Large check size pattern reversal visual evoked potentials – Full and sectorial field stimulation in multiple sclerosis and controls

Highlights

• Central field pattern reversal stimulus can improve the sensitivity of detecting optic neuritis.

• This can be performed with large check sizes (65′).

• Masking outside the central field of 4° radius is an easy modification to implement.

Abstract

Pattern reversal visual evoked potentials have been used to study optic neuritis. Although smaller check size in the central fields are more sensitive, larger check sizes can be advantageous for patient fixation and in poor vision. We compared sectorial central and peripheral hemisurround stimulus with commonly used full- and half- field stimulus using large check sizes (65′) in the context of optic neuritis and multiple sclerosis. Of 19 female and 16 male control subjects studied, females had shorter P100 latencies and larger amplitudes than males. In 9 of the 18 patients with VEP abnormalities, the central field abnormalities were greater than those recorded with the full field response. The results confirm for the first time that central field stimulation using large checks can show a greater extent of abnormality than can be appreciated with large check full field stimulation alone, and could be commonly employed to improve yield in the investigation of optic neuritis. The data suggest that it is necessary to collect separate gender-specific laboratory normal values using this check size.

Introduction

Pattern Reversal Visual Evoked Potentials (PRVEPs) are useful in the investigation of multiple sclerosis (MS). While visual function often recovers following an acute episode of optic neuritis (ON), VEP abnormalities often persist indefinitely and can be used to identify past or subclinical demyelination. Traditionally, the conventional VEP uses a large full field alternating black and white checkerboard pattern. Previous studies have suggested smaller stimulus fields that specifically target the central visual field may be more effective in detecting abnormalities in ON [1], [2], [3], [4]. To help the confident identification of the P100, some studies have shown that the paramacular P135 component is enhanced when selectively stimulating the peripheral regions (known as the ‘hemisurround’ field) [5], [6].

Large rather than small check sizes may be preferable in clinical settings. While smaller check sizes are more sensitive for targeting the fovea[7] producing larger P100 amplitudes[8] than if large checks were used, this only holds true if patient has optimal refraction. Large checks do not produce latency shifts even when visual acuity is reduced to 6/60 [8], [9], [10]. In addition to this, age related changes are more pronounced when smaller check sizes are used [11], [12]. Altering the VEP through defocusing is more difficult with larger checks [13]. As such, we have chosen to compare the different stimulus patterns in the assessment of ON using a large check size (65′), with the view that larger checks are more resistant to latency and amplitude changes caused by aging, refractive errors and defocusing.

Central field and hemisurround stimulus in addition to full field and half field stimulus, using this large check size, were used in this set of patients with multiple sclerosis and past ON for the first time. The aim was to determine which stimulus field detects the most abnormalities and to determine the effects of sex, age and head size on latency and amplitude in our control group with this method.