Intraventricular simple cysts
Intraventricular simple cysts, frequently referred to as intraventricular arachnoid cysts, are rare and usually asymptomatic. They represent an uncommon cause of an intraventricular cystic lesion.
It is important to realize (although this is of little clinical importance) that the term intraventricular arachnoid cyst is used as a blanket term for a number of cysts which share identical imaging and operative features but are histologically distinct. Some are indeed a type of arachnoid cyst whereas others are lined other tissue and are in fact ependymal cysts, neuroepithelial cysts or very large choroid plexus cysts 6.
Like arachnoid cysts elsewhere, they may be asymptomatic and discovered incidentally. When a cyst becomes symptomatic, the patient may present with headache, signs and symptoms of obstructive hydrocephalus, focal neurology or seizures2.
Intraventricular "arachnoid" cysts represent a number of different simple cystic structures, differing only in the histology of the wall.
Some are indeed lined by flattened arachnoidal epithelial tissue 6. Unlike the much more common arachnoid cysts located in the subarachnoid space, the origins of these lesions are controversial, since there should be no arachnoid tissue within the ventricular system. They are thought to arise from vascular mesenchyme or in some cases as an extension of a subarachnoid arachnoid cyst through the choroidal fissure and into the lateral ventricle 1-2.
Other cysts are lined by ependyma (known as ependymal cysts) and are lined by tall columnar epithelium, whereas still others are lined with cuboidal choroidal cells 6.
Intraventricular simple cysts have appearances similar to arachnoid cysts elsewhere (except for their location). They follow CSF on all modalities and sequences and have a very thin or imperceptible wall. They are most frequently encountered in the trigone of the lateral ventricles 6.
On CT the cyst itself can only be deduced to exist by the effect on the contour of the ventricle in which it arises. The ventricle is focally expanded. Any part of the ventricle proximal to it (e.g. the temporal horn of the lateral ventricle in lesions of trigone) may be dilated due to outflow obstruction.
CT ventriculography may be of benefit although seldom used since the advent of high-resolution MRI. If performed after injection of intrathecal contrast the cyst may be depicted as not filling with contrast at the same rate as the rest of the ventricle 5.
MRI is the modality of choice to fully characterize these lesions. The cysts, regardless of histology, will follow CSF on all sequences and demonstrate no enhancement or restricted diffusion. On high-resolution images (e.g. FIESTA / CISS) and even on good quality spin echo T2 images, the thin cyst wall may be visible.
Treatment and prognosis
Intraventricular arachnoid cysts are non-neoplastic, benign and usually asymptomatic. If large and/or thought to be symptomatic, these cysts can be fenestrated endoscopically 3-4.
The differential diagnosis is essentially that of other intraventricular cyst lesions including 6: