Stimulus-induced rhythmic, periodic, or ictal discharges (SIRPIDs): A common EEG phenomenon in the critically ill

Purpose: To describe an underrecognized EEG phenomenon in critically ill patients undergoing continuous EEG monitoring (cEEG). We named these EEG patterns stimulus-induced rhythmic, periodic, or ictal discharges (SIRPIDs). Methods: We reviewed 150 consecutive patients undergoing cEEG during a 9-month period and compared those with and without SIRPIDs. SIRPIDs were defined as periodic, rhythmic, or ictal-appearing discharges that were consistently induced by alerting stimuli. Results: We identified 33 patients with SIRPIDs (22%). SIRPID patterns included periodic epileptiform discharges in 21 patients (nine lateralized) and rhythmic patterns with evolution that fulfilled criteria for ictal discharges in 18 patients (12 unilateral). Eight patients had prior epilepsy; 24 had acute brain injury. Half the patients (17 of 33) had seizures, clinical or subclinical, during the acute illness in addition to SIRPIDs, and half (16 of 33) did not. No significant difference was found in the incidence of clinical seizures in patients with SIRPIDs (30%) compared with those without (45%). Clinical status epilepticus was more common in patients with focal (43%) or ictal-appearing (33%) SIRPIDs than in those without SIRPIDs (17%). Conclusions: Rhythmic, periodic, or ictal-appearing EEG patterns are commonly elicited by stimulation in critically ill, encephalopathic patients. Recording video, documenting patient stimulation, or repetitively examining patients during cEEG is necessary to recognize these patterns and to differentiate SIRPIDs from spontaneous seizures. Further research is necessary to determine the pathophysiologic, prognostic, and therapeutic significance of SIRPIDs.