Visual evoked potential abnormalities in compressive chiasmal lesions: the relevance of central visual field defects

Abstract

In healthy subjects, the pattern visual evoked potential (VEP) arises predominantly from the stimulation of the central 5 degrees of the visual field. This central-field paradigm is well confirmed in VEP studies of retrochiasmal lesions, but it appears to be more ambiguous in patients with compressive lesions of the chiasm. To further clarify how central and peripheral visual field defects affect VEP, we compared the Goldmann perimetric charts with the frequency and type of VEP abnormalities in 25 patients in whom a compressive chiasmal lesion had been confirmed by CT. Perimetric findings were compared with the full-field and half-field VEP records with regard to the presence or absence of defects within the 0-5 degrees of the visual field. Full-field VEP abnormalities were present in all eyes with absolute, in 81% of the eyes with relative central visual field defects, and in 59% of the eyes with no impairment of the central visual field. Half-field VEP abnormalities were found in all half-fields with absolute, in 89% of half-fields with relative central visual field defect, and in 77% of half-fields that manifested visual field defects without macular involvement. We also found abnormalities in 36% of half-field VEP recordings, obtained by stimulation of apparently normal visual half-fields. The analysis of the types of VEP abnormalities showed that Pi00 amplitude attenuation typically reflected the central field defect, while Pi00 asymmetry, Pi00 prolongation, and Pi35 appearance could be detected in both the presence or absence of the central field impairment. Our results confirm that central visual field defects are associated with VEP abnormalities. However, the high proportion of VEP abnormalities in eyes with field defects not affecting the central 5 degrees, and the VEP half-field abnormalities even in an absence of visual field defect indicate that compressive chiasmal lesions may be reflected in abnormal VEP well before the appearance of central-field defects in the Goldmann perimetric tests.

Key Words:

• Visual evoked potentials
• compressive chiasmal lesion
• Goldmann perimetry
• central visual field defects