Spontaneous regression of a cystic cavum septum pellucidum.

A persistent cavum septum pellucidum (CSP) is present in approximately 0.73% of adults, although its incidence ranges from 0.14 to 18.9% depending on the detection method. Cystic CSP is even rarer. A cyst causing mass within the CSP can obstruct the intraventricular foramen, leading to blockage of CSF flow and possible hydrocephalus, often justifying surgical intervention. We describe spontaneous decompression of a cystic CSP in a 36-year-old man. Initial MRI showed a cystic CSP with lateral bowing of the septal walls to 1.9 cm. Follow-up MRI 15 months later demonstrated no lateral bowing, and the septal wall width was 1.0 cm. This spontaneous decompression was not explained by the one previously described case report of cystic CSP regression.