Arachnoid cyst of the cavum velum interpositum in a septuagenarian: radiological features and differential diagnosis.

The cavum velum interpositum (CVI) is a thin, triangular-shaped cerebrospinal fluid (CSF)-filled space between the lateral ventricles that lies below the fornices and above the third ventricle. It is a normal variant seen in premature and newborn infants and usually disappears with brain maturation. CVI is rarely seen in adults as a persistent primitive structure. Although moderate cystic dilatation of the CVI may sometimes be observed, a true large cyst is extremely rare with only a handful of reported cases, mostly in children and adolescents. We describe the case of CVI arachnoid cyst diagnosed on imaging in a septuagenarian with the complaint of occasional headaches.