Pseudoseizures caused by hyperventilation resembling absence epilepsy.

During the 4-year period, 1982-1986, 18 patients presented to the Children's Hospital, Camperdown, Sydney, with the following features: (1) Recurrent "absences" clinically indistinguishable from childhood absence epilepsy, (2) Normal clinical examination, (3) Electroencephalogram (EEG) demonstrating normal waking background and sleep activity. On hyperventilation, "absences" occurred, characterized on EEG by a marked build-up of paroxysmal slow-wave activity unassociated with evidence of epileptic activity. We designate these attacks "pseudoseizures caused by hyperventilation resembling absence epilepsy." Individual cases demonstrated a variety of other symptoms consistent with the hyperventilation syndrome. There was an identifiable environmental stress in 13 of the 18 cases. Follow-up of 13 patients after a mean period of 20 months revealed that only two children continued to have occasional absences, associated with a clear history of breathing up when upset. Treatment did not influence outcome. On repeat hyperventilation with EEG and respiratory monitoring, five of the 13 had pseudoseizures. There was no indication that susceptibility to these episodes was associated with an abnormal CO2 response. It is postulated that the occurrence of pseudoseizures is related to cerebrovascular immaturity and an excessive vasoconstrictor response to a given level of CO2.