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A new type of test may be able to predict if people who enter a state of impaired consciousness after a brain injury will eventually recover.

The test, which involves seeing how people鈥檚 brainwaves respond to a general anaesthetic, was highly accurate when carried out on 12 people in a state of impaired consciousness 鈥� but now it needs to be trialled on more such people.

People who are in a coma after a brain injury, such as from a car accident or a stroke, sometimes emerge into a condition between coma and consciousness called a vegetative state. In this condition, their eyes may sometimes open, but they cannot respond to commands or move purposefully.

It is currently very hard for doctors to predict which of these people will recover, leading to great distress for families wondering if life support should be continued or stopped. 鈥淵ou need to make life-and-death decisions about someone you love, with minimal evidence 鈥� it鈥檚 typically agonising,鈥� says at McGill University in Montreal, Canada.

The new test exploits the fact that the brains of healthy people respond to anaesthesia with characteristic changes in their brainwaves, shown by placing electrodes on their scalp to carry out an electroencephalogram (EEG).

When healthy people are awake, their alpha brainwaves 鈥� which have a frequency of around 7 to 14 hertz 鈥� peak at the front of the brain first, with subsequent peaks happening progressively towards the back of the brain. If they lose consciousness under a general anaesthetic, the pattern flips, with the back leading the front.

Blain-Moraes wondered if this might be a useful sign of consciousness in people in a vegetative state. 鈥淚 suspect that nobody has tried this before because most people equate unresponsiveness with unconsciousness,鈥� she says. 鈥淲hy would you anaesthetise someone who you thought was already unconscious?鈥�

Her team carried out EEGs on 12 people who were either in a coma, a vegetative state or a similar condition called a minimally conscious state, in which people may occasionally show purposeful movements, like moving a finger when asked. The researchers used an injectable drug called propofol, often used to induce anaesthesia before surgery.

Four of the people responded to the injection in a similar way to healthy individuals. Three of them went on to recover full consciousness within three months of the test. The other person had their life support withdrawn soon after the investigation, so it is unknown if they would have recovered or not.

The study results weren鈥檛 available in time to be used as part of the medical decision-making for this person, and it would have been premature to change their treatment based on what was, at the time, an early-stage hypothesis, says Blain-Moraes.

The other eight people didn鈥檛 show the characteristic response. Of them, seven remained in their original condition and one died.

at the University of Li猫ge in Belgium says that while the approach needs to be tested in more people, the initial results are exciting. 鈥淚t鈥檚 a brilliant idea,鈥� he says. 鈥淎ny new approach is very much welcome. We now need to see if it鈥檚 true.鈥�

Another group has previously found that people in a vegetative state who respond to a bad smell by reducing how much air they inhale are more likely to show some recovery.

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