Responses to photic stimulation in patients with occipital spikes

Objective: To determine the effect of intermittent photic stimulation (IPS) and frequency of asymmetric driving responses in patients with occipital spikes. Methods: The amplitude of the driving response at 4 flash frequencies was measured from a referential montage in 60 patients with occipital spikes and in 60 normal EEG records from age-matched patients. Responses were classified as asymmetric if the amplitude at one occipital area was less than 50% of the amplitude at the other. Results: A measurable photic response occurred significantly less frequently in patients with occipital spikes (48%) compared to the control group (70%; Fisher's test P<0.05). The driving response was asymmetric in 7/36 patients (37%) with unilateral spike foci versus none in the control group (Fisher's test, P<0.001). The amplitude was suppressed ipsilateral to the focus in 5 patients, all of whom had an ipsilateral structural lesion or focal slowing. In two cases the amplitude was higher ipsilateral to the focus, neither having slowing or a structural lesion. Conclusions: Patients with occipital spikes have an increased frequency of asymmetric driving response. An attenuated response ipsilateral to the focus seems to be related to an underlying lesion while the presence of an epileptiform focus in some cases with no slowing on EEG and normal imaging studies may lead to an accentuation of this response.