Intermittent photic stimulation. Clinical usefulness of non-convulsive responses

The clinical usefulness of non-convulsive responses induced by intermittent photic stimulation (IPS) was studied in 46 patients whose evaluation included both electroencephalography (EEG) and computerized cranial tomography (CT scan). Three patterns of photic responses were identified and related to background EEG activity and type of lesion seen on CT scan. An asymmetry in photic driving manifested only by a consistent amplitude difference greater than 50% was rarely associated with other EEG changes such as focal slowing or significant asymmetry in the alpha rhythm. In contrast, an asymmetry in development of the photic response correlated well with ipsilateral focal slowing and CT scan evidence of parenchymatous brain disease. Bilateral, symmetrical high amplitude single spikes evoked by individual light flashes were seen only in patients with diffuse encephalopathies. In no patient with structural brain disease as judged by CT scan was an abnormality in IPS the only EEG finding. There was also no apparent relationship between the anatomical site of a focal cerebral lesion and the type of response induced by IPS. Photic stimulation is of limited value in non-epileptic patients. Little clinical significance can be attached to nonconculsive responses in the absence of associated EEG changes which by themselves are generally more informative.