Epilepsia, 45(2):109–123, 2004 Blackwell Publishing, Inc. C  2004 International League Against Epilepsy Stimulus-induced Rhythmic, Periodic, or Ictal Discharges (SIRPIDs): A Common EEG Phenomenon in the Critically Ill ∗ Lawrence J. Hirsch, ∗ †Jan Claassen, †Stephan A. Mayer, and ∗ Ronald G. Emerson ∗ Comprehensive Epilepsy Center and †Critical Care Division, Department of Neurology, Columbia University, New York, New York, U.S.A. Summary: Purpose: To describe an underrecognized EEG phe- nomenon 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 evolu- tion 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 sub- clinical, during the acute illness in addition to SIRPIDs, and half (16 of 33) did not. No significant difference was found in the in- cidence of clinical seizures in patients with SIRPIDs (30%) com- pared 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 neces- sary to determine the pathophysiologic, prognostic, and thera- peutic significance of SIRPIDs. Key Words: Critical illness— Encephalopathy—Continuous EEG monitoring—Periodic discharges—Convulsive status epilepticus—Alerting stimuli— Stimulus-responsive—SIRPIDs—PLEDs—Triphasic waves. Due to recent technological advances, it is now feasi- ble and practical to record 24-h continuous digital EEG (cEEG). This form of brain monitoring is being used in- creasingly often for patients with refractory status epilep- ticus and for other critically ill patients with abnormal mental status (1–3). In patients with refractory status epilepticus requiring continuous infusions of antiepileptic drugs (AEDs), cEEG monitoring is required, as the ma- jority of breakthrough and withdrawal seizures in these patients are subclinical (2,4). We have recently begun to record simultaneous digital video in many of these patients. This has been quite help- ful in differentiating artifact from abnormal brain activ- ity, identifying subtle clinical correlates to electrographic seizures, and assessing a patient’s state. By using cEEG with video and by frequently examining the patients at the bedside while recording cEEG, we have noted strik- ing EEG changes when stuporous or comatose patients Accepted October 1, 2003. Address correspondence and reprint requests to Dr. L. J. Hirsch at Columbia Comprehensive Epilepsy Center, Neurological Institute, Box NI-135, 710 W. 168 th Street, New York, NY 10032, U.S.A. E-mail: LJH3@columbia.edu are stimulated (e.g., by suctioning, examination). Many of these patterns appear ictal, yet are consistently elicited by stimulation. Without careful clinical–EEG correlation, it would not be possible to recognize that these patterns are stimulus induced or state related. Because of this dis- covery, we have been examining the effect of stimulation on the EEG routinely in our patients undergoing cEEG monitoring. The purpose of this article is to describe and increase awareness of the clinical and EEG features of this common phenomenon. We refer to these EEG patterns as SIRPIDs, an acronym for stimulus-induced rhythmic, periodic, or ic- tal discharges. We initially reported the same phenomenon under a slightly different acronym (4,5). Here we describe the clinical and EEG features of 33 consecutive patients with SIRPIDs. We also compared patients with SIRPIDs with those without to determine whether SIRPIDs correlate with clinical seizures, status epilepticus, or other clinical features. METHODS We reviewed clinical and EEG features of all pa- tients with SIRPIDs during cEEG monitoring (with or 109