[Intraventricular arachnoid cyst appearing with attacks of orbital pain: case report and review of the literature].

While there have been 5 cases of intraventricular arachnoid cyst published in the literature, the occurrence in the anterior horn of the lateral ventricle has not been reported. We report a case of intraventricular arachnoid cyst of the anterior horn causing attacks of orbital pain. A 30-year-old man was admitted with frequent attacks of orbital pain on his right side. Neurological examination revealed no abnormality. Plain CT showed a cystic dilatation of the anterior horn of the right lateral ventricle, and enhanced CT showed a deviation of the septal veins to the left side. T1-weighted MRI demonstrated a low-intensity mass in the anterior horn of the right lateral ventricle, and T2-weighted image demonstrated the mass as having high intensity. PEG in the sitting position showed no filling of air into the right lateral ventricle due to obstruction of the right foramen of Monro. The patient underwent an operation under a diagnosis of intraventricular benign cyst. The cyst wall was subtotally removed and the right foramen of Monro was opened. Histological examination of the specimen showed an arachnoid membrane with prolific collagen fibers. From an embryological point of view, the arachnoid membrane is derived from the arachnoid cell. We think intraventricular arachnoid cysts to originate from the remnants of the arachnoid cell on the tela choroidea or on the choroid plexus like intraventricular meningiomas.