole retina, reflects photoreceptor and inner nuclear layer retinal function, and allows separate functional assessment of the photopic and scotopic systems.
The ERG protocols now commonly adopted in most respectable laboratories include the recommendations by ISCEV, the ISCEV Standard ERG. In general, diseases that affect photoreceptor function will cause a-wave reduction accompanied by reduction in the amplitude of the b-wave, whereas diseases that have a maximal effect post-phototransductionally will give a "negative" ERG, so called because the overall waveform is dominated by the negative a-wave and where there is relative loss of the post-receptorally derived b-wave.

Restricted disease, such as sector RP or retinal detachment, will tend to give amplitude reduction but no change in implicit time.

The pattern electroretinogram (PERG) assesses the retinal response to a structured non-luminance stimulus such as a reversing black and white checkerboard. It provides useful information in the distinction between optic nerve disease and macular disease in patients with poor central visual acuity. This recording has a much lower amplitude than the full-field ERG, and signal extraction using computer averaging is necessary.
The PERG consists of two main components, P50 and N95, with N95 and much, but not all, of P50 probably arising in relation to central inner retina (ganglion cell). Analysis of the PERG concentrates on the latency and amplitude of P50, measured from the N35 trough, and the amplitude of N95 measured from the peak of P50.
Furthermore, when taken in conjunction with the full-field ERG, the PERG permits a distinction between macular dystrophy and cone or cone-rod dystrophy in the patient with an abnormal macula on ophthalmoscopy; in disease confined to the macula the PERG is abnormal but the ERG is unaffected. It should be remembered that a normal retinal or macular appearance does not necessarily imply normal function. Although there has been only limited application of the PERG to date, it is possible that changes in the PERG may assist in the early detection of central dysfunction in patients with retinal dystrophy and normal visual acuity. This may have prognostic implications.
EOG,
Conventional electro-oculography (the EOG slow oscillation) records the light induced rise in ocular standing potential following a period of dark adaptation. This reflects the rise in the potential across the retinal pigment epithelium (RPE) resulting from the progressive depolarisation of the basal membrane of the RPE which occurs in response to light adaptation. The clinical value of the EOG, and a reliable method for its measurement, were developed in the early 1960s by Arden's group
To record the EOG, surface electrodes are positioned at the medial and outer canthi of each eye. After a short period of pre-adaptation, the patient, usually seated at a Ganzfeld bowl with two light emitting diodes (LEDs) to provide fixation lights, performs fixed excursion lateral eye movements of approximately 30 degrees for 15-20 minutes during dark adaptation. During this time the standing potential, reflected in the amplitude of the signal measured between the lateral and outer canthus electrodes in relation to the eye movements, will usually reach a minimum value, the Dark Trough. The background light of the Ganzfeld is then switched on to create a full-field photopic environment, and the patient continues to make the same lateral movements during light adaptation until the gradual increase in standing potential which occurs has reached a maximum, usually at 7-10 minutes - the Light Peak.
The value of the amplitude of the Light Peak divided by the Dark Trough expressed as a percentage is known as the Arden index, and will be >170% in a normal subject. A normal EOG requires a normally functioning RPE and a normally functioning rod population with the retina in contact with the RPE.

Basic Information:

Program Type:	Non-Degree Based
Level Of study:	Short-Term Course
School / Faculty:	School of paramedical sciences
Department:	Optometry
Major (Name Of Program):	Electrophysiological testing of VEP – ERG and EOG

This program contact information:

Telephone:	00985138846710-19
Fax:
Mobile:	00989151581572
Email Address:	heraviansj@mums.ac.ir
Address:	Department of Optometry, School of Paramedical, Mashhad University of Medical Sciences, Mashhad Iran
Contact Person Name:	Dr.Samira Hasanzadeh

Program Detail:

Length of Training:	3 months
Language Requirement:	English
Admission Requirement:	BSc in Optometry

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