VISUAL-EVOKED POTENTIAL ABNORMALITIES IN COMPRESSIVE CHIASMAL LESIONS- THE RELEVANCE OF CENTRAL VISUAL-FIELD DEFECTS

In healthy subjects, the pattern visual evoked potential (VEP) arises predominantly from the stimulation of the central 5 degrees of the vis ual field, This central-field paradigm is well confirmed in VEP studie s of retrochiasmal lesions, but it appears to be more ambiguous in pat ients with compressive lesions of the chiasm. To further clarify how c entral and peripheral visual field defects affect VEP, we compared the Goldmann perimetric charts with the frequency and type of VEP abnorma lities in 25 patients in whom a compressive chiasmal lesion had been c onfirmed by CT, Perimetric findings were compared with the full-field and half-field VEP records with regard to the presence or absence of d efects within the 0-5 degrees of the visual field. Full-field VEP abno rmalities were present in all eyes with absolute, in 81% Of the eyes w ith relative central visual field defects, and in 59% of the eyes with no impairment of the central visual field, Half-field VEP abnormaliti es 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 t hat manifested visual field defects without macular involvement. We al so found abnormalities in 36% of half-field VEP recordings, obtained b y stimulation of apparently normal visual half-fields. The analysis of the types of VEP abnormalities showed that P100 amplitude attenuation typically reflected the central field defect, while P100 asymmetry, P 100 prolongation, and P135 appearance could be detected in both the pr esence or absence of the central field impairment, Our results confirm that central visual field defects are associated with VEP abnormaliti es. However, the high proportion of VEP abnormalities in eyes with fie ld 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 be fore the appearance of central-field defects in the Goldmann perimetri c tests.