¹⁸F-FDG PET in drug-resistant epilepsy due to focal cortical dysplasia type 2: additional value of electroclinical data and coregistration with MRI

Purpose: To assess the localizing value of ¹⁸F-FDG PET in patients operated on for drug-resistant epilepsy due to focal cortical dysplasia type 2 (FCD₂).

Methods: We analysed ¹⁸F-FDG PET scans from 103 consecutive patients (52 males, 7-65 years old) with histologically proven FCD₂. PET and MRI data were first reviewed by visual analysis blinded to clinical information and FCD₂ location. The additional value of electroclinical data and PET/MRI coregistration was assessed by comparison with pathological results and surgical outcomes.

Results: Visual analysis of PET scans showed focal or regional hypometabolism corresponding to the FCD₂ in 45 patients (44%), but the findings were doubtful or misleading in 37 patients and negative in 21. When considering electroclinical data, positive localization was obtained in 73 patients, and this increased to 85 (83%) after coregistration of PET and MRI data. Under the same conditions, MRI was positive in 61 patients (59%), doubtful in 15 and negative in 27. The additional value of PET was predominant in patients negative or doubtful on MRI, localizing the FCD₂ in 35 patients (83%). Interobserver agreement correlated with the grade of hypometabolism: it was good in patients with mild to severe hypometabolism (82-95%), but moderate in those with subtle/doubtful hypometabolism (45%). The main factors influencing positive PET localization were the grade of hypometabolism and the size of the FCD₂ (P < 0.0001). Misleading location (nine patients) was associated with a small FCD₂ in the mesial frontal and central regions. Following limited cortical resection mainly located in extratemporal areas (mean follow-up 5.6 years), a seizure-free outcome was achieved in 94% of patients, including Engel's class IA in 72%.

Conclusion: In this series, ¹⁸F-FDG PET contributed to the localization of FCD₂ in 83% of patients. This high localizing value was obtained by integration of electroclinical data and PET/MRI coregistration. This approach may help improve the surgical outcome in extratemporal epilepsy, even in patients negative on MRI.