Abstracts

Rationale: Non-neoplastic cystic lesions in the mesial temporal lobe are occasionally encountered on magnetic resonance imaging (MRI) of patients with epilepsy. These cysts are most commonly located in the choroidal fissure above the hippocampus. The role of these lesions in epileptogenesis is unknown. We studied the localization of the epileptogenic zone in relation to mesial temporal cysts in patients evaluated for epilepsy surgery. Methods: We searched the medical records and multidisciplinary surgery conference notes for any reference to a cystic lesion in the mesial temporal lobe in patients with drug-resistant partial epilepsy evaluated for epilepsy surgery at Vanderbilt University Medical Center between 1989 and 2011. Neoplastic lesions with cystic component were excluded. The MRI scans with a mesial temporal cystic lesion were mixed with control MRI scans and reviewed by a blinded neuroradiologist who confirmed the presence of cysts. Volumetric measurements of the cysts were obtained using manual tracing in Analyze 10.0 software. We noted the age of epilepsy onset, duration of epilepsy, results of pre-surgical workup, and for patients who had epilepsy surgery, the type and location of surgery and post-operative outcome. Results: We identified 13 patients with MRI evidence of mesial temporal cystic lesions. The majority of lesions were choroidal fissure cysts (10/13). In 9 patients, there were no other abnormalities on MRI. Hippocampal sclerosis was seen in 3 patients, ipsilateral to the lesion in one and contralateral in two patients. Mild increased T2 signal was seen around an anterior mesial temporal cyst in one case. Presurgical localization of the epileptogenic zone corresponded to the cystic lesion in 9/13 patients (64%); choroidal fissure cysts in these patients (6/9) had a relatively large volume (285-1328 mm3; mean 789±465 mm3). Six of these patients underwent amygdalohippocampectomy or anterior temporal lobectomy: two patients achieved long-term seizure freedom at 4 and 6 years of follow-up; two were seizure-free for 2-3 years before seizures recurred; one had only rare residual seizures but died one year after surgery due to an unrelated illness; and one patient did not have any improvement in seizure control. In the four cases where the epileptogenic zone had a discordant localization, the lesions were choroidal fissure cysts that were relatively small (75-267 mm3, mean 165±95 mm3). Two of these patients had contralateral hippocampal sclerosis. Conclusions: Our results demonstrate that non-neoplastic mesial temporal lobe cysts may represent epileptogenic lesions in some patients. The clinical significance of these lesions, specifically choroidal fissure cysts, appears to be related to their size- cysts smaller than 270 mm3 were not clinically relevant. While more studies are needed to confirm these findings, large mesial temporal cystic lesions should be considered potential epileptogenic lesions in patients with drug-resistant partial epilepsy.