Neurologic Critical Care Electroencephalogram silence ratio for early outcome prognosis in severe head trauma

Evidence has been found that electroencephalography can provide important prognostic information in patients with severe head trauma as stressed by Jordan (1). However, the pathologic alteration in the electroencephalogram induced by traumatic brain injury and rated for outcome prediction varies considerably. Burst suppression pattern is well known to correspond with poor outcome, although it is most commonly a consequence of hypoxic or metabolic brain damage that might have occurred after trauma (2). Gütling et al. (3) found a high correlation between electroencephalographic (EEG) reactivity to external stimuli and outcome after head injury whereas Evans and Bartlett (4) emphasized that recognition of sleep-related activity was followed by an adequate neurologic state. Rae-Grant et al. (5) developed a scale for outcome prediction rating various EEG abnormalities from previous investigations. However, all data were obtained from raw electroencephalograms, which are frequently difficult to interpret, especially for inexperienced observers. It would be useful if a metric variable derived from raw electroencephalograms could provide accurate information for early outcome prognosis. In several investigations, low amplitude EEG events have been found to correlate with poor outcome after brain injury (6–8). RaeGrant et al. (6) described episodic lowamplitude events lasting 0.5–4 secs with EEG total power ,5 mV showing different background activities. Synek and Synek (7) detected the same phenomenon combined with theta activity only. Brenner et al. (8) reported intermittent EEG inactivity lasting for seconds during dEEG. All authors stressed that these patterns can be clearly distinguished from burst suppression. The question arises whether this electrophysiological occurrence may be quantifiable and subsequently able to provide valuable information regarding early outcome prediction in patients with head trauma. Rampil et al. (9) introduced a variable called “burst suppression ratio” for quantifying the burst suppression phenomena induced by narcotic drugs. The algorithm described intervals of suppression as periods . 240 msecs during which the EEG voltage did not exceed 5 mV, then calculated as percentage of a given time period. This variable might be suitable for assessing quantitatively low-amplitude events in the electroencephalogram. The present study was performed to investigate whether the “burst suppression ratio” might be a useful variable for early outcome prognosis in patients with severe head trauma. To avoid misunderstandings with respect to classic burst suppression pattern, we renamed the term “burst suppression ratio” as “electroencephalogram silence ratio” (ESR), From the University Hospital of the Technical University of Dresden, Department of Anesthesiology (Drs. Theilen, Ragaller, Tschö, and Albrecht); and the University Hospital of the Technical University of Dresden, Department of Neurosurgery (Drs. May and Schackert), Dresden, Germany. Address requests for reprints to: Hermann J. Theilen, MD, Universitätsklinikum Carl Gustav Carus, Klinik und Poliklinik für Anästhesiologie und Intensivtherapie, Technische Universität Dresden, Fetscherstrabe 74, 01307 Dresden, Germany. E-mail: theilen@rcs.urz. tudresden.de Copyright © 2000 by Lippincott Williams & Wilkins Objective: To introduce the electroencephalogram silenceratio (ESR) as a variable derived from mathematically processed electroencephalogram for early outcome prognosis in patients with severe head trauma and to comparatively assess sensitivity, specificity and predictive value vs. somatosensory evoked potentials and brainstem auditory evoked potentials. Design: Prospective, interventional study. Setting: Intensive care unit of a university hospital. Patients: A total of 32 adults with severe acute head trauma (Glasgow Coma Scale score <8). Methods and Main Results: In all patients, electroencephalographic recording was continuously performed by frontomastoid electrode montage for 24–96 hrs after admission to the ICU. The data were subsequently computed by fast Fourier analysis and the ESR (intervals of suppression as periods >240 msecs during which the electroencephalographic voltage did not exceed 5 mV) was displayed and recorded on a computer for further evaluation. Somatosensory evoked potentials and brainstem auditory evoked potentials were elicited during the first 2 days after admission. Outcome evaluation was performed 6 months after trauma using the Glasgow Outcome Scale and the Rappaport Disability Rating Scale. After careful artifact exclusion, the ESR depicted the highest sensitivity, specificity, and positive predictive value compared with evoked potentials. Even a highly significant correlation between outcome and ESR was found (p < .0001). Conclusion: The ESR is a valuable variable showing a high reliability with respect to outcome prediction in severe head trauma with a higher predictive value than short latency somatosensory evoked potentials. Evidence exists that the ESR provides at least partial information regarding adequate cerebral oxygen delivery. (Crit Care Med 2000; 28:3522–3529)