Few other groups in the last few decades have made the study of coma pay off as much as the Belgian Coma Science Group under scientist Steven Laureys at the University of Liège. And from precisely this group now comes a new study, which has the potential to provide new foundations to diagnostics in clinically-defined consciousness. Mélanie Boly, a colleague of Laureys, has analysed, using sophisticated statistical tools, the “data transfer” in brains with limited or absence of consciousness. She comes to the conclusion that the associative cortex in the vegetative coma has a communication problem which is relatively specific to these patients and could potentially be detectable via EEG.
Persistent vegetative-state (PVS) patients are often still in a state of minimal awareness
But first things first. Laureys and his colleagues made headlines several years ago with a series of studies in which it could be proved that a not-inconsiderable proportion of patients who were regarded by clinicians as being in a vegetative state were actually in a state of minimal consciousness (Minimally Conscious State – MCS). In one of these studies the corresponding figure was over 40 percent. Similar data had already been collected by Nancy Child’s group from Austin, Texas.
The delineation of vigilant coma – which is also known as apallic syndrome or vegetative state – from MCS has been a much-debated topic. Patients in persistent vegetative state over years open their eyes during the daytime, but show no indication of intentional movements. In contrast, MCS patients react to their environment at least occasionally and can sometimes see or hear and respond to speech. The MCS can be a transition state in patients who slowly emerge from a coma. But it can also persist for years, in which case it is hardly clinically distinguishable from the vegetative state as MCS patients, just like vigilant coma patients, are not conscious in the usual sense. They often seem at first and even at second glance just like vegetative state patients.
Focus on frontal and temporal lobes
Clinically, the distinction is important because the chance of emerging from a MCS-state is significantly greater than the probability of awakening from a vigilant coma. Laureys and others have for that reason devoted themselves for years to the task of how persistent vegetative state and MCS can be better delineated. The classical EEG hasn’t helped until now at all. It’s true that a flatline EEG indicates brain-death with coma patients. PVS patients however are not brain dead. Imaging methods were therefore evaluated as an alternative, including functional MRI and Positron Emission Tomography (PET). The global rate of brain glucose measured using PET is significantly lower in the vegetative state than while fully conscious. There are nonetheless some cases of both healthy people with low glucose turnover and vegetative state patients in whom it is high. This offers therefore, clinically-speaking, nothing. More promising of success was the analysis of glucose turnover in the frontal and temporal lobes in a number of studies, that is in those areas in which the associative brain functions are located.
Consciousness makes longer loops
One problem with all imaging methods is that they are relatively impractical in clinical practice. Who wants to haul a coma patient into an MRI or PET scanner and carry out complex investigations? The findings from imaging studies could however contribute to less costly diagnostic methods experiencing a revival. That’s exactly what Mélanie Boly has made her task. As she has reported in the journal Science, she has analysed eight patients in a vegetative state,13 patients with MCS and 22 healthy control subjects for electrical potentials in the brain. These potentials provide information about what kind of communication a particular brain area exercised, in this case frontal and temporal lobes, in the different states of consciousness. She drew special attention to so-called evoked event-related potentials (ERP). They arise in response to sensory stimuli, such as sounds, and have different components that permit conclusions about what the frontal and temporal lobes do with the incoming “data”. In PVS patients, the relevant potentials were, at about 100 milliseconds, relatively short. In MCS patients, however – similarly to those in the healthy control subjects – ERP measures with long latency of 170 milliseconds were seen. The interpretation is now that with the vegetative state, while the sensory stimuli reach the frontal and temporal cortex, somehow the “backward-directed” further processing of stimuli from these associative regions towards deeper brain regions, however (unlike in patients with minimal residual consciousness) does not work.
“We need larger patient populations”
Responding to a request from DocCheck, Boly emphasised that an EEG-based distinction between the two states of consciousness would be at least possible: “An EEG is portable, and that’s a big advantage for clinical use. In principle these measurements could be integrated into a standard EEG-examination.” The expert stressed, however, that it is still much too early to be able to seriously discuss clinical use. “What we have now are data relating to a very limited number of patients. We need larger patient populations, and we must also urgently assess the prognostic value of the EEG response in these populations.”It’s also unclear for the time being whether the observed EEG responses to the acoustic stimulus are stable over longer periods, or if their type and expression are constantly changing“.