Nancy A. Melville

October 28, 2014

BALTIMORE, Maryland — Key measures of continuous electroencephalography (EEG) can offer important insights into patient consciousness that might otherwise not be evident with the more traditional measures of the Glasgow coma scale (GCS) in the acute care setting, according to new research.

"Our sample was small and selective, but these findings add to the evidence that [EEG] measures can be surrogates for both level of consciousness and prognosis, to some degree," coauthor Barry Gordon, MD, PhD, director of the Cognitive Neurology/Neurospychology Division of the Johns Hopkins Medical Institution Department of Neurology, Baltimore, Maryland, told Medscape Medical News.

Despite its role as the current gold standard for behavioral measures of consciousness, the GCS, which involves behavioral assessment, may fail to identify certain alterations of consciousness that have sensory, motor, or language deficits.

Continuous EEG has meanwhile increasingly gained ground as an important tool in evaluating cerebral electrical activity and helping to detect, monitor, and guide treatment of seizure activity, Dr Gordon explained.

In relation to measures of consciousness, delta and theta frequencies on EEG represent slower brain activity, while alpha and beta frequencies suggest faster activity.

"As a complex mental function such as consciousness presumably requires multiple underlying mental subcomponent functions, faster activity can be a surrogate for the physical substrate of such functions," the authors explained.

Previous studies have shown certain ratios of the measures to be particularly useful in assessing consciousness and coma in the chronic care setting.

The alpha-to-delta ratio (ADR) specifically, when measured poststimulus, can detect deteriorations of consciousness with sensitivity and specificity (Claassen et al, 2004), and the ratio of delta-plus-theta to alpha-plus-beta (DTABR) is prognostic in cases of decreased neurologic function (Sheorajpanday et al, 2011).

In research presented here at the American Neurological Association (ANA) 2014 Annual Meeting, Dr Gordon and colleagues evaluated EEG measures in relation to the GCS in the acute care setting. The study involved 21 patients receiving continuous EEG at Johns Hopkins Medical Center for transient alteration of awareness or change in mental status.

Inclusion criteria were having no seizures on any EEG, age 18 years or older, onset within less than 7 days, and no sedative medications at the time of evaluation.

In assessing the EEG and GCS data, the authors found significant correlations between GCS and the ADR (P = .001) as well as DTABR measures (P = .003), with higher GCS scores corresponding higher ADR and lower DTABR values.

The GCS and EEG measures also significantly correlated with patient outcome.

The authors noted that the study's limitations included the need to exclude various patients and more noise and artifacts on continuous EEG than desirable.

In addition, patients had widely ranging underlying diagnoses and often had more than one medical condition.

"Overcoming these challenges will be an important task for more thorough, prospective studies," they note.

The findings nevertheless point to the use of EEG as a potentially valuable tool in better assessing patient consciousness.

"These EEG measures may be of value in acute care settings, as adjuncts to behavioral measures, or as substitutes for behavioral measures," Dr Gordon said.

The study received support from the Therapeutic Cognitive Neuroscience Fund.

American Neurological Association (ANA) 2014 Annual Meeting. Abstract M1906. Presented October 13, 2014.

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