CLINICAL PROTOCOL

Preparation of the patient

Pattern stimuli for VEPs should be presented when the pupils of the eyes are unaltered by mydriatic or miotic drugs. Pupils need not be dilated for the flash VEP. Extreme pupil sizes and any anisocoria should be noted for all tests. For pattern stimulation, the visual acuity of the patient should be recorded and the patient must be optimally refracted for the viewing distance of the screen. With standard electrodes and any additional electrode channels attached, the patient should view the centre of the pattern field from the calibrated viewing distance.

Monocular stimulation is standard. This may not be practical in infants or other special populations, in such cases binocular stimulation may be used to assess visual pathway function from either eye. When a flash stimulus is used with monocular stimulation, care should be taken to ensure that no light enters the unstimulated eye. Usually this requires a light-tight opaque patch to be placed over the unstimulated eye. Care must be taken to have the patient in a comfortable, well-supported position to minimize muscle and other artifacts.

The ISCEV standard VEP waveforms

VEP waveforms are age-dependent. The description of standard responses below reflects the typical waveforms of adults aged 18-60 years of age. The time from stimulus onset to the maximum positive or negative deflection or excursion of the VEP will be referred to as the peak time. Historically, the term latency has been used to indicate the time from stimulus onset to the largest amplitude of a positive or negative deflection when referring to VEPs,. In most areas of physiological recording and in electroretinography the time from stimulus onset to the peak of a deflection has been referred to as the implicit time and latency referred to the time from stimulus onset to the beginning of a response. Recent ISCEV standards have tended to replace implicit time with peak time or peak latency [1, 3, 4], because the meaning of the terms is more immediately apparent. We recognize that other terms have been used for this same concept.

Pattern-reversal VEPs.

The pattern-reversal VEP waveform consists of N75, P100 and N135 peaks. These peaks are designated as negative and positive followed by the typical mean peak time (see Fig. 2). It is recommended to measure the amplitude of P100 from the preceding N75 peak. The P100 is usually a prominent peak that shows relatively little variation between subjects, minimal within-subject interocular difference, and minimal variation with repeated measurements over time. P100 peak time is affected by non-pathophysiologic parameters such as pattern size, pattern contrast, mean luminance, signal filtering, patient age, refractive error, poor fixation and miosis.

Figure 2: A normal pattern reversal VEP.

Pattern onset/offset VEPs show greater inter-subject variability than pattern reversal VEPs. Pattern onset/offset stimulation is effective for detection or confirmation of malingering and for evaluation of patients with nystagmus, as the technique is less sensitive to confounding factors such as poor fixation, eye movements or deliberate defocus. Standard VEPs to pattern onset/offset stimulation typically consists of three main peaks in adults; C1 (positive approximately 75 ms), C2 (negative approximately 125 ms) and C3 (positive, approximately 150 ms) (see Fig. 3). Amplitudes are measured from the preceding peak.

Flash VEPs

Flash VEPs are more variable than pattern VEPs across but are usually quite similar between eyes of an individual subject. They are useful for patients who are unable or unwilling to cooperate for pattern VEPs, and when optical factors such as media opacities prevent the valid use of pattern stimuli.

The VEP to flash stimulation consists of a series of negative and positive waves. The earliest detectable component has a peak time of approximately 30 ms post-stimulus and components are recordable with peak latencies of up to 300 ms. Peaks are designated as negative and positive in a numerical sequence (see Fig. 4). This nomenclature is recommended to differentiate the flash VEP from the pattern-reversal VEP. The most robust components of the flash VEP are the N2 and P2 peaks. Measurements of P2 amplitude should be made from the positive P2 peak at around 120 ms to the preceding N2 negative peak at around 90 ms.

Figure 4: A normal flash VEP.

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