The brain waves associated with sleep and dreaming could be helpful for distinguishing between people in a persistent vegetative state and those who are minimally conscious ? a distinction that could seal the fate of the individual.
The minimally conscious state, persistent vegetative state and coma are all disorders of consciousness caused by severe brain damage. Minimally conscious individuals tend to have better outcomes than vegetative individuals, but distinguishing between them is difficult and misdiagnosis is common. Getting it wrong can sometimes mean that a person who might otherwise recover has their life-support machine switched off.
Earlier this year, M?lanie Boly at the University of Li?ge in Belgium and colleagues reported that minimally conscious patients respond to sounds, whereas vegetative patients do not. The team has now found more evidence that some brain function is preserved in the minimally conscious state.
Rapid eye movement
The team used electroencephalography (EEG) to record the electrical activity of the brain in six minimally conscious and five vegetative people while they slept. These individuals had been diagnosed using traditional tests for reflex movements and responses to sounds and other stimuli.
Five of the six minimally conscious people exhibited brain-wave patterns that are typical of normal sleep, alternating between rapid eye movement (REM) and non-REM, or "slow-wave", sleep during the night.
This characteristic pattern of brain activity was not detected in the vegetative individuals. Although their sleep-wake cycles were preserved, these individuals' brain-wave patterns stayed in the non-REM state throughout sleep.
The monitoring of sleep may be an additional way of assessing brain function in severely brain-damaged patients, and could help clinicians to differentiate between the minimally conscious and vegetative states, says Boly.
Perchance to dream
Adrian Owen at the University of Western Ontario in London, Canada, thinks it's too soon to say whether the new technique can improve on existing methods, such as behavioural testing and brain scanning.
"Perhaps when they have tried this technique on much larger numbers of patients they will be able to show that it has other advantages. Perhaps it can assist with prognosis, although future studies will have to look into this."
Boly's findings also hint that minimally conscious individuals may be capable of dreaming, because REM sleep is normally associated with the emotional content and vivid imagery of dreams.
"It is certainly possible," Owen says. "But just because they show the same patterns of brain activity that are seen when healthy people dream does not necessarily mean they are dreaming."
Journal reference: Brain, DOI: 10.1093/brain/awr152
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