Low-frequency photoparoxysmal response in adults: an early clue to diagnosis.

Intermittent photic stimulation is performed during an EEG to evoke photoparoxysmal response. When they appear triggered by low-frequency stimulation in children, they are suggestive of rare diagnosis, that is, neuronal ceroid lipofuscinosis. Among adults, their significance is less well understood. Low-frequency (<5 Hz) intermittent photic stimulation was performed over a period of 5 years during adult standard EEG. This retrospective study included all patients exhibiting low-frequency photoparoxysmal response. Five cases were identified. Three of them presented with active epilepsy (two progressive myoclonus epilepsy, one unclassifiable), two had visual deficiency, and three had dementia. The etiologies were MELAS (two), Creutzfeldt-Jakob disease (one), Kufs disease (one), and remained undetermined for one patient. In all patients, low-frequency photoparoxysmal response was observed years or months before the final diagnoses have been reached. Low-frequency photoparoxysmal response, classically associated with childhood progressive myoclonus epilepsy, seems to have a wider etiological spectrum in adult population. Moreover, this neurophysiological feature could be present before the final diagnosis in most cases. Systematically testing low frequencies during intermittent photic stimulation even during adult EEG seems warranted, particularly in a context of severe progressive neurologic deterioration.