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Noninvasive Correlates of Subdural Grid Electrographic Outcome

Kalamangalam, Giridhar P.*†; Morris, Harold H. III*‡; Mani, Jayanthi; Lachhwani, Deepak K.*; Visweswaran, Shyam; Bingaman, William M.

Journal of Clinical Neurophysiology: October 2009 - Volume 26 - Issue 5 - p 333-341
doi: 10.1097/WNP.0b013e3181baaab9
Original Article

Purpose: To investigate reasons for patients not proceeding to resective epilepsy surgery after subdural grid evaluation (SDE). To correlate noninvasive investigation results with invasive EEG observations in a set of patients with nonlesional brain MRIs.

Methods: Retrospective study of adult epilepsy patients undergoing SDE during an 8-year period at Cleveland Clinic. Construction of semiquantitative “scores” and Bayesian predictors summarizing the localizing value and concordance between noninvasive parameters in a subset with nonlesional MRIs.

Results: One hundred forty patients underwent SDE, 25 of whom were subsequently denied resective surgery. In 10 of 25, this was caused by a nonlocalizing subdural ictal EEG onset. Eight of 10 such patients were nonlesional on MRI. Among all nonlesional patients (n = 34 of 140), n1 = 10 of 34 patients had nonlocalizing and n2 = 24 of 34 had localizing, subdural ictal onsets. As groups, n1 and n2 were statistically disjoint relative to their noninvasive scores. Bayesian measures predictive of focal invasive ictal EEG were highest for complete concordance of noninvasive parameters, decreasing with lesser degrees of concordance. A localizing scalp interictal EEG was a particularly good Bayesian prognosticator.

Conclusions: A small but significant proportion of SDE patients are denied subsequent therapeutic resective surgery. This is due to several reasons, including a nonlocalizing intracranial ictal EEG. The majority of such patients have nonlesional MRIs. The noninvasive data may be summarized by a semiquantitative score, as well as Bayesian likelihood ratios, which correlate with subsequent invasive outcome. This approach may find use in the selection and counseling of potential surgical candidates offered SDE.

From the *Section of Epilepsy, Department of Neurology, Cleveland Clinic, Cleveland, Ohio; †Department of Neurology, University of Texas Health Science Center, Houston, Texas; ‡Department of Neurology, University of Vermont College of Medicine, Burlington, Virginia, U.S.A.; §Department of Neurology, Bombay Hospital and Research Center, Mumbai, India; ∥Department of Biomedical Informatics, University of Pittsburgh, Pittsburgh, Pennsylvania; and ¶Department of Neurosurgery, Cleveland Clinic, Cleveland, Ohio, U.S.A.

This article is based in part on a paper read at the 58th Meeting of the American Epilepsy Society, New Orleans, U.S.A., December 2004.

Address correspondence and reprint requests to Dr. G. P. Kalamangalam, Department of Neurology, University of Texas-Health Science Center, 7.099 MSB, 6431 Fannin, Houston, TX 77030, U.S.A.; e-mail:

Copyright © 2009 American Clinical Neurophysiology Society