Aberrant wave forms to pattern reversal stimulation: clinical significance and electrographic 'solutions'.

Aberrant wave forms (AWFs) which are pattern visual evoked potentials (PVEPs) with ambiguous P2 peaks, were found in 13.7% of our patient population and not in our control group. The distribution of AWFs among the clinical entities of chronic myelopathy, questionable early MS, MS without visual lesions, MS with visual lesions and other neurological disorders, were similar to that of normally formed PVEPs with delayed P2 peaks. AWFs therefore appear to have a similar clinical significance to that of delayed, normally formed PVEPs, whether or not their latencies are within the normal range. Electrographic fields of non-aberrant PVEPs with well defined P2 peaks were studied to define the following obligate P2 characteristics: P2 always attained highest amplitude at the midline occipital position (Ox), hemifield stimulation using Fz as a reference always evoked a larger NPN complex over the hemisphere ipsilateral to stimulation than over the contralateral hemisphere. Multiple-channel analysis of both fullfield and hemifield stimulation and bipolar transverse montages reconstructed from referential recordings were employed to identify the P2 peak in AWFs. Bipolar reconstruction identified the P2 peak in 61% of all instances in which it was employed while hemifield stimulation did so in 84% of instances. In the 82 instances in which both were employed, hemifield stimulation identified the P2 in 84% as compared to only 33% for bipolar reconstruction. Therefore, although bipolar reconstruction may often identify the P2, hemifield stimulation is more effective for particularly complex situations.