Intermittent rhythmic delta activity (IRDA) morphology cannot distinguish between focal and diffuse brain disturbances

IRDA (intermittent rhythmic delta activity) is an abnormal generalized EEG pattern that is not specific to any single etiology and can occur with diff use or focal cerebral disturbances. To determine whether different electrog raphic features of IRDA and associated EEG findings can differentiate under lying focal from diffuse brain disturbances, we performed a blind analysis of 58 consecutive EEGs with an IRDA pattern, recorded from 1993 until 1996, in which we evaluated posterior background activity, focal slowing and IRD A characteristics (frequency, distribution, duration, symmetry and abundanc e). The clinical diagnosis, state of consciousness and CT brain findings we re retrieved from the patients' hospital records. There were 58 patients (3 3 females; mean age, 58+/-21 years). Twelve (21%) had only focal brain lesi ons, while 46 (79%) had diffuse brain abnormalities, (15 diffuse structural , 19 metabolic abnormalities, 12 postictal). Normal consciousness and focal EEG slowing were more frequent in patients with focal abnormalities, howev er, this was not statistically significant. Of the patients with focal abno rmality, 11 (92%) had normal posterior background activity either bilateral ly (n = 4) or contralateral to the focal lesion (n = 7). Bilaterally normal posterior background activity was observed in about 30% in both groups. Bi laterally abnormal posterior background activity was apparent in one patien t (8%) with focal brain lesion and in 31 patients (67%) with diffuse brain abnormalities (P<0.0001). There were no significant differences in IRDA ele ctrographic features between the focal group and the group with diffuse bra in disturbances. We conclude that IRDA morphology cannot distinguish betwee n focal and diffuse brain abnormalities. (C) 1999 Elsevier Science B.V. All rights reserved.