Epilepsia, **(*):1–7, 2007 doi: 10.1111/j.1528-1167.2007.01327.x FULL-LENGTH ORIGINAL RESEARCH Cyclic electrographic seizures in critically ill patients David E. Friedman, Catherine Schevon, Ronald G. Emerson, and Lawrence J. Hirsch Comprehensive Epilepsy Center, Columbia University Medical Center, New York, New York, U.S.A. SUMMARY Purpose: Prolonged electroencephalographic mon- itoring has facilitated the detection of noncon- vulsive seizures. Compressed displays of EEG frequency spectra (such as compressed spectral ar- ray, CSA) can facilitate interpretation of continu- ous EEG by allowing the reader to observe on a single screen patterns evolving over many minutes or hours. Methods: Patients were identified retrospectively over a 4-year period as displaying a cycling pattern of seizures on CSA. Results: We describe a pattern of seizures recur- ring in a cyclic fashion in a series of 13 critically ill patients of all ages. Several patients had a gradual buildup of EEG power prior to each seizure. Conclusion: We believe that while not rare, this pattern is difficult to recognize on standard EEG recording but it is readily apparent on CSA. The underlying pathophysiology of cyclic seizures is not known, but we speculate that cyclic seizures represent a form of status epilepticus in which the usual seizure terminating factors are present and transiently effective, but are inadequate to prevent resumption of the seizure activity. Studying these patients may provide insight into the pathophysiol- ogy of seizure initiation and cessation. KEY WORDS: Cyclic seizures, Compressed spec- tral array, Continuous EEG. Continuous EEG (CEEG) recording or monitoring is becoming increasingly common in critically ill patients with abnormal mental status and refractory status epilep- ticus (Jordan, 1995; Claassen et al., 2001; Scheuer, 2002). CEEG has enhanced the detection of nonconvul- sive seizures, which are now recognized as occurring in as many as 19% of critically ill patients with altered mental status (Claassen et al., 2004). In these patients, comput- erized techniques can complement standard visual analysis of EEG. By depicting on a single screen the major frequen- cies present in the EEG from moment to moment over sev- eral hours, the compressed spectral array (CSA) permits easy recognition of slowly evolving trends that are more difficult to identify by standard visual analysis. Routine use of CSA along with standard visual inter- pretation of the EEG has led to the identification of what we now recognize as fairly common phenomenon in pa- tients with nonconvulsive seizures: seizures recurring at relatively constant intervals from 30 s to 20 min for up Accepted August 19, 2007; Online Early publication xxxxxxx. Address correspondence to David E. Friedman at Baylor College of Medicine, Peter Kellaway Section of Neurophysiology, Department of Neurology (NB 302), One Baylor Plaza, Houston, TX 77030, U.S.A. E-mail: dfriedm@bcm.tmc.edu Blackwell Publishing, Inc. C  2007 International League Against Epilepsy to hours on end, or “cyclic seizures.” On the CSA, these seizures are often preceded by a gradual buildup of power; a power peak coincides with the seizure itself, to be fol- lowed by attenuation and then another buildup of power and seizure. The recurrence of the seizures at regular in- tervals (periodicity) and the gradual buildup of power are less easily appreciated by visual inspection of the raw EEG alone. METHODS Thirteen patients were identified retrospectively over a 4-year period from patients undergoing CEEG monitoring for refractory status epilepticus or abnormal mental status, as having cyclic seizures. These seizures appeared to be recurring regularly on visual inspection of the quantitative EEG. A quantitative definition of “cyclic” was not used. Complete CSA data were available from patients recorded during the past 12 months, and it was reviewed system- atically. Clinical and EEG features were reviewed, includ- ing the CSA. Clinical data included demographics, medical and neurological diagnoses, history of epilepsy, and neu- roimaging findings with CT, MRI, or both. EEG monitoring was performed using a 21-channel recording sampled at 512 Hz/channel, with electrodes placed according to the international 10–20 system. CSA 1