Introduction

This document presents the current (2004) standard for the visual evoked potential (VEP). The VEP is an evoked electrophysiological potential that can be extracted, using signal averaging, from the electroencephalographic activity recorded at the scalp. The VEP can provide important diagnostic information regarding the functional integrity of the visual system.

The current standard presents basic responses elicited by three commonly used stimulus conditions using a single, midline recording channel with an occipital, active electrode. Because chiasmal and retrochiasmal diseases may be missed using a single channel, three channels using the midline and two lateral active electrodes are suggested when one goes beyond the standard and tests patients for chiasmal and retrochiasmal dysfunction.

Pattern reversal is the preferred technique for most clinical purposes. The results of pattern reversal stimuli are less variable in waveform and timing than the results elicited by other stimuli. The pattern onset/offset technique can be useful in the detection of malingering and in patients with nystagmus, and the flash VEP is particularly useful when optical factors or poor cooperation make the use of pattern stimulation inappropriate. The intent of this standard is that at least one of these techniques should be included in every clinical VEP recording session so that all laboratories will have a common core of information that can be shared or compared.

Having stated this goal, we also recognize that VEPs may be elicited by other stimuli, including moving, colored, spatially localized, or rapidly changing stimuli. These stimuli may be used to stimulate neural subsystems or to assist in localizing visual field defects. VEPs may be recorded using a full montage of electrodes covering all head regions to enable source localization. In addition to the commonly used technique of signal averaging, a variety of procedures including kernel analysis and Fourier analysis may be used to extract the VEP from background EEG activity. Some of these specialized VEPs, not covered by the standard, are listed in table 20.5.1. Equipment manufacturers are encouraged to produce equipment that can perform as many of these specialized tests as possible. We particularly encourage the ability to record a minimum of five channels.

It is clear that this standard does not incorporate the full range of possibilities of VEP recording. However, in adopting the current standard for VEPs, the society, following a principle established in earlier standards^1–5, has selected a subset of stimulus and recording conditions which provide core clinical information and which can be performed by most of the world's clinical laboratories.

By limiting the standard conditions, the intention is that the standard method and responses will be incorporated universally into VEP protocols along with more specialized techniques (table 20.5.1) that a laboratory may chose to use. The standard does not require that all stimuli should be used for every investigation on every patient. In most circumstances a single stimulus type will be appropriate. However, it is not the purpose of the standards to impede research progress, which might demonstrate that other tests are of equal or greater usefulness. This standard will be reviewed periodically and revised as needed.

The organization of this report is as follows:

Basic Technology

Clinical Protocol