Signs of a Sleeping Brain Could Reveal Hidden Consciousness in Coma Patients

Comatose patients often have a questionable prognosis. Family and doctors are left to ask, what are the chances a loved one will awaken?

A team from Columbia University and New York-Presbyterian discovered signs of hidden consciousness can be detected using electroencephalography (EEG) scans by running tests at a more unusual time: at night, when a patient's brain might typically be asleep.

While comas are often compared to a deep sleep, unresponsive brains don't tend to go through typical wake-sleep cycles. Yet by catching signs of brain patterns called sleep spindles, researchers believe they can be confident regions of the brain that play vital roles in consciousness are still functional.

EEG scans have been used before in forming a prognosis on comatose patients, but the tests can be tricky to carry out, and the results aren't always reliable. Monitoring brains for other clues that correlate with these more difficult tests is an approach that's potentially more straightforward and even more accurate.

"We're at an exciting crossroads in neurocritical care where we know that many patients appear to be unconscious, but some are recovering without our knowledge," says Columbia University neurologist Jan Claassen.

"We're starting to lift the lid a little bit and find some signs of recovery as it's happening."

The researchers monitored electrical activity in the brains of 226 patients with acute brain damage in a number of regular overnight sessions. They also ran cognitive motor dissociation (CMD) tests on the same group: the more complex EEG-based analysis, where behaviorally unresponsive individuals are asked to respond to instructions to physically move.

Sleep spindles were identified by the researchers as predictors of whether or not someone would regain consciousness. These bursts of brain waves typically occur when conscious brains are in non-REM sleep, bouncing back and forth between areas of the thalamus in what's regarded as a process of sensory and memory consolidation.

What's more, they often appeared before signs of CMD, suggesting sleep spindles can predict recovery earlier. Further down the line, the researchers think manipulating brain waves during sleep could even improve the chances of consciousness returning.

"Spindles happen normally during sleep and they're showing some level of organization in the brain, suggesting circuits between the thalamus and cortex needed for consciousness are intact," says Claassen.

Sleep spindles weren't a fully accurate way of predicting someone coming out of a coma, but they could provide some big clues. Just over 40 percent of unresponsive patients with both well-formed sleep spindles and CMD recovered consciousness before leaving hospital, with more than three-quarters regaining neurological function within a year of the study period.

The researchers think sleep data could be combined with other related tests to detect signs of consciousness. It's an area we're making progress in: recent studies have also identified improved methods for detecting CMD, and found links between brain cell activity and consciousness.

"Families of my patients ask me all the time, will my mother wake up? How is my mother going to look in three, six, or 12 months?" says Claassen.

"Very often we cannot guide them very precisely, and it's crucial that we improve our predictions to guide their decision making."

The research has been published in Nature Medicine.