Epidermoid cyst involving the medial temporal lobe: surgical pathologic features of the epileptogenic lesion.

Epidermoid cysts in the middle fossa are rare and may involve the temporal lobe and lateral ventricle. Affected patients often suffer from seizures, but the pathomechanisms underlying the epileptogenic lesions have remained unclear. Here we report the surgical pathological features of the hippocampus in a 31-year-old woman with mesial temporal lobe epilepsy (mTLE), in whom an epidermoid cyst involving the right basal cistern and inferior horn of the lateral ventricle was evident. The ictal electrocorticogram indicated seizure onset at the parahippocampal gyrus. An anterior temporal lobectomy and amygdalohippocampectomy were performed. Histologically, the hippocampus showed marked atrophy with severe loss of pyramidal neurons in the cornu Ammonis subfields and granule cell loss in the dentate gyrus. At the ventricular surface of the hippocampus, there were small granulomatous lesions with spicularly anchored keratin substance. These features indicated multiple and chronic stab wounds by the cyst contents and consequent local inflammatory responses within the parenchyma. The predisposition to adhesion between the tumor and hippocampus may have caused neurons to develop abnormal irritability to certain chemical mediators present in the cyst. Epileptogenicity involving the atrophic hippocampus and medial temporal lobes nearby may have developed in association with these processes. This case appears to provide information that is useful for surgical planning in patients with mTLE and epidermoid cysts involving the medial temporal lobe.