Paradoxical masking of epileptic focus during hyperventilation

Hyperventilation (HV) has traditionally been used to activate epileptiform discharges in EEG, especially in generalized epilepsies. A young girl, with complex partial seizures secondarily generalised had frequent spike discharges (1spike/4.2 sec.) in left anterior temporal region in the resting record. During hyperventilation the spike discharges became less frequent (1 spike/15 sec.) and later disappeared. The spike discharges reappeared gradually in post-hyperventilation period and 2 minutes after stopping hyperventilation the spike frequency increased to 1/3.2 sec. The decrease in spike discharges may be due to hyperventilation induced arousal response or decrease in spike discharges may be due to hyperventilation induced arousal response or decrease in Cerebral blood flow resulting in suppression of spike focus. It is also possible that HV induced slow waves may obscure the spike discharges. Such paradoxical reduction of spike discharge during HV warrants cautions interpretation of the results of EEG when HV is used as a routine activation procedure.