Abstracts

Rationale: Temporal lobe encephaloceles represent an increasingly recognized and treatable structural cause of focal epilepsy, particularly in patients with adult-onset seizures and idiopathic intracranial hypertension. The standard imaging workup for adult focal epilepsy includes brain magnetic resonance imaging (MRI) and 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT). Though typically diagnosed on MRI (see example case, attached figure top left panel), we hypothesized that PET/CT should be complementary in detecting and characterizing encephaloceles as CT can show typical skull base remodeling (top right panel) and PET may demonstrate FDG avid herniated brain tissue with little background signal (bottom panels showing axial and sagittal grayscale PET images).

Methods: We identified 27 adult epilepsy patients at our institution with MRI documented encephaloceles and associated FDG-PET/CT scans from August 2020 through August 2024. The scans were retrospectively reviewed independently by two neuroradiologists and rated as to the presence (absent, questionable, probable, definite, definite with distortion/encephalomalacia), number, and location of encephaloceles on PET/CT.

Results: The neuroradiologists agreed on the presence of encephaloceles as probable or definite based on FDG-PET/CT in 63% of cases (17 of 27) and questionable or absent in 15% (4 of 27), even questioning the accuracy of reference MRI in these latter cases.  The raters disagreed on the presence of encephaloceles in 22% of cases (6 of 27).  Agreed upon encephaloceles were found along the temporal poles in 88% of cases (15 of 17) and skull base foramina in 35% (6 of 17), noting lesions in multiple locations in some patients and bilateral lesions in 53% (9 of 17). Confidence did vary based on the quality and resolution of the PET scanner used.

Conclusions: Even with high resolution MRI or temporal bone CT, temporal lobe encephaloceles may be missed or the degree of brain herniation may be uncertain. PET/CT can be complementary and confirmatory and should be scrutinized for the presence of these lesions. We found known encephaloceles to be readily identifiable on PET/CT in most cases, though this does depend on the relative severity of brain herniation among cases in our sample. We did not assess the propensity for false positive prediction. Especially in cases with multiple lesions, future work should also focus on the reliability of FDG hypometabolism in distinguishing epileptogenic encephaloceles in relation to seizure semiology, electroencephalography, and surgical outcomes.

Funding: None