Coma patients, be it inflicted from trauma or initiated by doctors to preserve bodily functions, have their brain activity regularly monitored using electroencephalography (EEG). When in a deep coma the brain activity is described by a flat-pattern signal- basically minimal to no response, one of the limits that nearly prompts  establishing brain death. A group of physicians at University of Montreal, however, have discovered an up until now never before seen type of brain activity that kicks in after a patient’s EEG shows an isoelectric (“flat line”) EEG.

The discovery was first spurred by the findings of Dr. Bogdan Florea who was caring for a human patient in an extreme deep hypoxic (deprived of oxygen) coma under powerful anti-epileptic medication, typically used to control seizures. Instead of just a flatline, though, Florea also observed some unusual signals – anything that wasn’t flat was basically weird at this point. So Florea contacted the University of Montreal team and explained his peculiar situation.

Flat line and Nu-complex signals (credit: Daniel Kroeger et al./PLoS ONE)

The Montreal researchers found, after analyzing the patient’s records, “ that there was cerebral activity, unknown until now, in the patient’s brain,” said Dr. Florin Amzica. To test whether or not this was a measuring glitch of some sort, Amzica and team performed an experiment. The team recreated the initial patient’s coma state in cats (the model animal for neurological studies) by drugging them with a higher dose of isoflurane anesthetic than normal. This effectively placed the cats in a deep coma and the EEG showed the expected flat (isoelectric) EEG line. Things were all normal until then. However, after a while strong oscillations were observed.

When pinpointing their origin, the researchers found the signal’s origin was in the hippocampus, the part of the brain responsible for memory and learning processes. The researchers concluded that the observed EEG waves, or what they called “Nu-complexes,” were the same as those observed in the human patient.

Besides its peculiar nature, the finding might prove to be extremely important. For one, there are many cases in which doctors intentionally induce certain patients into coma to protect their bodies and brain. This may be technically faulty in practice. A deep coma, based on the experiment on cats, might be better suited since it preserves a certain brain activity.

“Indeed, an organ or muscle that remains inactive for a long time eventually atrophies. It is plausible that the same applies to a brain kept for an extended period in a state corresponding to a flat EEG,” says Professor Amzica.

“An inactive brain coming out of a prolonged coma may be in worse shape than a brain that has had minimal activity. Research on the effects of extreme deep coma during which the hippocampus is active is absolutely vital for the benefit of patients.”

“As these functions fade at the onset of unconsciousness, the orchestrating powers are relinquished to more basic structures such as the thalamus (in the case of sleep) or the limbic system [per the current data in the experiment],” the researchers said in the paper. “When these structures are released from neocortical influence, they begin to pursue activity patterns on their own and proceed to impose these patterns on other brain regions including the neocortex.”

Findings were reported in the journal PLoS ONE.

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