We wished to determine if the degree of
hypocapnia correlates with increased frequency of absence
seizures and if there is a critical pCO2 at which absence
seizures are reliably provoked. Twelve untreated children with newly diagnosed
absence epilepsy were continuously monitored by EEG and end-expiratory CO2 recording during quiet respiration and
hyperventilation (to
absence seizure or exhaustion) while breathing four gas mixtures: (a) room air, (b) 100% O2, (c) 4% CO2 in room air, or (d) 4% CO2 + 96% O2). In quiet respiration, a reduction in number of spike and wave bursts and total seconds of spike and wave was noted in children breathing supplemental CO2 (
gases c and d vs.
gases a and b), p < 0.05. Supplemental O2 had no effect. Eight subjects had absence
seizures elicited with each trial of
hyperventilation. All subjects had their own critical pCO2, ranging from 19 to 28 mmHg. Three children had no
seizures, two despite
hypocapnia to pCO2 of 19 and 21 and 1 who achieved a pCO2 of only 25. In 1, absence
seizures were provoked in only six of nine
hyperventilation trials to pCO2 of 17-23. In 67% of subjects, absence
seizures were reliably provoked by
hypocapnia. Critical pCO2 varied among children with absence. Determination of whether variation in sensitivity to
hypocapnia may be helpful in determining response to
antiepileptic drugs (AEDs) or remission of
seizures will require further study.