Anything going on in there? (comascience.org/ULg)

It’s the nightmare scenario: people think you are in a vegetative state when you are not. While some people with serious brain damage are totally unaware of their surroundings, others are in a “minimally conscious” state (MCS). These patients have some level of awareness but may be unable express it to those around them because of the injuries to their brain.

The discovery of a signalling pathway in the brain that is different in the two conditions could open the way to an easy and objective way of telling whether a particular patient has any remaining consciousness, even if they can’t respond in an obvious way.

Being able to distinguish between these states would make it easier to decide whether or not to turn off life support-machines.

At present, making these assessments is time-consuming and subjective, and misdiagnoses are common, says Mélanie Boly from the University of Liège in Belgium.. An estimated 40 per cent of those thought to be in a vegetative state may actually be conscious on some level.


EEG test

Now Boly and her colleagues think they have discovered a new way of telling these patients apart. They played irregular sounds to eight people previously diagnosed as being in a vegetative state, 13 people in a minimally conscious state and a healthy control group. All of the subjects were wired up to an electroencephalograph (EEG) machine, which can read brain activity through electrodes placed on the scalp.

When Boly’s team compared how the three groups responded to the sounds, they found that the brains of healthy and MCS patients generated a much longer signal than patients in a vegetative state.

To explain this difference, Boly turned to a type of mathematics called dynamic causal modelling. “It uses the information we know about the brain – how neurons are connected for example – to see which processes might explain this activity at the scalp level.”

The analysis pointed to a specific feedback mechanism between the temporal cortex, which is responsible for auditory processing, and a higher-level brain area called the frontal cortex. Both healthy and MCS subjects were able to send signals between these brain areas in both directions. In vegetative state patients, signals could pass from the auditory area to the frontal cortex, but not in the opposite direction.

Boly believes the loss of this so-called “top-down” process is what causes unconsciousness in brain-damaged patients, and that it can therefore be used to identify MCS patients. If further research backs up these findings, EEG could be used at the bedside of severely brain-damaged patients to diagnose their state of consciousness, Boly says.

Neuroscientist Nicholas Schiff at Weill Cornell Medical College in New York advises caution, however. The findings are ambiguous, he says, because of the small number of patients involved in the study. “Before it can be accepted as a diagnostic tool we need to test more patients,” Boly says.

Journal reference: Science, DOI: 10.1126/science.1202043