Hyperventilation-induced high-amplitude rhythmic slowing: A mimicker of absence seizures in children

Hyperventilation (HV) in children can lead to HV-induced high-amplitude rhythmic slowing (HIHARS) on the EEG (electroencephalogram) which is sometimes associated with altered awareness (AA) and concomitant semiological features. Our aims were to determine the frequency of HIHARS in children, to assess if the associated semiological features were temporally related to HV, and to evaluate if specific semiological features can differentiate HIHARS with AA from absence seizures.

Consecutive children with suspected new onset seizure(s) underwent HV and awareness testing during video-EEG acquisition. Hyperventilation-induced high-amplitude rhythmic slowing was defined as 2.5- to 5-Hz generalized rhythmic slowing with amplitude ≥100 μv lasting for ≥3 s. The associated semiological features were compared between the group of children with HIHARS and AA, an age- and gender-matched control group without HIHARS, and in children who experienced absence seizures during HV.

One hundred sixteen children with a mean age of 9.8 years were included. Hyperventilation-induced high-amplitude rhythmic slowing occurred in 39 children (33.6%) with AA documented in 30 (76.9%). The probability of developing AA during HIHARS was significantly and positively correlated with the HIHARS duration. The frequencies of HIHARS were not significantly different between children diagnosed with seizure(s) and those with nonepileptic spells. Hyperventilation cessation and staring did not occur in any child of the control group. Fidgeting and yawning were significantly more common in the group with HIHARS with AA while staring and blinking were significantly more frequent in the group of children with absence seizures.

We ascertained that HIHARS with AA is a relatively common occurrence in children and most likely represents an age-related nonepileptic phenomenon. When associated with fidgeting or yawning, it can help differentiate this phenomenon from absence seizures. However, recording the concomitant presence of generalized spike wave discharges on the EEG remains essential to confirm the diagnosis of absence seizures.