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PET scans offer new hope in brain injury

Research reveals PET scans best for determining which patients will recover consciousness

Mark Gould

Wednesday, 16 April 2014

For the first time researchers have tested the diagnostic accuracy of functional brain imaging techniques in clinical practice. Writing in The Lancet* they found that positive emission tomography (PET) scans are the most promising tool for determining which severely brain injured patients in vegetative states have the potential to recover consciousness.

Study leader Professor Steven Laureys from the University of Liége in Belgium compared traditional bedside clinical examinations, functioning Magnetic Resonance Imaging (fMRI) and PET.

Prof Laureys said that traditionally bedside clinical examinations have been used to decide whether patients are in a minimally conscious state (MCS), in which there is some evidence of awareness and response to stimuli, or are in a vegetative state (VS) also known as unresponsive wakefulness syndrome, where there is neither, and the chance of recovery is much lower. But up to 40% of patients are misdiagnosed using these examinations.

The study assessed whether PET with the imaging agent fluorodeoxyglucose (FDG) and functional MRI (fMRI) during mental imagery tasks—could distinguish between vegetative and MCS in 126 patients with severe brain injury (81 in a MCS, 41 in a VS, and four with locked-in syndrome—a behaviourally unresponsive but conscious control group).

The researchers then compared their results with the well-established standardised Coma Recovery Scale–Revised (CSR-R) behavioural test, considered the most validated and sensitive method for discriminating very low awareness.

Overall, FDG-PET was better than fMRI in distinguishing conscious from unconscious patients. Mental imagery fMRI was less sensitive at diagnosis of a MCS than FDG-PET (45% vs 93%), and had less agreement with behavioural CRS-R scores than FDG-PET (63% vs 85%). FDG-PET was about 74% accurate in predicting the extent of recovery within the next year, compared with 56% for fMRI.

Importantly, the researchers say a third of the 36 patients diagnosed as behaviourally unresponsive on the CSR-R test who were scanned with FDG-PET showed brain activity consistent with the presence of some consciousness. Nine patients in this group subsequently recovered a reasonable level of consciousness.

Prof Laureys says: “Our findings suggest that PET imaging can reveal cognitive processes that aren't visible through traditional bedside tests, and could substantially complement standard behavioural assessments to identify unresponsive or “vegetative” patients who have the potential for long-term recovery."

He says the research confirms “that a small but substantial proportion of behaviourally unresponsive patients retain brain activity compatible with awareness".

But he and his co-authors stress that the study was done in a specialist unit focusing on the diagnostic neuroimaging of disorders of consciousness and therefore roll out might be more challenging in less specialist units.

* Stender J, Gosseries O, Bruno MA, et al. Diagnostic precision of PET imaging and functional MRI in disorders of consciousness: a clinical validation study. The Lancet, Early Online Publication, 16 April 2014. doi:10.1016/S0140-6736(14)60042-8

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