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Hippocampal Spindles and Barques Are Normal Intracranial EEG Entities

Key findings

  • This study assessed in humans whether hippocampal spindles and barques on intracranial EEG are markers of epileptogenicity
  • Pairs of pathologic and pathology-free groups were created on both the patient level (75 epilepsy patients with suspected temporal lobe epilepsy) and hippocampal level (102 hippocampi implanted, of which 43 were found to be epileptogenic)
  • No significant differences in spindle and barque incidence were found among patients with and without temporal lobe epilepsy or in the corresponding hippocampi
  • On both the patient and hippocampal levels, spindle frequency was similar between patients who were seizure free after surgery and those without temporal lobe epilepsy, while barques occurred more frequently in extratemporal lobe epilepsy
  • Proper interpretation of hippocampal spindles and barques may reduce unnecessary resection during epilepsy surgery

In 2020, writing in Frontiers of Human Neuroscience, Massachusetts General Hospital researchers established the term "hippocampal barques" for previously described waveforms on intracranial EEG (iEEG) that resemble epileptic discharges. Barques are rhythmic sequences of asymmetric 14 Hz high-amplitude spikes that are often overlaid on low-amplitude 6 Hz slow waves.

Barques have been considered the intracranial generator of a benign scalp EEG variant known as "ctenoids" or "14&6/sec positive spikes." Now, Vasileios Kokkinos, PhD, neurophysiologist and Instructor in the Department of Neurosurgery, and Mark Richardson, MD, PhD, director of the Functional Neurosurgery Program at Mass General, and colleagues provide the first direct evidence that hippocampal barques—and hippocampal spindles—are non-epileptogenic. Their report appears in Clinical Neurophysiology.

Study Methods

The researchers used traditional criteria to form pathologic and pathology-free groups according to clinical diagnosis:

  • 27 patients diagnosed with temporal lobe epilepsy (TLE, well known to involve the hippocampus)
  • 27 with TLE+
  • 21 with extratemporal TLE (lowest likelihood of pathology)

Between November 2011 and June 2019, these 75 patients underwent stereo-EEG in 102 hippocampi. The researchers also grouped the hippocampi into pathologic and pathology-free groups based on seizure onset involvement; 43 hippocampi proved to be epileptogenic on the ictal onset EEG and 59 were not.

Incidence of iEEG Entities

Spindle and barque incidence did not differ significantly among the three diagnostic groups at either the patient level or the hippocampal level.

Engel Class

59 of 75 patients were subsequently surgically treated. Among the 29 who achieved freedom from disabling seizures (Engel class I), the incidence of spindles was similar in the patient-based analysis and the hippocampal-based analysis.

Extratemporal Epilepsy

Compared with the two other diagnostic groups, barques were significantly more frequent in extratemporal epilepsy in both the patient-based and hippocampal-based analyses.

Implications for iEEG Interpretation and Surgical Treatment

These results suggest both spindles and barques are normal entities on hippocampal iEEG. In fact, the higher incidence of barques in patients without hippocampal pathology (those with extratemporal epilepsy) suggests they may signify a healthy non-epileptogenic hippocampus.

Mistaking barques and other normal hippocampal iEEG entities as epileptogenic may lead to unnecessary incorporation of the hippocampus into the surgical treatment plan.

Proper interpretation may improve outcomes and ultimately overcome the current underutilization of epilepsy surgery.

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