Hyperventilation in functional seizures: Evidence for subtypes.

PURPOSE: Functional seizures (FS) are heterogenous, with no agreed way to subdivide them. One FS subtype frequently observed during EEG is those whose seizures are provoked by hyperventilation. We wished to see whether this subtype might reflect a different seizure mechanism. METHODS: We analysed the video-EEG/ECGs of all patients with FS from two hospitals in Melbourne from 2010-6. RESULTS: We identified 120 patients during the study period, 107 of whom had usable recordings. Examining those 11 (10%) whose seizures had been induced by hyperventilation, we compared the heart rates of those where the seizure occurred during the hyperventilation, and those where they occurred afterwards. The during-hyperventilation group had a higher baseline heart rate which increased prior to their seizure; the after-hyperventilation group had a lower baseline heart rate and no pre-ictal increase. In those patients whose seizures were not hyperventilation-induced, the same two heart rate patterns could be found: those with a higher baseline heart rate showed increasing heart rate prior to seizure onset, while those with a lower baseline heart rate did not. Cluster analysis showed the sample was optimally divided into these two groups based on their pre-onset heart rate alone. CONCLUSION: Patients with FS show two distinct patterns of pre-ictal heart rate, which may reflect two distinct seizure mechanisms.