The electroencephalogram following Western and St. Louis encephalitis

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Abstract
Electroencephalograms obtained from 183 persons who had had Western or St. Louis encephalitis (about one-third of the patients in an encephalitis clinical follow-up study) were studied in relation to age at onset of illness, severity of illness, convulsions during and since the illness, effect on the tracing of the time interval between the illness and recording, and the type and severity of sequelae. Some observations and hypotheses resulting from this study follow. Because of recognized limitations in the data, these observations are not considered conclusive but may suggest areas for further investigation. Among the early follow-up electroencephalograms from patients less than 1 year of age at onset of encephalitis, normal tracings outnumbered abnormal tracings by almost 3 to 1, yet severe illness and severe neurologic damage occurred most frequently in infant patients. Possible explanations are [1] over-conservatism in interpretations and [2] inability of the immature brain to reflect abnormalities through the electroencephalogram. In some patients ill in infancy, the first electroencephalogram was normal and later tracings were abnormal, which supported the hypothesis that the immature brains of infants were unable to reflect abnormalities through the electroencephalogram. The proportion of abnormal to normal follow-up recordings was highest in the patients whose ages were 1 to 19 years at onset of illness, which would indicate the more mature brain was better able to reflect brain damage through the electroencephalogram. The proportion of abnormal tracings in the follow-up studies among patients 20 years of age or older at onset of Western or St. Louis encephalitis approximated the 15% expected in the general population. If the electroencephalogram made a year or more after the illness was abnormal, later tracings were also abnormal. Findings on recordings made after recovery correlated with neither the severity of the acute illness nor convulsions during the acute illness. Electroencephalo-graphic findings did not closely parallel the clinical state of the patient after the acute phase. Some patients with severe neurologic sequelae had normal tracings, and some patients with no objective evidence of brain damage had abnormal tracings. Abnormal recordings correlated more closely with emotional disturbances than with motor or intellectual impairment. Of 14 patients who had recurrence of convulsions, only 4 had abnormal tracings during the period of this follow-up study. All 14 were 13 months of age or younger at onset of encephalitis. The lack of a consistent correlation between follow-up electroencephalographic findings and the clinical status of the patient either during or after acute illness suggests that [1] Western and St. Louis encephalitides do not produce uniform changes in the electroencephalogram and [2] the tracing does not necessarily reflect the effects of a viral encephalitis.

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