Type II focal cortical dysplasia: electroclinical phenotype and surgical outcome related to imaging

Purpose: Type II focal cortical dysplasia (TTFCD), a highly epileptogenic lesion with severe epilepsy curable by surgery, is missed by magnetic resonance imaging (MRI) in about one third of cases. Little is known about the electroclinical presentation in these MRI‐negative patients and a poor surgical outcome is frequently reported. We compared the clinical and neurophysiologic features in MRI‐negative and MRI‐positive cases in order to better identify candidates for surgery. Methods: Among 62 consecutive TTFCD patients (38 male, 24 female; 7–52 years old; 22 children) operated for intractable epilepsy, 25 (40%) presented negative MRI findings. We compared the history of epilepsy; the type, frequency, and distribution of seizures; neurologic examination cognitive and psychiatric impairment; interictal‐ictal electroencephalography (EEG) and stereo‐EEG (SEEG) data, fluorodeoxyglucose positron emission tomography (FDG‐PET) data, neuropathologic findings; and surgical outcome in the MRI‐negative and the MRI‐positive groups. Key Findings: Severe partial epilepsy beginning in childhood, high seizure frequency including status epilepticus, stereotyped seizures suggestive of precise brain localization, extratemporal location and functional area involvement were characteristic and similarly found in both groups. On EEG, pseudorhythmic activity was found in about 40% of patients in each group. SEEG recordings demonstrated the typical pattern characterizing TTFCD in both groups. FDG‐PET had a localization value in 84% of the MRI‐negative cases and helped to delineate the dysplastic cortex in 65% of the MRI‐positive cases. The combination of imaging and neurophysiologic data allowed us to perform safe and restricted resections, limited to a single gyrus in more than half of all cases. In addition, we were able to avoid invasive monitoring in most MRI‐positive cases and even in some selected MRI‐negative cases. The proportion of patients with a favorable surgical outcome was comparable in both groups (88% in MRI‐negative and 94% in MRI‐positive cases). The main difference between the groups was a significantly higher frequency of sleep‐related epilepsy in the MRI‐negative group (p = 0.028). This phenotypic characteristic provides a new argument for TTFCD in MRI‐negative extratemporal epilepsy. Significance: These results lead us to consider that children or adult patients in whom electroclinical data suggest TTFCD, are highly suitable for surgery, especially for cryptogenic sleep‐related epilepsy.