Abstracts

Rationale: Photic driving is a physiologic response (PDR) consisting of rhythmic activity elicited over the parietooccipital regions. It is time locked to the flash frequency during photic stimulation. It can be evoked a few hours after birth. The amplitude of PDR is usually tends to increase in elderly people. Destructive cortical lesions may cause unilateral PDR depression, whereas irritative lesions with epileptic potential may produce increased PDR on the side of the lesion. In this study we investigated the clinical significance of PDR in patients during routine electroencephalography. Methods: This is a retrospective analysis of the routine EEG records to investigate photic driving responses. EEGs were performed based on the international 10-20 system (NicoletOne EEG system, Natus Medical Incorporated, San Carlos, CA, USA). Minimum 20 minutes artifact-free recording were required for each patient. At least 3 minutes after hyperventilation, all patients had received photic stimulation evoked by light flashes at frequencies of 1,2,8,10,15,18,20,25,30,40,50,60Hz. Asymmetry of PDR was defined as absence or more than 50% reduction of the amplitude on one side. Results: EEGs of 381 patients who were evaluated during the period of January-April 2014 were included in this study. Only 133 of the patients had some degree of PDRs. 116 (87%) of these PDRs were symmetrical, while the remaining 17(13%) were asymmetrical. The patients with asymmetric PDRs were further analyzed for accompanying EEG abnormalities. Nine patients had ipsilateral and the other four of them had contralateral EEG abnormalities to the low voltage or absent PDRs. Two patients did not have any EEG abnormalities and the remaining 2 had generalized EEG abnormalities. Conclusions: Careful interpretation of EEGs is necessary and brings important information for accurate localization. In the light of the foregoing we will discuss importance of asymmetric PDRs with some case examples.