Validity of a revised EEG coma scale for predicting survival in anoxic encephalopathy

A revised EEG grading scale in coma has been introduced previously. This scale is based on the internationally accepted 5 grade scale but also contains uncommon patterns such as spindle, alpha and theta pattern comas. By defining 15 separate grades and subgrades it was possible to reduce the number of patterns of uncertain prognostic significance for survival to 4 out of the 15 possible patterns. It is also hoped that by a clear definition of individual grades and subdivisions the scale will assist electroencephalographers who lack experience of coma cases. The validity of the scale has been tested in a group of 63 patients who suffered cardiac arrest in excess of 7 minutes' duration. The EEG was performed 24 to 36 hours after the onset of coma. The accuracy of prediction for survival was 98.4%, a rate higher than in previously published studies. The mortality rate was 70%, 19 patients surviving. There was a statistically significant correlation between mortality rate and duration of cardiac arrest and also between mortality rate and increasing age. Four suggested scale subdivisions did not occur in the material studied but these subdivisions are more common in traumatic encephalopathies. This study shows that an EEG performed 24 to 36 hours after cardiac arrest provides significant information for the prediction of survival. No specific patterns of microscopic changes in the brain were found to correlate with individual EEG abnormalities in fatal anoxic encephalopathies.