Evoked potential and electroencephalographic assessment of central blindness due to brain abscesses in a steer.

Central blindness in a 3.5-year-old crossbred steer was evaluated by electrodiagnostic techniques. When admitted the steer was depressed, head pressed, and circled to the left. Ptosis and absence of menace response were seen with the right eye, while the left eye was normal; direct and consensual pupillary light responses were normal in both eyes. Brainstem auditory evoked potentials and electroretinograms were essentially normal. The flash visual evoked potential (VEP) was greatly reduced upon stimulation of the right eye, while no VEP peaks could be recognized after flash stimuli were presented to the left eye. The amplitude of the electroencephalogram was depressed over right occipital cortex. Multiple brain abscesses were detected on postmortem examination adjacent to the left thalamus, in the left caudal cerebrum, and right frontal cerebrum. Corynebacterium pyogenes was cultured from abscess exudate; however, no origin for the infection could be determined. Both eyes were microscopically normal. The thalamic abscesses were postulated to have produced the EEG depression. Correlations between the VEP abnormalities and the abscess locations are discussed, based on proposed central nervous system generators of the VEP.