Objective VEP perimetry in glaucoma: Asymmetry analysis to identify early deficits

Purpose: The multifocal visual evoked potential (VEP) shows markedly symmet rical responses between the two eyes of control subjects. Patients with gla ucoma and patients considered at high risk for glaucoma were examined to de termine if VEP asymmetry could be identified and used for diagnosis and det ection of early damage. Methods: Multifocal pattern VEP recordings were performed using a single ch annel bipolar occipital electrode position and the Visual Evoked Response i maging System (VERIS). Then were 125 subjects: 24 control subjects, 70 pati ents with glaucoma, and 31 patients considered at high risk for glaucoma. A between-eye relative asymmetry coefficient (RAC) was determined for each o f the 60 test points in the VEP field. The RAC for patients with glaucoma a nd patients considered at risk for glaucoma were compared with values from control subjects. Correlation between Humphrey thresholds and RAC scores wa s performed. Results: Patients with glaucoma and patients considered at risk for glaucom a both showed significantly larger mean quadrant RAC values. When point by point analysis was performed, 69 out of 70 scotomas were identified with a cluster of at least 3 points of P < 0.05. For those considered at high risk for glaucoma, 10 out of 31 patients had abnormal areas in the VEP field. T here was a strong correlation (r = 0.82) between quadrantic RAC mean values and Humphrey quadrant threshold scores in an asymmetric glaucoma subgroup. Abnormal VEP responses were identified in parts of the visual field that w ere still normal on perimetry. Conclusions: Asymmetry analysis correctly identifies patients with glaucoma tous field loss and shows abnormalities in many patients considered at high risk for glaucoma who still have normal fields. Asymmetry analysis is able to identify objectively the extent of glaucomatous damage and may be able to detect changes before subjective field loss occurs.