Pattern-onset and OFFset visual evoked potentials in the diagnosis of hemianopic field defects

Background Visual evoked potentials (VEPs) assess the function of the visual pathway from the retina to the primary visual cortex. There is much evidence that monocular pattern-reversal and flash VEPs can distinguish dysfunction due to chiasmal and post-chiasmal afferent pathway lesions. There is less evidence about the use of pattern-onset/OFFset VEPs to identify post-chiasmic dysfunction. Methods We present nine patients with a range of visual pathway defects that caused dense hemianopic field defects. These patients had pattern onset-OFFset VEPs recorded from an array of occipital electrodes referred to a mid-frontal electrode to checks that appeared for 230 ms and disappeared for 300 ms into a background of mean luminance, in a stimulus field of 30 degrees. Results We found pattern-onset VEP components lateralise to occipital electrodes overlaying the functional hemisphere, whereas pattern-OFFset VEP components demonstrate the paradoxical lateralisation phenomenon, described in reversal VEPs, and are maximal over the contralateral occiput. Conclusion Our findings show how extending the recording time window to include an OFFset VEP facilitates identification of hemianopic visual field defects. We advocate the pattern-onset/OFFset VEP in the assessment of patients with hemianopia, having particular value for patients who are otherwise unable to perform more demanding half-field electrophysiology, imaging or psychophysical testing.