The significance of amplitude asymmetry in clinical electroencephalography

Forty-two EEG recordings with bilateral generalized slow background activity, but asymmetric because of suppression of the amplitude over one hemisphere, were selected for the study. This pattern was prospectively correlated with neurological findings as well as neuroimaging techniques. In 76% of cases, the suppression of the amplitude could be lateralized correctly to the site of cerebral insult. It is suggested that: (a) unilateral attenuation of the amplitude of the background EEG correlates highly with ipsilateral focal cerebral acute or subacute large lesion; (b) in cases where bihemispheric slow activity is present, the hemisphere with the lower amplitude is likely to be the site of such a structural lesion; and (c) this study also supports a previous report that amplitude depression has the same pathophysiological significance in EEG as delta activity.