- This study examined the spatial distribution of barques in 51 patients who had stereo-electroencephalography electrodes implanted simultaneously in the anterior and posterior hippocampus as part of evaluation for therapeutic epilepsy surgery
- Barques were predominantly noted in the posterior hippocampus
- When barques occurred in the anterior hippocampus, they were always concurrent and synchronized with posterior barques, with lower amplitude/power
- Barques presented with phase reversal in electrode contacts that resided either immediately mesial or immediately lateral to the posterior hippocampal parenchyma, which seems to explain the positive polarity of scalp EEG 14&6/sec spikes
- These findings are important for differentiating barques from interictal epileptic activity, allowing more informed surgical decisions when mesial-temporal involvement is suspected, and reducing the risk of unnecessary hippocampal resections
Positive-polarity 14/sec and 6/sec spikes are well-known normal variants on scalp electroencephalography. Researchers at Massachusetts General Hospital previously reported that the intracranial correlates of these variants are hippocampal barques, which manifest as a series of high-amplitude spikes of about 14 Hz, having a ramping-up and often ramping-down profile. Mass General researchers also previously demonstrated that barques are normal variants of the intracranial EEG (iEEG), not a marker of epileptogenicity.
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Vasileios Kokkinos, PhD, epilepsy surgery neurophysiologist and instructor in the Department of Neurosurgery, Mark Richardson, MD, PhD, director of the Functional Neurosurgery Program, and colleagues have since determined barques show high specificity for the posterior human hippocampus. This observation, reported in Clinical Neurophysiology, is important for identifying barques and differentiating them from interictal epileptic activity.
The 51 participants in the study (mean age 38) were implanted with stereo-EEG electrodes as part of the evaluation for therapeutic epilepsy surgery. The implantation plan called for at least two electrodes unilaterally in either hippocampus: one covering the anterior hippocampus and one covering the ipsilateral posterior hippocampus.
All told, 70 hippocampi (32 left and 38 right) were implanted with simultaneous anterior and posterior coverage.
The principal observations were that:
- In all hippocampi where barque activity was identified, barques predominated in the posterior part of the hippocampus
- When barque activity was observed in the anterior hippocampus, it was always concurrent and synchronized with posterior barque activity, with lower amplitude/power
- Barques presented with phase reversal in electrode contacts that resided either immediately mesial or immediately lateral to the posterior hippocampal parenchyma
Interpreting the Findings
The anterior-posterior gradient in the spatial distribution of barques suggests they could be generated by CA3 pyramidal neurons in the posterior hippocampus.
The posterior-lateral hippocampal barque phase reversal explains the positive polarity of scalp 14&6/sec spikes.
Recognizing barques as normal will support more informed surgical decisions when mesial-temporal involvement is suspected and should reduce the risk of unnecessary hippocampal resections.
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