Trapped fourth ventricle
This patient clearly has shunt dysfunction accounting for the presentation. The differential for the posterior fossa cyst includes:
A trapped fourth ventricle is by far the most likely etiology in this case given that the patient has had prior shunt placement which implies A) absence of a large 4th ventricular cystic mass at that stage, and B) prior episode(s) of meningitis / hemorrhage, both common antecedents to ventricular isolation.
Arachnoid cysts are frequent anomalies of the CNS. They are benign lesions within the arachnoid membrane and have been reported to occur in virtually all locations where arachnoid is present. An intraventricular location, however, is rare and occurrence within the fourth ventricle is particularly uncommon. The first report was published in 1979 in a pediatric patient. Since then, only a few further examples have been reported. Most of these patients presented with hydrocephalus. Shunting procedures were performed, but did not afford long-term improvement of symptoms. Definitive treatment consisted of open resection of the cyst-wall.
The Blake's Pouch Cyst is a cystic malformation of the posterior fossa. It represents persistence and expansion of the normally transient Blake pouch which normally regresses during the fifth to eighth gestational weeks. It is described as posterior ballooning of the inferior medullary velum into the cisterna magna, and is communicating with open fourth ventricle below and posterior to the vermis. In this case the vermis appears intact behind the cyst and is therefore thought most unlikely.