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Predicting Seizure-Free Status for Temporal Lobe Epilepsy Patients Undergoing Surgery: Prognostic Value of Quantifying Maximal Metabolic Asymmetry Extending over a Specified Proportion of the Temporal Lobe

Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, UCLA, Los Angeles, California

Correspondence: For correspondence contact: Daniel H. Silverman, MD, PhD, Ahmanson Biological Imaging Division, CHS, AR-144, UCLA Medical Center, MC694215, Los Angeles, CA 90095-6942. E-mail: dsilver{at}ucla.edu Guest Editor: Andrew Newberg, University of Pennsylvania

Conventional visual analysis of brain ¹⁸F-FDG PET scans is useful for predicting postsurgical improvement for temporal lobe epilepsy (TLE) patients, but prognostic value for identifying patients who will achieve seizure-free status is considerably lower. We aimed to develop an approach with which to quantitatively assess prognostically pertinent aspects of metabolic asymmetry in presurgical PET scans for forecasting postsurgical seizure-free clinical outcomes. Methods: Presurgical brain PET scans of 75 TLE patients were examined using a display/analysis tool that quantified maximal metabolic asymmetry in a specified proportion (x%) of the temporal lobe pixels in the most asymmetric plane, generating a temporal lobe asymmetry index (T-AI_x). Results of this analysis were compared with patients' actual postsurgical outcomes after an average of approximately 4 y of clinical follow-up. The investigation was divided into 2 main steps: The PET scans examined in the first step, selected by chronological order of scan acquisition dates, comprised just less than two thirds of the patient group studied (n = 47) and were used to look for parameters predicting seizure-free postsurgical outcome; in the second step, the predictive value of the parameters suggested by the analysis in the first step was independently examined using the set of remaining PET scans (n = 28) to check for wider applicability of the approach. Results: Of the 75 patients studied, 42 became seizure free after surgery, whereas 33 continued to seize beyond the immediate postoperative period, during a mean 3.8-y follow-up interval. The specified proportion of temporal pixels with which to assess maximal asymmetry that provided the highest prognostic value with respect to achieving seizure-free status was 20%. Across the study population, those patients with scans having lower T-AI₂₀ values (corresponding to <40% difference in pixel intensities between left and right temporal lobes, among the 20% most asymmetric left–right pixel pairs measured in the most asymmetric plane) were only half as likely to continue to have seizures postsurgically as those with scans having higher T-AI₂₀ values (positive likelihood ratio for achieving seizure-free outcome, 1.98; 95% confidence interval, 1.07–3.67). Overall, those patients with greater maximal asymmetry, as indexed by higher T-AI₂₀ values, had a significantly decreased chance of achieving seizure-free status after surgery than those with lower degrees of asymmetry (P = 0.017), and this same tendency was observed for both the first and second series of PET scans examined. Conclusion: A quantifying approach to assessing maximal temporal asymmetry over a specified proportion of the temporal lobe may help to predict whether patients will likely be free of seizures during the years after neurosurgical resection of epileptogenic tissue.

Key Words: temporal lobe epilepsy • metabolic asymmetry • quantitative assessment • seizure-free status • ¹⁸F-FDG • PET • presurgical evaluation

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