Beyond the Third Ventricle: Suprasellar Arachnoid Cyst

Abstract

Suprasellar arachnoid cysts (SACs) are benign congenital collections of cerebrospinal fluid (CSF), accounting for approximately 9% of all arachnoid cysts [1–4]. These cysts progressively enlarge from an abnormality in the interpeduncular cistern or in the membrane of Liliequist. They can be classified as communicating cysts; that is, cystic dilatations of the interpeduncular cistern, and as non-communicating intra-arachnoid cysts of the diencephalic portion of the membrane of Liliequist [5]. In communicating cysts, the basilar artery bifurcation is located inside the cyst with no overlying membrane, whereas in a non-communicating cyst, the basilar artery and its branches are pushed posteriorly against the brainstem, and the cyst can be observed through a transparent membrane (diencephalic portion of the membrane of Liliequist). A new recent classification divides SACs into three types: SAC-1, SAC-2, and SAC-3. SAC-1 may arise from an expansion of the diencephalic portion of the membrane of Liliequist. SAC-2 shows a dilatation of the interpeduncular cistern and corresponds to a defect of the mesencephalic portion of the membrane of Liliequist. SAC-3 is an asymmetrical form that expands to other subarachnoid spaces [6]. However, even on a high-definition intraoperative view it can be difficult to distinguish these types of SACs, because the diencephalic portion of the membrane of Liliequist is very thin and transparent. Due to this difficulty this classification is not applied to the intraoperative images shown here.