Evolution of stimulus-induced rhythmic EEG discharges in three patients with encephalopathy

Stimulus-induced rhythmic EEG discharges (SIRDs) is a recently reported phenomenon in critically ill patients and little is known about their evolution. We found SIRDs in three patients with encephalopathy and followed them with serial EEGs. SIRDs appeared between 4 and 13 days after the onset of illness and persisted for 2-3 days. The discharges were elicited by tactile or nociceptive stimuli and lasted for 20-120 s. They were detected in 2/6, 1/3 and 2/11 EEGs performed between 9 and 32, 2 and 4 and 3 and 15 days, respectively, after the onset of illness. Their morphology varied: blunt triphasic waves, rhythmic delta activity and rhythmic sharp wave complexes. The background EEG activity was slowed or suppressed in all. One patient had acute disseminated encephalomyelitis (ADEM) with good recovery and the other two had fatal hypoxic ischemic encephalopathy. SIRDs appear to be a transient phenomena occurring in patients with encephalopathy, appearing hours to few days after the onset of illness. This is the first report of SIRDs in ADEM. Serial EEGs and repeated testing of EEG response to tactile and nociceptive stimuli is required for their detection. Larger number of patients with SIRDs need to be studied to assess their prognostic significance.