Abstract:

Visual Evoked Potentials (VEPs) can provide important diagnostic information regarding the functional integrity of the visual system. This document updates the ISCEV standard for clinical VEP testing and supersedes the 2004 standard. The major change in this revision is that test parameters have been made more precise to achieve better consistency of results within and between test centers.

The ISCEV standard VEP protocols are defined for a single recording channel with a midline occipital active electrode. These protocols are intended for assessment of prechiasmal function; additional electrode sites are recommended for evaluation of chiasmal and post-chiasmal function. ISCEV has selected a subset of stimulus and recording conditions that provide core clinical information and can be performed by most clinical electrophysiology laboratories throughout the world. These are:

• 1. Pattern-reversal VEPs elicited by checkerboard stimuli with large 1 degree (60 min of arc) and small 0.25° (15 min of arc) checks.
• 2. Pattern Onset/Offset VEPs elicited by similar checkerboard stimuli with identical checkerboard stimuli with large 1 degree (60 min of arc) and small 0.25° (15 min of arc) checks.
• 3. Flash VEP elicited by a brief luminance increment, a flash, which subtends a visual field of at least 20 deg.

INTRODUCTION

VEPs (visual evoked potentials) are visually evoked electrophysiological signals extracted from the electroencephalographic activity in the visual cortex recorded from the overlying scalp. As visual cortex is activated primarily by the central visual field, VEPs depend on functional integrity of central vision at any level of the visual pathway including the eye, retina, the optic nerve, optic radiations and occipital cortex.

This document updates the ISCEV standard for clinical VEP testing and supersedes the 2004 VEP standard [1]. The major change in the current standard compared with the previous VEP standard is that test parameters have been made more precise to achieve more consistency of results within and between test centers. Clinical VEP laboratories are encouraged to use the current Standard Method where possible. Reports of VEP recordings performed to the Standard Method given here should cite this 2009 Standard. Where a method is used which deviates from the Standard Method, the deviations should be stated, together with any normative or reference data. Where the method used conforms to a previous ISCEV VEP Standard, this may be cited instead.

The waveform of a VEP depends upon the temporal frequency of the stimulus. At rapid rates of stimulation, the waveform becomes approximately sinusoidal and is termed steady-state. At low temporal frequencies, the waveform consists of a number of discrete deflections and is termed a transient VEP. All ISCEV standard VEPs are transient.

This standard presents minimum protocols for basic clinical VEP recording. Three standard stimulus protocols are defined. The ISCEV standard VEP protocols are defined for a single recording channel with a midline occipital active electrode. If chiasmal or retrochiasmal disease is suspected, a three channel montage, using the midline and two lateral active electrodes, is recommended in addition to the basic standard tests. Following a principle established in earlier standards [2-6], ISCEV has selected a subset of stimulus and recording conditions which provide core clinical information that can be performed by most clinical electrophysiology laboratories throughout the world. These are:

• 1. Pattern-reversal VEPs elicited by checkerboard stimuli with large 1 degree (60 min of arc), and small 0.25° (15 min of arc) checks.
• 2. Pattern Onset/Offset VEPs elicited by similar checkerboard stimuli with identical checkerboard stimuli with large 1 degree (60 min of arc), and small 0.25° (15 min of arc) checks.
• 3. Flash VEP elicited by a brief luminance increment, a flash, which subtends a visual field of at least 20° deg.

Pattern-reversal is the preferred stimulus for most clinical purposes. Pattern-reversal VEPs are less variable in waveform and timing than the VEPs elicited by other stimuli. The pattern onset/offset stimulus is best suited for the detection of malingering and for use in patients with nystagmus. Flash VEPs are useful when poor optics, poor cooperation or poor vision makes the use of pattern stimulation inappropriate. To comply with this standard at least one standard protocol 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.

ISCEV recognizes that VEPs may be elicited by a wide range of stimulus protocols that are not covered in the standard. Some of the widely used specialized VEPs and extended VEP protocols are listed in Table 1. Manufacturers are encouraged to produce equipment that can perform as many of these specialized tests as possible.

By limiting this standard to three protocols, the intention is that standard VEPs will be incorporated universally into clinical VEP testing along with additional tests and extended protocols that a laboratory may chose to use (Table 1). The standard does not require that all three protocols should be used for every investigation on every patient. In many circumstances, a single stimulus protocol will be appropriate. ISCEV actively encourages the use of additional protocols for clinical research, which may 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

Electrodes

Stimulation

Pattern stimuli

Flash stimulus

Recording parameters

Amplification and filtering

Averaging and signal analysis

Clinical Protocol

Preparation of the patient

Description of the ISCEV standard VEPs

Pattern-reversal VEPs

Pattern onset/offset VEPs

Flash VEPs

VEP measurement and reporting

Normal values

VEP reporting

VEP interpretation

Specialized Procedures and additional tests

Pediatric VEP recording

Multi-channel recording for assessment of the central visual pathways

-- TWikiAdminUser - 22 May 2009

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