How epileptogenic are temporal encephaloceles?

Encephaloceles are spontaneous or acquired abnormal herniations of the brain into congenital or acquired osseous-dural defects of the skull base or cranial vault. When located in the middle fossa, they may present with medically intractable epilepsy, recurrent meningitis, CSF fistulas, middle ear fluid, or unilateral hearing loss. Bony deficits may involve either the body or the greater wing of the sphenoid bone. Anteroinferior temporal encephaloceles, resulting from deficits of the horizontal portion of the greater sphenoidal wing laterally to the cranial base foramina of the sphenoidal bone, may be asymptomatic or lead to drug-resistant temporal lobe epilepsy. It has long been recognized that the epilepsy associated with such encephaloceles can be surgically treated. Small encephaloceles may be overlooked on standard neuroimaging, and the literature consists only of case reports and small series. For these reasons, it has been unclear how often these lesions are found in the surgical workup, and what the clinical characteristics of the associated epilepsy syndrome and the outcomes of surgical treatment are. These issues are addressed in this issue of Neurology® by a timely article by Saavalainen et al. that describes a large series of 23 patients from an epilepsy registry in Finland with anteroinferior temporal encephaloceles (7 bilateral) associated with epilepsy identified between 2006 and 2013. These 23 cases represented 0.3% of MRIs in new-onset patients and 1.9% among drug-resistant patients at their institute. They describe the MRI characteristics of these lesions, the ictal phenomena in 19 patients who underwent video-EEG monitoring, and results of surgery in 5 who received an anterior temporal lobectomy with amygdalohippocampectomy and 7 others who had a lesionectomy. Of the 12 patients who received resection, 9 were seizure-free and 3 had rare seizures with a 2.8-year mean follow-up. This work demonstrates that anteroinferior temporal encephalocele is less rare than previously reported, that its clinical presentation and ictal semiology is not particularly distinct from other temporal lobe epilepsies, that surgical outcome is very good, and that bilateral temporal encephalocele can also be amenable to surgery. It is somewhat surprising that a full 10% of temporal lobe resections in this series had encephaloceles. This may reflect better identification of these lesions, but could also be influenced by referral patterns and the surgical selection process at their center. In addition, encephaloceles may also have been seen more often because they might be becoming a bit more common, secondary to intracranial hypertension resulting from increasing obesity in the developed world. Although higher resolution MRI is typically used in the presurgical workup to search for subtle lesions such as cortical dysplasias, 3T MRI creates more distortion at the skull base, impeding detection of subtle encephaloceles. Perhaps a more reasonable approach should include both high-resolution MRI for parenchymal imaging and, as suggested by the authors, a high-resolution skull base 3D CT, which would add little to the workup cost. Not unexpectedly, when we look more carefully for these encephaloceles, we find them. Since review of this article, one of us (J.J.V.G.) has begun to routinely explore the circumference of the temporal lobe intraoperatively prior to removal. The figure illustrates a temporal encephalocele detected in this fashion. Most epilepsy-associated encephaloceles are due to medial sphenoid and medial temporal bone defects, as contrasted with temporal encephaloceles associated with CSF otorhinorrhea that are lateral over the tegmun tympani, with seizures reported in only 1%–2% of cases at this location. This difference in epilepsy occurrence between these 2 groups might be related to higher epileptogenicity of mesial temporal allocortex. However, it is also possible that anteroinferior encephaloceles may go undetected and untreated longer because there is no CSF leak, allowing eventual development of an overt seizure focus over many years. These “little brain hernias” in the anterior and inferior temporal lobe can cause big epilepsy, with disabling, drug-resistant complex partial seizures. They can be easily overlooked without proper neuroimaging and fastidious examination of these studies. If they are not seen on imaging, finding them by