Distinguishing between the vegetative state (VS) and the minimally conscious state (MCS) in clinical practise is currently subject to various difficulties, giving rise to cases of misdiagnosis. The diagnostic process is based on behavioural tests of awareness - VS patients exhibit no awareness of self or the external environment, whilst MCS patients exhibit limited awareness of both on a sliding scale.
Recent developments in technology and associated techniques have uncovered possibilities for aiding the diagnosis of VS and MCS: fMRI (functional magnetic resonance imaging) and EEG (electroencephalography) have both shown promising results. fMRI has been used to reveal signs of awareness in a small number of VS diagnosed patients and EEG has been found to predict negative outcome or even awakening in other small patient groups, however these techniques have not yet proved to be definitive diagnostic methods.
Diffusion tensor imaging (DTI) is a relatively new imaging technique based on MRI. In a recent study, 25 VS and MCS patients were examined using DTI and conventional behavioural testing to investigate whether differences between the states could be reliably identified with this technique. Specific differences in 'mean diffusivity' (a measure taken with DTI) between the VS and MCS patient groups were found in the thalamus and subcortical white matter. In this study VS and MCS patients were effectively diagnosed with 95% accuracy using the DTI measurements.
Although the exact meaning behind the mean diffusivity measure is not yet fully understood and DTI interpretation methods are still in development, this research demonstrates a promising breakthrough. In future DTI may, following further research, be used in conjunction with current clinical tests to reliably diagnose and differentiate VS patients from MCS patients, reducing the number of misdiagnosed cases seen today.
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Main reference:
Fernández-Espejo, D., Bekinschtein, T., Monti, M. M., Pickard, J. D., Junque, C., Coleman, M. R., Owen, A. M. (2011). Diffusion weighted imaging distinguishes the vegetative state from the minimally conscious state, Neuroimage. 54, 103-112.
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